NIST Technical Note 1697 

Energy Consumption of Automatic 

Ice Makers Installed in Domestic 

Refrigerators 

Revision 1 

David A. Yashar 

Ki-Jung Park 
ii

NIST Technical Note 1697 

Energy Consumption of Automatic 

Ice Makers Installed in Domestic 

Refrigerators 

Revision 1 

David A. Yashar 

Ki-Jung Park 

Energy and Environment Division 

Engineering Laboratory 

National Institute of Standards and Technology 

Gaithersburg, MD 20899-7321

November 2011 

U.S. Department of Commerce 

John Bryson, Secretary 

National Institute of Standards and Technology 

Patrick D. Gallagher, Director 
iii

ENERGY CONSUMPTION OF AUTOMATIC ICE MAKERS INSTALLED IN 

DOMESTIC REFRIGERATORS

David A. Yashar and Ki-Jung Park 

National Institute of Standards and Technology 

Gaithersburg, MD 20899 

 

Abstract 

This study examines the energy consumption of automatic ice makers installed in 

domestic refrigerators. The purpose of this research is to determine a method of 

measuring the energy consumption of automatic ice makers that will generate a 

repeatable and reproducible result. This study examined four refrigerator-freezers of 

different configurations, one top-mount unit, one side-by-side unit, and two French-door 

units with bottom freezers. The components and operational modes of each ice maker 

varied among the test subjects. This study examined each automatic ice maker and its 

components, and discussed how the operation of each component contributed to the 

overall energy consumption. 

Two of the units had a single-speed compressor that maintained the compartments within 

a range of temperatures by switching on and off (cycling). Since this type of unit draws 

considerable power when the compressor operates and very little when it doesn’t, a good 

representation for energy consumption can only be obtained over a number of complete 

compressor cycles. However, the unit’s ice maker also produces cyclic variation in 

power draw; therefore accurate measurement of the ice maker energy consumption 

should also be obtained over a number of complete ice making cycles. Since the units’ 

compressor cycles and the ice making cycles are not coincident, it is difficult to 

determine the ice making energy using a predefined test period. 

The other two units employed inverter-driven, variable-speed compressors, which 

maintained a constant, part-load condition without cycling on and off. Determination of 
iv

the ice making energy consumption for these units was rather straightforward since data 

could be examined over several complete ice making cycles. 

We also examined the conditions which affect the ice making energy consumption. The 

results of these tests showed that the ice making energy consumption is influenced by the 

operating temperatures inside the cabinets. The results also showed that when their ice 

maker is switched from a mode where it is not producing ice to one where it is actively 

producing ice, some units exhibit temperature changes inside the cabinets without 

changing the thermostat settings. For these reasons, it may not be feasible to prescribe an 

ice making test at a specific thermostat setting, rather the results of multiple tests should 

be interpolated to a predefined set of compartment temperatures. 

The measurements showed that only approximately one half of the energy used by a 

refrigerator to operate an automatic ice maker is actually used to freeze water into ice; the 

other half is a result of using heaters to free the ice from the ice maker. The process of 

freeing ice from the molds may be accomplished by other, less energy intensive means; 

therefore, it is expected that introducing an ice making energy test will result in 

significant reductions of field energy use. 

Our measurements indicate that the ice making energy consumption varies widely from 

product to product. The most efficient product tested consumed approximately 

0.249 kWh per kilogram of ice produced under the test conditions used in this study, 

while the least efficient product consumed approximately 0.375 kWh per kilogram of ice. 

Variations in ice maker design and control algorithms played a large role in the ice 

production rate, which influenced the ice-making energy consumption. The annual 

energy consumption attributed to automatic ice makers added approximately 11 % - 20 % 

to the rated energy consumption value for each refrigerator, as determined by a regulatory 

test method. This value is based on the product of the measured ice making energy 

consumption per unit mass of ice produced and a consumer driven household ice 

consumption factor provided by a trade organization. 
v

This study shows that it is possible to develop a test method for rating the energy use of 

automatic ice makers. In order to develop a test method that results in a repeatable and 

reproducible test result, the test must compare the energy consumed by a refrigerator with 

and without the ice maker being operated and the refrigerator maintaining comparable 

temperature conditions. Furthermore, data must be collected over a sufficiently long test 

period in order to reduce the influence of the refrigerator’s compressor cycles operating 

at a different frequency from the ice maker’s cycles. 

Keywords: energy consumption, ice maker, refrigerator 
vi

Preamble 

In April 2011, we published NIST Technical Note 1697 which described energy 

consumption tests of automatic ice makers installed in four domestic refrigerators. After 

the completion of that study and publication of the results, we determined through 

discussions with manufacturers of the test subjects that two of the tested refrigerators 

were defective. It is important to note that, although defective, the test subjects 

performed their functions without indication that would be obvious in field operation. In 

order to assure the utility of the information disseminated from this study, we replaced or 

repaired the defective refrigerators and repeated the tests. This report supersedes the 

original NIST Technical Note 1697 and replaces the results and analysis derived from the 

tests of the defective test units. 

During the original study we noted that the refrigerator, known as the side-by-side 

refrigerator-freezer, was not producing as much ice as was expected. Through 

examination of this unit, the source of the malfunction was identified as a leaky 

connection between the solenoid valve that controls the flow of water to the refrigerator 

and the tube that delivers the supplied water to the ice-maker. This leaky connection was 

located at the exit point of the solenoid, near the bottom of the refrigerator, and water that 

dripped from the connection was ultimately routed to the same reservoir that receives the 

water runoff from the periodic defrosting of the evaporator. This leaky connection was 

repaired with an adhesive and the refrigerator was reexamined. 

The manufacturer of first refrigerator used in this study, known as the top mount 

refrigerator-freezer, also brought it to our attention that the ice production rate under the 

test conditions was much lower than the rate it was designed to produce. We examined 

the refrigerator and were unable to determine the cause of the malfunction without 

destroying it; therefore we purchased a new refrigerator of the same model as a 

replacement. 
vii

These two units were tested according to the methods outlined in the Technical 

Note 1697, but the results are quite different since these units are not defective. 

Chapter 3 and Chapter 4 of this study were rewritten with data and analysis determined 

from the new data sets. Minor changes were also made to Chapter 7, Chapter 8, and 

Chapter 9 of this report in consideration of the updated information. 

We also made one additional minor change to the document. In the initial study, we 

explored the possibility of using a purely calculation based approach as an alternative to 

measuring the energy consumed due to the production of ice. To perform these 

calculations, a value for the coefficient of performance (COP) of a refrigerator was 

needed. A value of 1.8 was used to perform the calculations, since this was the best 

estimate that could be provided by manufacturers. The use of this value was cautioned in 

the previous publication by the statement “It should be emphasized that this assumed 

COP is merely a placeholder assumption and has no verifiable basis.” Since the original 

publication was released, additional information was shared between several 

manufacturers which suggest that this value is too high; therefore the calculations were 

performed again with a COP value of unity. This new value has resulted in minor 

changes in Chapter 1, Chapter 3, Chapter 4, Chapter 5, and Chapter 6.
viii

Acknowledgement 

This work was sponsored by the United States Department of Energy. Dr. W. Vance 

Payne contributed to the test apparatus design, operation, and data acquisition. Mr. John 

Wamsley provided technician support throughout various phases of this project. Ms. 

Natascha Milesi Ferretti of NIST and Dr. Detlef Westphalen of Navigant Consulting Inc. 

provided technical commentary on the draft of this report. 
ix

Table of Contents 

Abstract .............................................................................................................................. iii 

Preamble ............................................................................................................................ vi 

Acknowledgement ........................................................................................................... viii 

Table of Contents ............................................................................................................... ix 

List of Figures ................................................................................................................... xii 

List of Tables ................................................................................................................... xiv 

Nomenclature .....................................................................................................................xv 

1. Introduction ......................................................................................................................1 

2. Test Setup and Data Acquisition ......................................................................................6 

3. Experimental Results for Top Mount Refrigerator Freezer .............................................8 

 3.1 Top Mount Refrigerator Freezer with No Ice Production .........................................8 

 3.1.1 Mid Setting Results .............................................................................................8 

 3.1.2 Warm Setting Results .........................................................................................9 

 3.1.3 Combined Test Results for Top Mount Refrigerator Freezer with No Ice 

 Production ..........................................................................................................9 

 3.2 Top Mount Refrigerator Freezer with Ice Production .............................................10 

 3.2.1 Mid Setting Results ...........................................................................................10

 3.2.2 Cold Setting Results ..........................................................................................12

 3.2.3 Mixed Setting Results .......................................................................................14 

 3.3 Calculation Based Method .......................................................................................16 

 3.4 Test Length Determination ......................................................................................17 

 3.5 Summary ..................................................................................................................22 

4. Experimental Results for Side-by-Side Refrigerator Freezer ........................................24 

 4.1 Side-by-Side Refrigerator Freezer with No Ice Production .....................................25 

 4.1.1 Mid Setting Results ...........................................................................................25

 4.1.2 Cold Setting Results ..........................................................................................25

 4.1.3 Combined Test Results for Side-by-Side Refrigerator with No Ice 

 Production ........................................................................................................27 

 4.2 Side-by-Side Refrigerator Freezer with Ice Production ...........................................27 

 4.2.1 Mid Setting Results ...........................................................................................28
x

 4.2.2 Cold Setting Results ..........................................................................................30

 4.3 Calculation Based Method for Estimating Ice-Maker Energy .................................31 

 4.4 Test Length Determination ......................................................................................32 

 4.5 Summary ..................................................................................................................36 

5. Experimental Results for French Door Refrigerator Freezer #1 ....................................37 

 5.1 French Door Refrigerator Freezer #1 with No Ice Production ................................38 

 5.1.1 Mid Setting Results ...........................................................................................38

 5.1.2 Cold Setting Results ..........................................................................................39

 5.1.3 Mixed Setting Results .......................................................................................40 

 5.1.4 Combined Test Results for French Door Refrigerator Freezer #1 with 

 No Ice Production .............................................................................................40 

 5.2 French Door Refrigerator Freezer #1 with Ice Production ......................................40 

 5.2.1 Mid Setting Results ...........................................................................................40

 5.2.2 Cold Setting Results ..........................................................................................43

 5.2.3 Mixed Setting Results .......................................................................................44 

 5.3 Calculation Based Method .......................................................................................46 

 5.4 Summary ..................................................................................................................46 

6. Experimental Results for French Door Refrigerator Freezer #2 ....................................49 

 6.1 French Door Refrigerator Freezer #2 with No Ice Production ................................50 

 6.1.1 Mid Setting Results ...........................................................................................50

 6.1.2 Cold Setting Results ..........................................................................................50

 6.1.3 Mixed Setting Results .......................................................................................51 

 6.1.4 Combined Test Results for French Door Refrigerator Freezer #2 with 

 No Ice Production .............................................................................................51 

 6.2 French Door Refrigerator Freezer #2 with Ice Production ......................................51 

 6.2.1 Mid Setting Results ...........................................................................................52

 6.2.2 Cold Setting Results ..........................................................................................54

 6.2.3 Mixed Setting Results .......................................................................................55 

 6.3 Calculation Based Method .......................................................................................57 

 6.4 Summary ..................................................................................................................58 

7. Observations ..................................................................................................................60 
xi

8. Recommendations for Ice-Making Energy Consumption Test .....................................62 

9. Open Items for Future Consideration ............................................................................63 

10. References ....................................................................................................................64 

Appendix : Uncertainty Analysis .......................................................................................65 
xii

List of Figures 

3.1 Top Mount Refrigerator Freezer ..................................................................................8 

3.2 Power Signatures of Top Mount Unit under Test ......................................................10 

3.3 Temperature Response to Ice Maker Operation ........................................................11 

3.4 Refrigerator Power at Cold Setting with Ice Production ...........................................13 

3.5 Graphical Representation of Triangular Interpolation ...............................................15 

3.6 Average Power Draw Over Full Compressor Cycle – Top Mount Unit Test #1 .......18 

3.7 Instantaneous and Cumulative Average Power Draw 

 Top Mount Unit Test #1 ............................................................................................19 

3.8 Ice Production Rate - Top Mount Unit Test #1 .........................................................20 

3.9 Instantaneous and Cumulative Average Power Draw 

 Top Mount Unit Test #2 ............................................................................................21 

3.10 Ice Production Rate - Top Mount Unit Test #2 .........................................................21 

4.1 Side by Side Refrigerator Freezer ..............................................................................24 

4.2 Power Signature of Side-by-Side Unit - Cold Setting ...............................................26 

4.3 Temperature Response of Side-by-Side Unit - Cold Setting .....................................26 

4.4 Refrigerator Power at Mid Setting with Ice Production 

 for Side-by-Side Refrigerator Freezer .......................................................................28 

4.5 Temperature Response to Mid Setting with Ice Production 

 for Side-by-Side Refrigerator Freezer .......................................................................29 

4.6 Temperature and Power of Side-by-Side Unit under Coldest Setting .......................30 

4.7 Average Power Draw Over 12 Whole Compressor Cycle 

 Side-by-Side Unit Test #1 ..........................................................................................32 

4.8 Instantaneous and Cumulative Average Power Draw 

 Side-by-Side Unit Test #1 ..........................................................................................33 

4.9 Ice Production Rate - Side-by-Side Unit Test #1 .......................................................34 

4.10 Instantaneous and Cumulative Average Power Draw 

 Side-by-Side Unit Test #2 ..........................................................................................35 

4.11 Ice Production Rate - Side-by-Side Unit Test #2 .......................................................36 

5.1 French Door Unit #1 ..................................................................................................37 

5.2 Refrigerator Power during Defrost – French Door Unit #1 .......................................39 
xiii

5.3 Refrigerator Power During Ice Production at Mid Setting 

 for French Door Unit #1 ............................................................................................41 

5.4 Temperature Response During Ice Production at Mid Setting for 

 French Door Unit #1 ..................................................................................................42 

5.5 Power Response During Ice Production at Cold Setting French Door Unit #1 .........43 

6.1 French Door Unit #2 ..................................................................................................49 

6.2 Power Response for Ice Production at Mid Setting for French Door Unit #2 ...........52 

6.3 Temperature Response to Ice Production at Mid Setting for 

 French Door Unit #2 ..................................................................................................53 

6.4 Power for Ice Production at Cold Setting French Door Unit #2 ................................54 

6.5 Refrigerator Power with Ice Production at Mixed Setting for 

 French Door Unit #2 ..................................................................................................55 

6.6 Refrigerator and Freezer Compartment Temperatures with Ice Production at 

 Warm/Cold Setting for French Door Unit #2 .............................................................56 
xiv

List of Tables 

2.1 Measurement uncertainty ...............................................................................................6 

3.1 Ice making energy by various methods – top mount refrigerator freezer ....................23 

4.1 Ice making energy by various methods – side by side refrigerator freezer .................36 

5.1 Ice making energy by various methods – french door unit #1 .....................................47 

5.2 Energy consumption error associated with first batch – french door unit #1 ..............48 

6.1 Ice making energy by various methods – french door unit #2 .....................................58 

6.2 Energy consumption error associated with first batch – french door unit #2 ..............59 
xv

Nomenclature 

E Energy Consumption ......................................................................................... kWh/yr 

P Power .......................................................................................................................... W 

T Temperature ................................................................................................................. oC 

m Mass ............................................................................................................................kg 

U Uncertainty .............................................................. consistent with measured quantity 
1

1: Introduction 

Cyclic type automatic ice makers produce ice within one of the compartments of a 

domestic refrigerating appliance. They are directly connected to a source of water and 

continuously produce batches of ice and store it in a low temperature bin. At the present, 

these devices are rendered inoperative during the regulated energy consumption test (10 

Code of Federal Regulations Part 430, Subpart B, Appendix A1, 2010) therefore the 

energy used to make ice is not measured. This is because ice making energy was not 

included in the basic energy consumption test (AHAM, 1979). The use of cyclic 

automatic ice makers do, however, have a significant impact on the product’s energy use, 

which effectively goes undetected under current regulatory test procedure. 

A cyclic type automatic ice maker consists of several components that enable repetitive 

cycling of three distinct sequential processes: (1) delivering of water into the ice maker, 

(2) freezing of water into ice, and (3) harvesting or removal of the ice from the molds in 

which it was frozen. The operation of these devices is halted when a sensor indicates that 

the ice storage bin is filled to its limit and can no longer accept new ice. The energy 

consumption associated with cyclic type automatic ice makers can be broken down into 

four categories. The first category is the energy used to deliver water into the ice maker; 

this is generally accomplished by energizing a solenoid valve. The second category is 

related to the additional load the refrigerator’s vapor compression system must handle to 

extract thermal energy from the water in order to cool and freeze it. This second category 

of energy consumption is equal to the extracted thermal energy, the enthalpy difference 

between the water entering the system and the frozen ice, divided by the appliance’s 

Coefficient of Performance (COP). The third category consists of the energy required to 

free the ice from the molds and deliver it to the storage bin. Most commonly, this is done 

by using electric resistance heaters to melt the interface between each ice cube and the 

mold where it was formed. The last category is related to the additional load to the 

appliance’s vapor compression system needed to remove any thermal energy introduced 

to the cabinet through the other steps. Most commonly, this is the thermal energy 

introduced by ice heaters used to free the ice from the molds divided by the appliance’s 

COP. 

Limited research exists in literature on energy consumption measurement of cyclic 

automatic ice makers. Meier and Martinez (1996) attempted to quantify the energy 

consumed by automatic ice makers by producing approximately 500 g of ice and 

subtracting off the energy consumed under similar conditions with the ice maker 

inoperative. The approach used in this study, however, did not consider the variability of 

the mass of ice produced during the test, which was as much as 12 %, nor did it consider 

the temperature changes in the cabinets during ice maker operation. Another study by 

Haider et al. (1996) showed a more comprehensive evaluation of ice maker energy 

consumption. In this study, the researchers compared the energy consumption with and 

without the ice maker operating under two temperature settings and normalized the ice 

maker energy to the measured quantity of ice produced during the test. Although these 

authors did acknowledge substantial differences in compartment temperatures when the 

ice maker was rendered operative while using similar thermostat settings, and substantial 
2

differences in ice production rates with different thermostat settings, they did not 

interpolate the results of their measurements to a common condition. 

These studies, however, were quite useful and have shown that the energy use associated 

with cyclic automatic ice makers is quite substantial compared to the total energy used by 

a domestic refrigerating appliance. These studies also suggested that energy used to cool 

and freeze water into ice does not constitute the majority of their energy consumption. 

Both studies show that the energy required to cool and freeze water into ice is 

approximately half of the total energy consumption of cyclic automatic ice makers. 

Considerations for Developing an Automatic Ice Maker Energy Test 

Theoretically, it should be relatively simple to extract the energy used by a cyclic 

automatic ice maker simply by performing the basic energy consumption test with and 

without the ice maker operating, and examining the difference of the measurement results. 

In practice, however, there are several issues that complicate the development of a 

repeatable test. Some of the specific points are outlined below. 

1) UIce-making cycles are not typically coincident with the appliance’s compressor 

cyclesU. Most domestic refrigerator-freezers use a single-speed compressor that 

operates intermittently to maintain the compartment(s) within a range of 

temperatures. In order to accurately measure the energy consumption for a unit 

using this type of compressor, the power must be averaged over a whole number 

of complete ‘on’ and ‘off’ compressor cycles during an energy consumption test. 

Attempting to quantify the energy consumption with anything short of a whole 

number of compressor cycles introduces systematic error. Each ice-making cycle 

produces a discrete amount of ice, which is not obtained until the harvest step. 

Therefore, attempting to characterize the energy consumption of a cyclic 

automatic ice maker with anything short of a whole number of ice-making cycles 

will also introduce error. Ice-making cycles are generally independent of the 

compressor cycles; therefore any method used to measure an automatic ice 

maker’s energy consumption must compromise between these two constraints. 

The error caused by this tradeoff can be reduced by extending the test period, 

which increases the number of whole compressor and ice making cycles under 

consideration. 

Furthermore, heat is removed from water at varying rates during the ice making 

process. Most cyclic automatic ice makers operate in such a way that water is 

frozen by cooling it with cold air. During the compressor ‘on’ time periods, the 

cold air near the ice molds is usually being circulated by fans; therefore the heat 

transfer mode used to remove thermal energy is forced convection. The heat 

transfer coefficients are roughly two orders of magnitude lower during the 

compressor ‘off’ periods because the fans are not running and heat is transferred 

to the surrounding air via free convection. Also, the air temperature oscillates 

during the compressor cycles (decreasing temperatures during the on cycles and 

increasing temperatures during the off cycles) and the temperature of the water in 

the molds is continuously decreasing as it approaches a frozen state. Therefore, 
3

as the water is cooled and frozen into ice, the heat transfer rate governing the 

production of ice varies widely because of the variations in heat transfer 

coefficients, water temperature, and air temperature. This aspect has an influence 

on the repeatability of an ice-making energy measurement and it compounds the 

effect of the non-coincident compressor and ice-making cycles because it is 

dependent on the temporal phase shift between the ice-making cycles and the 

compressor cycles. 

2) UCabinet temperatures may depart from steady-state operation in response to 

ice-maker operationU. When an automatic ice maker inside an appliance is 

rendered operative, one or more of the appliance’s compartments may exhibit 

some type of temperature change. This response varies by product configuration 

and control algorithm. Certain units respond in such a way that water introduced 

into the ice maker’s mold in one compartment results in a much higher 

temperature in that compartment due to the thermal load of the warm, unfrozen 

water. Other units respond in such a way that they overcompensate for this load 

and actually cool this compartment below the temperature that was maintained 

before ice making was initiated. Another variation is that the cabinet’s control 

system will simply have difficulty balancing the load between the fresh food and 

freezer compartments when making ice, therefore inducing a temporary departure 

from the steady-state temperature of both the fresh food and frozen compartments. 

It should be noted that the most highly sophisticated state-of-the-art units with 

electronic controls generally do not exhibit temperature changes of the same 

magnitude as the traditional mechanically-controlled units. This aspect raises the 

question of whether or not to consider cabinet temperature departures during ice 

making. This can be done by interpolating the results of multiple ice-making tests 

to calculate the ice-making capacity and energy consumption at a set of standard 

temperatures. This could then be a uniform basis for comparison with the basic 

energy test. The aspect of temperature departure should be considered in some 

way; however, there are at least two reasonable methods to consider this aspect. 

One method is to perform multiple ice-making tests and interpolate the results of 

these tests to a given target temperature. Another method is to perform a single 

ice-making energy consumption measurement and use the recorded temperatures 

from that data as a target set of temperatures for interpolation with the baseline 

energy consumption measurements (without making ice). Therefore this could 

consist of a single ice-making measurement in addition to the already existing 

energy consumption measurements. 

3) UDetermination of an appropriate inlet water temperatureU. The temperature of 

water supplied to the cabinet has an impact on the overall ice-making energy 

consumption because it is directly related to the amount of energy that must be 

extracted to freeze it into ice. In a typical household installation, water is supplied 

to the cabinet via direct connection to a water source generally maintained 

between 10 oC and 21 oC (50 oF and 70 oF). However, water in the supply lines to 

the cabinet moves very slowly which causes the water to reach thermal 

equilibrium with the environment before entering the cabinet. Therefore, 
4

attempting to supply water to the cabinet at any temperature other than the 

ambient temperature in the test chamber is not feasible. The ambient room 

temperature during the current energy consumption tests is 32.2 oC (90.0 oF). 

Using 32.2 oC (90.0 oF) as the temperature of the water entering the ice maker is 

an atypical condition and may present a biased challenge to a unit under test; 

however, using a temperature more representative of field conditions would result 

in greater test burden. 

4) UDetermination of how to treat interactions with defrost cyclesU. Most domestic 

refrigerators periodically undergo a cycle that removes accumulated frost from the 

evaporator. During this period of operation, the power consumption and cabinet 

temperatures undergo deviations from steady state operation. When a defrost 

cycle occurs during a time when an ice maker is rendered operational, analyzing 

the energy consumption measurement data can get quite complicated because 

these are two transient phenomena working against each other within the cabinet. 

Without going into a great level of detail, it is undesirable to use data that 

encompasses any portion of a defrost sequence when examining the ice making 

energy. This is a complication because the test operator generally has no control 

over the timing of a defrost cycle and cannot easily structure data collection to 

avoid defrost periods. Furthermore, in many cases the production of ice shortens 

the time between successive defrost sequences because many refrigerators employ 

some type of demand defrost and the supply of water into the ice maker often 

generates an accelerated demand for defrosting. 

5) UNon-standard ice production rate. Every model of automatic ice maker produces 

ice at its own rate, therefore it may produce a biased evaluation to rate such a 

device on the basis of energy consumed per unit of time. The more appropriate 

approach is to rate them by ice-making energy per unit mass of ice produced. 

This metric can later be converted to energy/time based on a standardized usage 

factor of mass of ice per unit time. With this in mind, it is important to note that 

while testing in a 32.2 oC (90 oF) ambient would reduce the test burden, the 

amount of energy that must be removed from the water to freeze it is greater than 

what would normally be expected in field operation, and the refrigeration system 

COP would be lower as well – both effects increase the measured energy use 

associated with ice making. Therefore, the rate at which the unit can produce ice 

under test conditions would likely be smaller than in the field. 

Because of all of these known difficulties, there have also been suggestions that a purely 

calculation-based approach for determining ice maker energy use might yield a better 

result. The suggested approach is to tabulate the energy used by each of the cyclic 

automatic ice maker’s components and add that to the additional compressor work which 

is the total thermal load (energy removal from water/ice plus the load from any heaters 

used) divided by the system’s COP. This approach will also be investigated as an option 

to gauge ice maker energy consumption. 
5

This study examines the ice-making energy associated with cyclic automatic ice makers 

installed in four different styles of domestic refrigerator-freezers. The objective of this 

study is to facilitate the establishment of a robust, repeatable ice making energy 

consumption measurement method that would be applicable to cyclic automatic ice 

makers and would not substantially increase the test burden beyond that of the currently 

regulated energy consumption test. This study characterizes the energy consumption 

attributed to four automatic cyclic ice makers by examining the difference between the 

energy consumption of each refrigerator-freezer with the ice maker operative and 

inoperative. Various interpolation methods are examined and their merits and drawbacks 

are compared. Specifically, we examined the energy consumption of an automatic ice 

maker by (1) examining the ice-making and non ice-making energy consumption of a 

unit at a single set of temperature settings; (2) examining the ice-making and 

non ice-making energy consumption of a unit using multiple measurements and 

interpolating the results to a common set of operational conditions, this was performed 

using two different interpolation methods; and (3) a purely calculation based approach. 

During the data analysis, the individual data sets were examined to determine an 

appropriate length of time for data collection, since steady state operation does not 

generally exist. 
6

2: Test Setup and Data Acquisition 

The test setup was constructed in accordance with the Department of Energy’s (DOE) 

test procedure outlined in 10 Code of Federal Regulations Part 430, Subpart B, 

Appendix A1, 2010. There was one slight modification to this setup; a continuous water 

supply was connected to the unit under test to supply water to the automatic ice maker. 

Two identical test cells were constructed to simultaneously measure the energy 

consumption of two refrigerators. Each test cell consists of a thermally non-conductive 

platform and a vertical wall adjacent to one edge of the platform. Each refrigerator is 

mounted on top of the platform with its rear adjacent to the vertical wall. All faces of the 

test cells were painted dull black to minimize the radiant heat transfer to and from each 

refrigerator during testing. 

The test cells were placed in an environmental chamber, which was large enough to 

house two test cells and all of the necessary data acquisition hardware. This 

environmental chamber was capable of providing controlled ambient temperature and 

humidity over long periods of time with little supervision, as necessitated by the lengthy 

test periods of domestic refrigerator energy consumption measurements. 

Water supply lines were connected to each test unit. The temperature of the water 

supplied to each unit was not controlled, but the tube connecting each line to a 

refrigerator was sufficiently long to allow the water to equilibrate with the temperature in 

the chamber. This was verified by inserting two T-type thermocouples into each water 

line just upstream of the refrigerator connection. 

All of the temperature and humidity data were gathered using a personal computer and a 

multiplexed data acquisition unit. The electrical energy input was monitored using a 

separate personal computer dedicated to two digital power meters, one connected to each 

test unit. All temperatures were sampled every 30 s, and the power was sampled every 

two seconds. Table 2.1 lists the measured quantities and the uncertainty associated with 

95 % confidence. The equations used to calculate the measurement uncertainty are 

shown in the Appendix. 

Table 2.1: Measurement uncertainty 

Measured quantity Measurement device Uncertainty at 95 % 

confidence 

Temperature Thermocouples ± 0.1 °C (0.2 °F) 

Power Watt-meter ± 0.5 % of reading 

Energy Watt-meter ± 0.5 % of reading 

Mass Digital scale ± 5 g 

The dry-bulb temperature in the environmental chamber was maintained at 32.2 oC 

(90.0 oF) in accordance with the US DOE test procedure. Throughout the duration of the 

tests, the analysis was performed to characterize the performance of the test subjects 

while maintaining cabinet temperatures of 3.9 oC (39 oF) in the refrigerator compartment 

and -17.8 oC (0 oF) in the freezer compartment. These target temperatures are different 

from the current Department of Energy test procedure, 7.2 oC (45 oF) in the refrigerator 
7

compartment and -15 oC (5 oF) in the freezer compartment, but will be incorporated in the 

new test procedure, which will be required for rating products starting in 2014 

(75FR78810), therefore it is of greater interest to use these new temperatures. It is 

important to note, however, that the test units in this study were rated using the current 

target temperatures, therefore it is expected that they consume more energy during the 

non-ice making tests than their rated specification at different temperatures. 

In total, four domestic refrigerator-freezers were used for this study. The units were 

selected to provide information spanning a variety of designs and features that are 

expected to influence ice making energy consumption. The first unit in this study is a 

top-mount refrigerator-freezer with an ice maker located in the freezer compartment. The 

second unit is a side-by-side refrigerator-freezer with an automatic ice maker mounted on 

the door of the freezer, and it dispenses ice through the freezer door. The third and fourth 

units are French door style units, whose freezer compartments are located beneath their 

refrigerator compartments. The third unit produces ice by directing some cold air from 

the freezer to an insulated ice-making sub-compartment in the refrigerator compartment. 

Lastly, the fourth unit produces ice by means of a separate ice making evaporator in an 

insulated ice making sub-compartment located in the refrigerator compartment 
8

3: Experimental Results for Top Mount Refrigerator Freezer 

The first test unit is a 600 L (21 ft3) top mount, energy star rated model shown in Fig. 3.1. 

It has variable defrost control, mechanical thermostat control, and a single-speed 

compressor. The unit has a factory installed cyclic automatic ice maker, which was 

connected to an external water supply and produces and stores ice in a bin located in the 

freezer compartment. The unit was outfitted with three thermocouples in the refrigerator 

compartment and three thermocouples in the freezer compartment, as shown below. The 

compartment temperatures used for the calculations that follow were the average of the 

time averaged values reported from the three thermocouples located in each compartment 

during the test duration. 

Figure 3.1 Top Mount Refrigerator Freezer 

The minimum and maximum time between defrost cycles for this unit is 8 hours and 96 

hours of compressor run time, respectively. According to the procedure outlined in the 

current DOE energy test procedure, these values result in a calculated average time 

between defrost of 30 hours of compressor run time. 

3.1 Top Mount Refrigerator Freezer with No Ice Production 

U3.1.1 Mid Setting Results 

During the first test, the energy consumption was measured with the thermostats set to 

the median setting for each compartment and the icemaker rendered inoperative. The 

following results were obtained: 
9

Steady state cyclic operation time: 10801 seconds = 03:00:01 

Measured refrigerator compartment temperature: (4.0 ± 0.1) oC 

Measured freezer compartment temperature: (-18.4 ± 0.1) oC 

Energy expended during the test period: (164.2 ± 0.8) watt-hours 

Alternatively, this can be expressed as an average steady state power of (54.72 ± 0.27) W 

at the measured temperatures. 

The defrost sequence for this unit consisted of (a) defrost heater switching on, (b) defrost 

heater switching off, (c) period of no power draw, (d) compressor on – recovery cycle, 

and (e) the following compressor off cycle. Measurements during this sequence yielded 

the following results: 

Defrost time: 8153 seconds = 02:15:53 

Energy expended during defrost period: (257.4 ± 1.3) watt-hours 

Combining the results of these two portions of the test, assuming that the average time 

between defrosts is 30 hours of compressor run time, we arrive at the total energy 

consumption of (498.9 ± 2.5) kWh/yr at the measured temperatures. 

U3.1.2 Cold Setting Results 

Since the temperatures in the refrigerator compartments for the first test was warmer than 

the target temperature, the thermostat was set to the coldest setting and the measurements 

were performed again. The following results were obtained: 

Steady state cyclic operation time: 12069 seconds = 03:21:09 

Measured refrigerator compartment temperature: (3.0 ± 0.1) oC 

Measured freezer compartment temperature: (-20.6± 0.1) oC 

Energy expended during the test period: (199.2 ± 1.0) watt-hours 

Alternatively, this can be expressed as an average steady state power of (59.4 ± 0.3) W at 

the measured temperatures. 

Defrost time: 9858 seconds = 02:44:18 

Energy expended during defrost period: (298.7 ± 1.5) watt-hours 

These values resulted in the total energy consumption of (540.3 ± 2.7) kWh/yr at the 

measured temperatures. 

U3.1.3 Combined Test Results for Top Mount Refrigerator Freezer with No Ice Production 

We then combined the results of the measurements at the first thermostat setting with the 

results at the second thermostat setting and interpolated to the target temperatures. When 

interpolating to the refrigerator target temperature of 3.9 oC, the calculated energy 

consumption was (503.0 ± 2.5) kWh/yr; when interpolating to the freezer target 

temperature of -17.8 oC, the energy consumption was (491.4 ± 2.5) kWh/yr. The 
10

maximum of these two values is declared as the baseline energy consumption: 

(503.0 ± 2.5) kWh/yr. 

It is also useful for later calculations to note that the energy consumption obtained 

through interpolation if the defrost energy was not considered. In this case, the 

calculations were performed with the steady state operational average power of 

(54.7 ± 0.3) W ((479.2 ± 2.4) kWh/yr) from the mid/mid setting and (59.4 ± 0.3) W 

((520.4 ± 2.6) kWh/yr) from the cold/cold setting rather than the total values which 

included the contribution of the defrost cycles. Here, negating the energy from periodic 

defrosting shows the energy consumption for this unit to be (483.5 ± 2.6) kWh/yr. 

3.2 Top Mount Refrigerator Freezer with Ice Production 

With the baseline energy consumption characterized, we next performed a series of 

measurements to examine the energy consumption attributed to the production of ice by 

the automatic ice maker. 

U3.2.1 Mid Setting ResultsU

The thermostat was set to the median position for each compartment and the ice maker 

was rendered operative. Water was supplied to the unit at a temperature of 32.2 oC. The 

power signature of this unit showed a few differences compared to the measurements 

with the ice maker inoperative. Fig. 3.2 shows the power drawn by the unit with the 

thermostats set to the median setting with (a) ice maker inoperative and (b) ice maker 

operative. 

Figure 3.2 Power Signatures of Top Mount Unit under Test 
11

The first difference between the signatures for the two different ice-maker settings is that 

the compressor on cycle is longer while the unit is producing ice; this is due to the 

additional cooling load. The second difference is the power consumed by the ice-maker 

components. There are two separate energy-consuming actions that are engaged during 

the process of ice making and harvesting. The first block of energy is consumed by the 

ice mold ejection heater, which draws approximately 190 W for a period of 90 seconds. 

This heater is used to heat up the tray in which the ice is formed so that it can be freed 

and dropped from the mold into the storage bin. The second device is a solenoid valve, 

which is powered up to allow water to flow from the supply into the ice molds; this 

solenoid draws approximately 28 W for a period of 10 seconds. It is also very important 

to note that the ice making cycle, which consists of the time period starting from the 

solenoid valve fill through the operation of the ice ejection heater, is not synchronized 

with the compressor cycle. Since the results of all measured quantities are averaged over 

a whole number of successive compressor cycles, it is desirable to record data over the 

longest attainable time period in order to increase the number of whole compressor cycles. 

Extending the data set in this manner increases the accuracy of the results by reducing the 

effects of the incomplete ice making cycles. 

Another aspect of the ice-making test data was that the additional load to the freezer 

compartment resulted in an elevated temperature in this compartment. Fig. 3.3 shows the 

temperatures measured during the ice-making test. At a certain time (approximately 

130 000 seconds on the graph) the ice maker automatically halted ice production due to 

the ice storage bin reaching its fill limit. After this point the freezer compartment 

temperature decreased by approximately 2 K which resulted in a temperature similar to 

that seen during the test at the same thermostat setting test with the ice maker inoperative. 

Figure 3.3 Temperature Response to Ice Maker Operation 
12

The data recorded during the time period encompassing 31 whole compressor cycles and 

all ice production showed the following results: 

Steady state cyclic operation time: 127278 seconds = 35:21:18 

Measured refrigerator compartment temperature: (3.7 ± 0.1) oC 

Measured freezer compartment temperature: (-16.2 ± 0.1) oC 

Mass of ice produced: (2.46 ± 0.005) kg 

Ice production rate: (69.6 ± 0.2) grams per hour 

Energy expended during the test period: (2.41 ± 0.01) kWh 

Alternatively, this can be expressed as an average steady state power of (68.21 ± 0.34) W 

at the measured temperatures. 

Single Data Point Method for Measuring Ice-Maker Energy Consumption 

The first proposed approach to quantifying the ice-making energy is to compare the 

steady state energy consumption with both thermostats set to their median setting for both 

ice producing and ice maker inoperative conditions. In this case, the difference between 

the average power with and without ice making was: 

(68.21 ± 0.34) W – (54.72 ± 0.27) W = (13.49 ± 0.43) W 

The energy used to produce ice in this configuration can then be calculated from the mass 

of ice, the differential power and the test period time. 

( )( )

( )

0.0135 kW 35.355 h

2.46 kg = (0.194 ± 0.006) kWh/kg 

The uncertainty of ± 0.006 kWh/kg was calculated using the individual uncertainties of 

the measured quantities. This notation is used throughout this report. 

This, however, may not be the best approach to estimate the ice-making energy because 

the temperatures realized in this unit changed considerably in response to the activation 

of the ice maker. 

U3.2.2 Cold Setting Results

The next approach was to attempt to use an interpolated energy (and ice production rate) 

using a second set of measurements. Since the internal temperature of the freezer 

compartment was warmer than the target temperature, we set the thermostats of each 

compartment to the coldest setting. When this unit was operated with the compartments 

set to their coldest temperatures and warm water introduced into the freezer through the 

ice maker, the automatic defrost control algorithm made the unit operate with the 

compressor running 100 % of the time and with a very short time between defrost periods. 

This mode did not allow the unit to operate in a steady state mode long enough to 

produce a significant set of useable data. Therefore, the thermostats were set to a 
13

position slightly warmer than the coldest position as a means to achieve a steady cyclic 

operation. 

The power measurements are shown in Fig. 3.4. The useable portion of this data set 

occurs between the two defrost events. The operation of the ice mold ejection heater is 

clearly visible in this figure, and its deployment can be used as a counter for the number 

of ice making cycles during the test. 

Figure 3.4 Refrigerator Power at Cold Setting with Ice Production 

The data from this set of measurements must be used to estimate the marginal energy 

consumption due to ice making; therefore we have to consider only the steady state 

operation portion, i.e. that which in not influenced by either defrost events. We need to 

calculate the relevant values averaged over a whole number of compressor cycles 

operating at steady state; therefore, we prorated the ice making events of the entire data 

collection period to determine the ice produced during the 11 events that occurred during 

the period of steady operation. 

Single cycle operation time: 38765 seconds = 10:46:04 

Measured refrigerator compartment temperature: (-2.4 ± 0.1) oC 

Measured freezer compartment temperature: (-18.1 ± 0.1) oC 

Mass of ice produced: (0.961 ± 0.005) kg 

Ice production rate: (89.2 ± 1.4) grams per hour 

Energy expended during the test period: (836.2 ± 4.2) watt-hours 
14

Average steady state power of (77.65 ± 0.4) W 

Two Data Point Method for Measuring Ice-Maker Energy Consumption 

Now, the energy consumption at the specified target temperatures is calculated based on 

the results of the mid/mid and cold/cold setting tests. For this calculation, the 

interpolated values for a freezer temperature of -17.8 oC are an energy consumption of 

(667.2 ± 3.9) kWh/yr. Since the ice production rate varied between these two tests, it was 

necessary to estimate the ice production at the target temperatures by some means. 

Therefore, the ice production rate at the target temperatures was estimated using a linear 

interpolation approach, this effectively weighted the measured ice production rates in the 

same proportion as the measured energy. The ice production rate at the target conditions 

was calculated as (754.3 ± 7.8) kg/yr. However, these interpolated values occur at a 

refrigerator compartment temperature of 2.6 oC, which manifests itself as a penalty on the 

energy consumption value because it is colder than the target temperature. Regardless, 

comparing these values to the non-defrost influenced data for the tests without ice 

production results in the following values: 

Energy consumption difference: 

(667.2 ± 3.9) kWh/yr - (483.5 ± 2.4) kWh/yr = (193.7 ± 4.6) kWh/yr 

Ice production rate: (754.3 ± 7.8) kg/yr 

Ice production energy: (0.257 ± 0.006) kWh/kg 

It should again be noted that this unit has difficulty controlling the internal compartment 

temperatures during the ice production phase, and that operating this unit at the same 

thermostat settings with and without ice production does not result in the same 

compartment temperatures. This is because ice production places a large processing load 

onto the duty of the freezer compartment without any addition to the refrigerator 

compartment’s duty. Also, the mechanical thermostat used by this unit has difficulty 

balancing the cooling load requirements between the compartments in ice production 

mode. 

U3.2.3 Mixed Setting ResultsU

In order to attempt to obtain a more accurate representation of the ice-making energy, 

another data set was taken with the freezer compartment set at the coldest setting and the 

refrigerator compartment set at its median setting. By using three data points, a more 

accurate triangular interpolation method (ASNZ, 2007) can be applied in lieu of the linear 

interpolation method. The following values were obtained from this data set. 

Steady state cyclic operation time: 39379 seconds = 10:56:19 

Measured refrigerator compartment temperature: (4.7 ± 0.1) oC 

Measured freezer compartment temperature: (-17.2 ± 0.1) oC 

Mass of ice produced: (0.962 ± 0.005) kg 

Ice production rate: (87.9 ± 1.1)) grams per hour 

Energy expended during the test period: (854.4 ± 4.3) watt-hours 

Average steady state power: (78.1 ± 0.43) W 
15

Three Data Point Method for Measuring Ice Maker Energy Consumption 

The triangular interpolation method is based on the idea that one can estimate the energy 

consumption of a two compartment cabinet at a specific pair of internal temperatures by 

using three data points that form a triangle around that pair of temperatures. Using the 

three measured data points, the position of a theoretical fourth data point (T4) is 

calculated, which lies at the intersection of a line joining two of the data points with a line 

joining the third data point to the target set of temperatures. This theoretical data point 

serves as a basis to calculate the energy consumption at the target set of temperatures. 

Fig. 3.5 below shows the data points in graphical form. For this particular unit, however, 

the mechanical thermostat had limited ability to control the compartment temperatures 

during ice production. Therefore, we were unable to acquire a third data set that 

graphically forms a triangle with the other data sets that contains the set of target 

temperatures of 3.9 oC and -17.8 oC; this is why the target temperatures lie outside of the 

triangle formed by the three measured data sets. The mathematics outlined in this 

method can still be applied to extrapolate the data to the target temperatures. 

Figure 3.5 Graphical Representation of Triangular Interpolation 

Using this approach, values for the energy consumption and ice production rates were 

calculated as if they were obtained at the target temperature conditions. 

Energy consumption: (700 ± 12) kWh/yr 

Ice production rate: (808 ± 21) kg/yr 
16

Comparing this to the baseline energy consumption value without ice productions, the 

result is: 

Energy consumption difference: 

(700 ± 12) kWh/yr - (483.5 ± 2.4) kWh/yr = (216.5 ± 12) kWh/yr 

Ice production energy: (0.268 ± 0.015) kWh/kg 

In this case, the value determined by three point triangular interpolation is similar to the 

value obtained through two point linear interpolation. This is because the refrigerator 

temperature realized through two point interpolation, 2.6 oC, is relatively close to the 

target refrigerator temperature of 3.9 oC. 

All of the above methods estimate the amount of energy consumed for the purposes of 

making ice while excluding the interactions between the ice production and the defrost 

sequence. It is very difficult to separate out the ice-making energy if these interactions 

are considered; particularly since ice making may, but does not necessarily occur during 

the defrost sequence. Also, this particular unit shows that operating the ice maker 

influences the automatic defrost control scheme, which may not be the case for all 

refrigerator freezers, and it is therefore questionable whether this interaction could be 

neglected. Furthermore, since ice making is prompted by user interaction and 

automatically halted when the storage bin is filled, it is difficult to determine the 

significance of these interactions during field use. Obviously, ice production is a much 

less efficient process during the time period that the defrost heater is operating, but this 

situation may have limited occurrence. 

3.3 Calculation Based Method 

The last method examined for determining ice-maker energy consumption is a simplified 

calculation approach. There are four components contributing to energy consumption 

during ice production: (1) energy used by the solenoid valve to deliver water to the ice 

molds, (2) the energy consumed to run the refrigeration cycle to meet the demand of 

removing heat from the water, (3) the energy used by the ice mold ejection heaters to free 

the ice from the molds, and (4) the energy used to run the refrigeration cycle to meet the 

demands of the additional load contribution of the ejection heaters. 

Each data set taken with the ice maker operative was started with the ice bin empty and 

logged until the bin reached its shut off limit. The first data set showed 3.28 kg of ice 

produced through 23 cycles, the second showed 2.99 kg in 21 cycles, and the third 

showed 2.85 kg in 20 cycles. Therefore, this unit produces an average of 142.5 grams of 

ice during each cycle. 

The solenoid valve draws 28 W for a period of 10 seconds to deliver water to the molds, 

which results in 280 joules per ice-making cycle. The ice ejection heater draws 190 watt 

for a period of 90 seconds to free the ice from the molds, which results in 17,100 joules 

per ice-making cycle. The total energy consumed by these components is therefore 

17,380 joules per ice-making cycle. Considering the mass of ice produced per cycle, this 

can be expressed as 122.0 kJ/kg of ice produced. Also, the energy consumed by the ice 
17

ejection heater is ultimately dissipated as heat into the freezer cabinet; therefore it must 

be considered an additional heat load to be processed by the refrigeration system. 

The energy required to cool 32.2 oC water to freezing is comprised of three parts. First, 

approximately 135 kJ/kg of energy is required to reduce the water’s temperature to 0 oC. 

Then 334 kJ/kg of energy is required to freeze it into ice. Lastly the ice must be cooled 

down to the freezer temperature (-17.8 oC) by expelling an additional 36 kJ/kg. In total, 

this accounts for 505 kJ of energy per kilogram of ice. 

The proposed calculation method relies on an assumed value for a Coefficient of 

Performance (COP). At any given set of operating conditions, modern products operate 

with a somewhat broad range of COP’s and it is difficult to select a representative value 

that is applicable for all appliances. Also, it should be noted that the COP used in the 

calculation approach is not the COP of the test unit itself, but rather the COP of ice 

making within the test unit which will be lower that the system COP due to other 

thermodynamic losses between the evaporator and the ice maker. For the sake of 

exploring the calculation approach, we will use an assumed COP value of unity, i.e. that 

the system can remove one unit of heat for every unit of electrical input. It should be 

emphasized that this assumed COP is merely a placeholder assumption and has no 

verifiable basis. 

While making ice, this refrigeration system must meet the additional demand of 627 kJ of 

heat per kg of ice produced (505 kJ for cooling and freezing the water plus 122.0 kJ for 

the load from the ejection heaters). Using the assumed value of the COP, the additional 

electrical input to the system required to meet this load is 627 kJ/kg of ice produced. 

Adding in the energy consumed by the solenoid and ejection heater brings the total load 

to 749 kJ/kg; or in alternative units 0.208 kWh/kg. 

3.4 Test Length Determination 

The main point of this exercise is to examine the power response from the unit while 

making ice in a little more detail in order to determine how much data needs to be 

recorded to properly calculate the energy consumption. Fig. 3.6 shows the average power 

drawn during each full compressor cycle while the unit was operating at the mid/mid 

setting and producing ice. 
18

Figure 3.6 Average Power Draw Over Full Compressor Cycles – Top Mount Unit Test #1 

Throughout this test period, there was either zero or one batch of ice harvested during 

each compressor cycle. The compressor cycles that included a harvest event drew, on 

average, about 5 W more than the compressor cycles that didn’t include a harvest event. 

Fig. 3.7 shows this same data plotted as the cumulative average power draw versus time; 

the instantaneous power is also shown in the figure. Here, the beginning of this data is 

coincident with the compressor switching on; therefore each local minimum point on the 

plot is indicative of another compressor on switch. Consequently, each local minimum 

also represents the cumulative average power draw over a whole number of compressor 

cycles. We can see that as time elapses, the local minima of cumulative average power 

draw asymptotically approaches a value of approximately 68.2 W. This value represents 

the average power draw needed to maintain the temperatures in the cabinet and 

simultaneously produce ice over a sufficiently long test period, which is one of the values 

that is of interest to this study. It is interesting to note that the running average stays 

within 1 % of the final value after only 2 compressor cycles and 1 ice ejection cycles 

were recorded. This means that the average power could be determined in a very short 

test period. 
19

Figure 3.7 Instantaneous and Cumulative Average Power Draw 

Top Mount Unit Test #1 

Fig. 3.8 shows the ice production rate, defined as the cumulative quantity of ice produced 

since the beginning of the test period divided by the test period length. Fig. 3.8 shows 

that while the cumulative average power is stable after 4 compressor cycles, the ice 

production rate is not as stable on a time scale. Instead, the ice production mechanism is 

directly related to the amount of time that the compressor is running; therefore the 

periods of time when the compressor is not running are effectively null or slow bands in 

the production of ice. 

The value of 69.6 grams per hour was reported for this data set based on the total length 

of the data set, however, the saw tooth pattern indicates that if we were able to extend this 

data set for an infinite amount of time, the reported ice production rate would lie 

somewhere between 68.5 grams per hour and 71.6 grams per hour. Since the reported 

value is less than 3 % from either of these limits; however, the data required to achieve 

this took over 35 hours to collect. 
20

Figure 3.8 Ice Production Rate - Top Mount Unit Test #1 

We also performed a similar analysis on the second set of ice making data. During the 

collection of this data set, the moisture addition to the cabinet from the operation of the 

ice maker caused the unit to undergo defrost cycles more frequently, thereby limiting the 

amount of useful data. Fig. 3.9 shows the instantaneous and cumulative average power 

draw for this data set. This figure shows that the average power can be determined to 

within 1 % of the final value after the third compressor cycle, less than 5 hours of data 

collection. Fig. 3.10, however, shows that the ice production rate is not as easy to 

determine by these methods. This figure shows that the saw tooth pattern is still 

oscillating between 84.9 grams per hour and 95.5 grams per hour. The reported value of 

89.2 grams per hour determined by the entire length of the useable data set, is less than 

7 % from either of these limits. 
21

Figure 3.9 Instantaneous and Cumulative Average Power Draw 

Top Mount Unit Test #2 

Figure 3.10 Ice Production Rate 

Top Mount Unit Test #2 
22

A similar analysis was also performed on the third data set. Similar to the other data sets, 

the average power remained within 1 % of the reported value after only three compressor 

cycles, less than 5 hours of data collection. The ice production rate, however, was still 

bounded by the limits of 81.7 grams per hour and 91.9 grams per hour at the end of the 

data set. 

3.5 Summary 

The ice-making energy was calculated by four methods as shown in Table 3.1. Also 

shown in the table is the energy impact of producing a representative consumer usage 

factor of 0.816 kg of ice per day (AHAM, 2009). It should be noted that this usage factor 

was based on a very limited sample of survey data and will likely be revised after further 

studies have been completed. 

The first method was to compare the difference in energy consumed with the thermostats 

set to their median positions. This method resulted in a large departure from the non-ice 

producing freezer temperature because of the control system used in this cabinet. The 

freezer temperature in this compartment was more than 2 oC warmer during the ice 

production process, which effectively biased the energy consumption results because it 

reduced the cooling load during the ice-making test. 

The second method attempted to characterize the ice-making energy by using the twopoint interpolation method for two energy consumption measurements, and comparing 

these results to the baseline energy test results without accounting for the defrost energy. 

This method attempted to mitigate the effects of the elevated freezer temperature during 

ice production. Because of the nature of the thermostat used in this system, it was 

difficult to finely control the temperatures during the ice-making tests, and this 

interpolation method resulted in a value that would represent the energy required to make 

ice while maintaining the appropriate freezer temperature and a refrigerator temperature 

that was more slightly colder than the target temperature. 

The third method attempted to characterize the ice-making energy by using the threepoint triangular interpolation method. The controllability of the cabinet made it 

impossible to actually triangulate around the specified set point; however, extrapolation 

of these data points should result in a reasonable estimate of the ice-making energy, 

depending on the linearity of the relationship between ice production rates and 

compartment temperatures. Upon initial examination of the results, it is surprising that 

this method returned a value of ice making energy that is higher than the two point 

interpolation method. After considering the limits on our ability to determine the ice 

production rates for the second and third data sets, however, the results seem reasonable. 

The final method was a simple calculation method based on the amount of energy 

required to process the ice-making load, and to power the auxiliary devices required to 

make and store ice. This method relies heavily on an assumed COP value which is a 

parameter that is not readily available for a given test subject. 

 
23

Table 3.1 Ice making energy by various methods – top mount refrigerator freezer 

Method Ice making energy 

kWh/kg kWh/yr % of baseline energy 

1 Median position only 0.194 58 12% 

2 Median and coldest 

positions 

0. 257 77 15 % 

3 Triangular Interpolation 0. 268 80 16 % 

4 Calculation 0. 208 62 12 % 

A point for consideration is the ice maker’s influence on the defrost controller. 

Alteration of the defrost control period will have influence the overall energy 

consumption. There is a wide variety of input that the current technology may use for 

variable automatic defrost controllers. The input used can range from measurement of 

ice on the evaporator, internal compartment temperatures, ambient temperatures, air 

humidity levels, door openings, data collected during previous operation, etc.; therefore, 

some defrost control algorithms may be strongly affected by the operation of ice makers 

and some may be completely unaffected. This particular unit was strongly influenced by 

the ice maker’s operation, but this effect was not considered in this analysis. This effect 

is worthy of investigation, however, and it should be revisited at some point in the future. 
24

4: Experimental Results for Side-by-Side Refrigerator Freezer 

This unit is a 760 L (27 ft3) side-by-side, energy star rated model. It has variable defrost 

control, electronic thermostat control, and a single-speed compressor. The unit has a 

factory installed ice maker, which is connected to an external water supply and produces 

and stores ice in a bin located on the inside of the door to the freezer compartment. This 

ice maker was equipped with an ON/OFF switch and is capable of producing ice at three 

different rates. For the purposes of these tests, all non-ice production test data were 

recorded with the ice maker switch set to the “ON” position and the bail arm forced to 

sense a full bin condition. The ice production rate was set to the highest setting. 

The unit was outfitted with three thermocouples in the refrigerator compartment and five 

thermocouples in the freezer compartment, as shown in Fig. 4.1. The compartment 

temperatures used for the calculations that follow are the average of the time averaged 

values reported from each thermocouple during the test duration. 

Figure 4.1 Side-by-Side Refrigerator Freezer 

The manufacturer provided the minimum and maximum time between defrosts for this 

unit as 7 hours and 50 hours of compressor run time, respectively. According to the 

method outlined in the current DOE energy test procedure, these values result in a 

calculated average time between defrosts of 22.4 hours of compressor run time. 
25

4.1 Side-by-Side Refrigerator Freezer with No Ice Production 

U4.1.1 Mid Setting Results 

This unit’s electronically controlled thermostat was capable of setting the freezer 

temperature between -21.1 oC and -13.3 oC, and the refrigerator temperature between 

0.6 oC and 7.8 oC. During the first measurement, each compartment’s thermostat was set 

to its median temperature setting, -17.2 oC for the freezer compartment and 3.9 oC for the 

refrigerator compartment. Examination of the test data showed that the temperatures in 

the refrigerator and freezer compartment temperature changes were not in sync; i.e. the 

refrigerator compartment would get colder and the freezer got warmer and vice versa. 

This suggests that this unit supplies cold air to the compartments in an alternating pattern. 

From an energy test point of view, this feature showed itself as a series of secondary 

cycles (both power and temperature) that occurred while the compressor was operating. 

For the purposes of these tests, each period during which the compressor was running 

was considered as a single compressor “ON” cycle, regardless of the number of times it 

shifted the cooling load between compartments. The following results were obtained: 

Steady state cyclic operation time: 14290 seconds = 03:58:10 

Measured refrigerator compartment temperature: (4.2 ± 0.1) oC 

Measured freezer compartment temperature: (-17.7 ± 0.1) oC 

Energy expended during the test period: (401.1 ± 2.0) watt-hours 

Alternatively, this can be expressed as an average steady state power of (101.1 ± 0.5) W 

at the measured temperatures. 

The defrost sequence for this unit consisted of (a) defrost heater switching on, (b) defrost 

heater switching off, (c) period of no power draw, (d) compressor on – recovery cycle, 

and (e) the following compressor off cycle. Measurements during this sequence yielded 

the following results: 

Defrost time: 9355 seconds = 02:35:55 

Energy expended during defrost period: (399.9 ± 2.0) watt-hours 

Combining the results of these two portions of the test, assuming that the average time 

between defrost is 22.4 hours, we arrive at the total energy consumption of 

(912 ± 4.6) kWh/yr at the measured temperatures. 

U4.1.2 Cold Setting Results 

Since the temperatures in both compartments for this test were warmer than the target 

temperatures, the thermostat was set to the coldest setting for each compartment, 0.6 oC 

for the refrigerator and -21.1 oC for the freezer, and the measurements were performed 

again. The power signature from this data set and the temperatures recorded in the 

refrigerator and freezer compartment are shown in Figs. 4.2 and 4.3. 
26

Figure 4.2 Power Signature of Side-by-Side unit - Cold Setting 

Figure 4.3 Temperature Response of Side-by-Side unit - Cold Setting 
27

By examining the temperature response to the compressor power draw, we can see that 

the unit shifts its cooling load between refrigerator and freezer compartments a number of 

times during each compressor “ON” cycle. The cyclic operation shows that the electrical 

load increases and the freezer temperature increases while cooling is supplied to the 

refrigerator compartment, and that the electrical load decreases and the refrigerator 

temperature increases while it is not. 

The following results were obtained: 

Steady state cyclic operation time: 20715 seconds = 05:45:15 

Measured refrigerator compartment temperature: (-2.0 ± 0.1) oC 

Measured freezer compartment temperature: (-21.5 ± 0.1) oC 

Energy expended during the test period: (729.9 ± 3.6) watt-hours 

Alternatively, this can be expressed as an average steady state power of (126.8 ± 0.6) W 

at the measured temperatures. 

Defrost time: 18882 seconds = 05:14:42 

Energy expended during defrost period: (776.8 ± 3.9) watt-hours 

These values resulted in the total energy consumption of (1133 ± 6) kWh/yr at the 

measured temperatures. 

U4.1.3 Combined Test Results for Side-by-Side Refrigerator with No Ice Production 

The results of the measurements at the first thermostat setting were combined with the 

results at the second thermostat setting and interpolated to the target temperatures. When 

interpolating to the refrigerator target temperature of 3.9 oC, the calculated energy 

consumption was (923 ± 3) kWh/yr; when interpolating to the freezer target temperature 

of -17.8 oC, the energy consumption was (918 ± 6) kWh/yr. The maximum of these two 

values is declared as the baseline energy consumption: (923 ± 3) kWh/yr. It is important 

to note that the interpolation results using the refrigerator temperature target and that 

using the freezer temperature target are very close to each other, therefore using the 

triangular interpolation method will not result in improved accuracy. 

It is also useful for later calculations to note the energy consumption obtained through 

interpolation if the defrost energy was not considered. In this case, the calculations were 

performed with the steady state operational average power of (101.1 ± 0.5) W (mid/mid 

setting) and (126.8 ± 0.6) W (cold/cold setting) rather than the total values. Here, 

negating the energy from periodic defrosting shows the energy consumption for this unit 

to be (897 ± 3) kWh/yr. 

4.2 Side-by-Side Refrigerator Freezer with Ice Production 

With the baseline energy consumption characterized, we next performed a series of 

measurements to examine the energy consumption attributed to the production of ice by 

the automatic ice maker. 
28

U4.2.1 Mid Setting Results 

The thermostat was set to the median position for each compartment and the ice maker 

was rendered active. Water was supplied to the unit at a temperature of 32.2 oC. 

Figs. 4.4 and 4.5 show the power and temperatures measured during the test. These 

figures demonstrate the complexity of the operation of this unit. The first point to note is 

that the compressor cyclic pattern changes periodically, this is because of the nature of 

the electronic thermostatic controller used in this system. This controller monitors the 

temperatures in both compartments and supplies cooling to each one based on its needs. 

Satisfying both of these compartments needs simultaneously results in the operational 

mode switching as shown in the figures. 

The power signature of this unit also shows the operational energy consumed by the ice 

making components. The ice ejection heater operates at approximately 130 W for a 

period of 150 seconds (19.5 kJ). After the ice is freed from the mold, a plastic rod is 

swept through each mold and pushes the frozen cubes to the storage bin. Then the 

solenoid valve opens to deliver water to the molds; this process consumes approximately 

32 W for 4 seconds (128 J). 

Figure 4.4 Refrigerator Power at Mid Setting with Ice Production 

for Side-by-Side Refrigerator Freezer 
29

Figure 4.5 Temperature Response to Mid Setting with Ice Production 

for Side-by-Side Refrigerator Freezer 

The data was recorded for the maximum amount of time that could be logged while 

avoiding a defrost cycle. The data was also reduced to allow a whole number of 

compressor cycles; the ice production cycles were again treated as secondary. 

Steady state cyclic operation time: 80801 seconds = 22:26:41 

Measured refrigerator compartment temperature: (3.1 ± 0.1) oC 

Measured freezer compartment temperature: (-17.7 ± 0.1) oC 

Mass of ice produced: (1.81 ± 0.005) kg 

Ice production rate: (80.6 ± 0.23) grams per hour 

Energy expended during the test period: (2.91 ± 0.01) kWh 

Average Power drawn during test period: (129.5 ± 0.6) W 

Single Data Point Method for Measuring Ice-Maker Energy Consumption 

The first proposed approach to quantifying the ice-making energy is to compare the 

steady state energy consumption with both thermostats set to their median setting for both 

ice producing and ice maker inoperative conditions. Since this particular unit had an 

electronic thermostat, the compartment temperatures measured during the tests at the 

median temperature settings were similar regardless of whether the ice maker was 

producing ice or not; therefore, it is thought that this method should provide a fairly good 

comparison. In this case, the differential power consumption was: 

(129.5 ± 0.6) W – (101.1 ± 0.5) W = (28.4 ± 0.8) W 
30

The energy used to produce ice in this configuration can then be calculated from the mass 

of ice, the differential power and the test period time. 

( )( )

( )

0.0284 kW 22.44 h = 1.81 kg (0.352 ± 0.01) kWh/kg 

U4.2.2 Cold Setting Results 

The next approach was to attempt to use an interpolated energy (and ice production rate) 

using a second set of measurements. Since the internal temperature of the freezer 

compartment was slightly warmer than the target freezer temperature, the thermostats of 

each compartment were set to the coldest setting for the second measurement. This 

caused the unit to operate with the compressor running 100 % of the time and caused the 

unit to initiate a defrost cycle after every 7 hours of compressor run time; the operation is 

shown graphically in Fig. 4.6. 

Figure 4.6 Temperature and Power of Side-by-Side Unit under Coldest Settings 

The compressor does not cycle under these conditions and with this very short time 

between defrost cycles this unit never achieves steady state operation as defined in the 

test procedure. We must therefore select a test period that consists of the longest period 

of time during which ice was actively produced without the influence of the defrost cycle. 

The first and second ice ejection events occur shortly after the deployment of the defrost 

heater and the freezer compartment is undergoing its recovery, therefore it is not 

desirable to use data prior to either of these instances. It is also important to note that the 
31

freezer and refrigerator compartment temperatures exhibit a cyclic pattern as the cooling 

load is periodically shifted between these compartments. We will therefore select the test 

period that begins at the instance the unit begins to supply cold air to the freezer 

compartment after the second ice ejection event and ends at the instance that the unit 

begins to supply cold air to the freezer compartment just prior to the next defrost event. 

The following results were obtained: 

Steady state cyclic operation time: 15198 seconds = 04:13:18 

Measured refrigerator compartment temperature: (-2.3 ± 0.1) oC 

Measured freezer compartment temperature: (-19.4 ± 0.1) oC 

Mass of ice produced: (0.467 ± 0.005) kg 

Ice production rate: (110.5 ± 0.6) grams per hour 

Energy expended during the test period: (0.625 ± 0.003) kWh 

Average Power drawn during test period: (148.0 ± 0.8) W 

Two Data Point Method for Measuring Ice-Maker Energy Consumption 

Now, the energy consumption at the specified target temperatures is calculated based on 

the results of the mid/mid and cold/cold setting tests. For this calculation, the results 

show that the interpolated values for a freezer temperature of -17.8 oC are an energy 

consumption of (1144 ± 9) kWh/yr and an interpolated ice production rate of 

(721.5 ± 14.5) kg/yr. The interpolation results using the refrigerator compartment 

temperatures are again very close to the interpolation results using the freezer 

temperatures. Comparing these values to those calculated from the tests without ice 

production (not considering defrost periods) results in the following values: 

Energy consumption difference: 

(1144 ± 9) kWh/yr - (897 ± 3) kWh/yr = (247 ± 10) kWh/yr 

Ice production rate: (721.5 ± 14.5) kg/yr 

Ice production energy: (0.342 ± 0.013) kWh/kg 

4.3 Calculation Based Method for Estimating Ice-Maker Energy 

The next method of estimating ice-maker energy was a simplified calculation approach. 

Each data set taken with the ice maker operative was started with the ice bin empty and 

logged until the bin reached its shut off limit. The first data set showed 1.90 kg of ice 

produced through 20 cycles, the second showed 1.96 kg in 21 cycles. Averaging the 

results of these two tests yields a production rate of 94.1 grams of ice per cycle (3.86 kg 

in 41 cycles). 

As mentioned earlier, the solenoid valve consumes 128 Joules per cycle, or 1.36 kJ/kg of 

ice produced. The ice ejection heater consumes 19.5 kJ, or 207.2 kJ/kg, which is then 

dissipated into the freezer compartment. Therefore, the total energy consumed by these 

components is 208.6 kJ/kg of ice produced. Considering the heat load for freezing water 

into ice and the removal of the dissipated heat from the ejection heaters, the total demand 

of additional cooling to the refrigeration system is 713.6 kJ/kg. Assuming that the 

refrigerating system operates with a COP of 1.0, this translates to 713.6 kJ/kg. Finally, 
32

adding in the energy consumed by the solenoid, ejection rod, and ejection heater brings 

the total load to 922.2 kJ/kg; or in alternative units 0.256 kWh/kg. 

4.4 Test Length Determination 

As with the previous test subject, we examined the power consumed by the unit while 

making ice in a little more detail in order to determine how much data needs to be 

recorded to properly calculate the energy consumption. Fig. 4.7 shows the average power 

consumed during each full compressor cycle while the unit was operating at the mid/mid 

setting and producing ice. 

Figure 4.7 Average Power Draw Over 12 Whole Compressor Cycle – Side by Side Unit 

Test #1 

This data set encompasses 12 compressor cycles that occurred while the unit was 

producing ice. Eight of these compressor cycles each produced two batches of ice, while 

the remaining four compressor cycles each produced one batch. It is interesting to note 

that while there was variation in the number of batches of ice produced during each 

compressor cycle, the average power consumed during each compressor cycle did not 

vary by much. The compressor cycle with the largest deviation from the mean was 

within 2 % of the average of all 12 compressor cycles. 

Fig. 4.8 shows the data plotted as both instantaneous power (pink) and cumulative 

average power draw (blue) versus time for this cabinet under the mid/mid thermostat 

setting. This figure shows that the cumulative average power draw changes by less than 

1 % between successive compressor cycles throughout the entire data set. The results 
33

showed that, under these conditions, it is possible produce a reasonable energy 

consumption result using only one compressor cycle, about 2 hours of data. 

Figure 4.8 Instantaneous and Cumulative Average Power Draw 

Side-by-Side Unit Test #1 

The ice production rate, however, is not as quick to stabilize. Fig. 4.9 shows the ice 

production rate, defined as the cumulative quantity of ice produced since the beginning of 

the test period divided by the test period length, versus time for the same data set. The 

value of 80.6 grams per hour was reported based on the total length of the test period, 

however, the saw tooth pattern in the figure suggests that the true value lies somewhere 

between 80.4 grams per hour and 84.9 grams per hour. 
34

Figure 4.9 Ice Production Rate - Side-by-Side Unit Test #1 

An analysis of the ice making energy to quantity of ice produced was performed using 

the second data set. This data set did not encompass any compressor off cycles, since 

operating this unit at the coldest setting with ice production caused it the compressor to 

run 100 % of the time. Therefore, we used an alternative approach based on temperature 

cycles. Fig. 4.10 shows the data plotted as both instantaneous power (red) and 

cumulative average power draw (blue) versus time for this cabinet under the coldest 

thermostat setting. This figure shows that the oscillations in cumulative average power 

draw change by less than 1 % between successive temperature cycles after the fourth 

cycle. The results showed that, under these conditions, it is possible produce a 

reasonable energy consumption result using about 3 hours of data. 
35

Figure 4.10 Instantaneous and Cumulative Average Power Draw 

Side-by-Side Unit Test #2 

Fig. 4.11 shows the ice production rate for this same data set. The value of 110.5 grams 

per hour was reported based on the total length of the test period, however, the saw tooth 

pattern in the figure suggests that the true value lies somewhere between 92.0 grams per 

hour and 114.9 grams per hour. 
36

Figure 4.11 Ice Production Rate - Side-by-Side Unit Test #2 

4.5 Summary 

Table 4.1 shows the ice making energy was calculated by three methods. Also shown in 

Table 4.1 is the annual energy impact of producing 0.816 kg of ice per day. 

Table 4.1 Ice making energy by various methods – side by side refrigerator freezer 

Method Ice making energy 

kWh/kg kWh/yr % of baseline energy 

1 Median position only 0.352 105 11 % 

2 Two point interpolation 0.342 102 11 % 

3 Calculation 0.256 76 8 % 
37

5: Experimental Results for French Door Refrigerator Freezer #1

This unit is an 800 liter (28 cubic foot) French door, energy star rated model with through 

the door ice and water service. It has variable defrost control, electronic thermostat 

control, and a variable-speed compressor. The unit has a factory installed ice maker 

which is located in the upper portion of the refrigerator compartment. The ice maker is 

connected to an external water supply and produces and stores ice in a bin also located in 

the refrigerator compartment. The ice maker for this unit has a dedicated evaporator and 

produces ice without displacing cold air from the freezer compartment. The unit was 

outfitted with three thermocouples in the refrigerator compartment, as shown below, and 

five thermocouples in the freezer compartment. The compartment temperatures used for 

the calculations that follow are the average of the time averaged values reported from 

each thermocouple during the test duration. 

Figure 5.1 French Door Unit #1 

The minimum and maximum time between defrost for this unit are 6 hours and 96 hours 

of compressor run time, respectively. According to the procedure outlined in the current 
38

DOE energy test procedure, these values result in a calculated average time between 

defrost of 24 hours of compressor run time. 

This unit also has an automatically controlled anti-sweat heater, which responds to the 

ambient humidity condition. The humidity levels were maintained below 8 oC dew point 

for all tests performed in this study in order to ensure that this feature would not consume 

electricity outside the scope of this set of experiments. 

5.1 French Door Refrigerator Freezer #1 with No Ice Production 

U5.1.1 Mid Setting ResultsU 

During the first test, the energy consumption was measured at with the thermostat set to 

the median setting for each compartment. The fact that this unit had a variable-speed 

compressor facilitated the characterization of energy consumption because the 

compressor did not undergo periodic cycles; rather it maintained part load continuous 

operation to stabilize the temperatures in the cabinets. The following results were 

obtained: 

Steady state operation time: 40145 seconds = 11:09:05 

Measured refrigerator compartment temperature: (6.9 ± 0.1) oC 

Measured freezer compartment temperature: (-17.9 ± 0.1) oC 

Energy expended during the test period: (629.2 ± 3.1) watt-hours 

Alternatively, this can be expressed as an average steady state power of (56.42 ± 0.28) W 

at the measured temperatures. In alternative units, (494.3 ± 2.5) kWh/yr. 

The defrost sequence for this unit consisted of (a) defrost heater switching on, (b) defrost 

heater switching off, (c) period of no power draw, (d) a recovery cycle – high compressor 

power, and (e) return to steady state – part load compressor power. The sequence is 

shown below in Fig. 5.2. 
39

Figure 5.2 Refrigerator Power during Defrost – French Door Unit #1 

Measurements during this sequence yielded the following results: 

Defrost time: 3598 seconds = 00:59:58 

Energy expended during defrost period: (205.3 ± 1.0) watt-hours 

Combining the results of these two portions of the test, assuming that the average time 

between defrost is 24 hours, we arrive at the total energy consumption of 

(521.5 ± 2.6) kWh/yr at the measured temperatures. It should be noted that the 

calculations used to arrive at this number are based on an assumption that the compressor 

operates 50% of the time; which is not valid for a variable speed compressor which 

operates 100% of the time under part load conditions. 

U5.1.2 Cold Setting ResultsU 

The second set of tests were conducted with the thermostat for each compartment set to 

the coldest setting. The following results were obtained: 

Steady state operation time: 89901 seconds = 24:58:21 

Measured refrigerator compartment temperature: (4.4 ± 0.1) oC 

Measured freezer compartment temperature: (-22.7 ± 0.1) oC 

Energy expended during the test period: (1756.4 ± 8.8) watt-hours 

Alternatively, this can be expressed as an average steady state power of (70.33 ± 0.35) W 

at the measured temperatures, or (616.1 ± 3.1) kWh/yr. 
40

Measurements during this sequence yielded the following results: 

Defrost time: 5426 seconds = 01:30:26 

Energy expended during defrost period: (266.7 ± 1.3) watt-hours 

Incorporating the defrost energy results in an overall energy consumption value of 

(645.4 ± 3.2) kWh/yr at these temperatures. 

U5.1.3 Mixed Setting ResultsU 

Since the thermostat for this unit was programmed to optimize performance while 

simultaneously maintaining temperatures of 7.2 oC and -15.0 oC in the refrigerator and 

freezer, respectively, a third data set was taken with the refrigerator set to the coldest 

setting and the freezer set to the warmest setting. The third data set resulted in the 

following values: 

Steady state operation time: 39733 seconds = 11:02:13 

Measured refrigerator compartment temperature: (2.4 ± 0.1) oC 

Measured freezer compartment temperature: (-15.7 ± 0.1) oC 

Energy expended during the test period: (673.5 ± 3.4) watt-hours 

Alternatively, this can be expressed as an average steady state power of (61.0 ± 0.3) W at 

the measured temperatures, or (534.5 ± 2.7) kWh/yr. Measurements during this sequence 

yielded the following results: 

Defrost time: 4304 seconds = 01:11:44 

Energy expended during defrost period: (250.9 ± 1.3) watt-hours 

Incorporating the defrost energy results in an overall energy consumption value of 

(567.0 ± 2.8) kWh/yr at these temperatures. 

U5.1.4 Combined Test Results for French Door Refrigerator Freezer #1 with No Ice 

Production 

The steady operation values from these three data sets were used to calculate the energy 

consumption at -17.8 oC and 3.9 oC using the three point interpolation method; this 

resulted in an energy consumption value of (544 ± 5) kWh/yr. The total energy 

consumption, including the defrost energy, was (574 ± 5) kWh/yr. 

5.2 French Door Refrigerator Freezer #1 with Ice Production 

With the baseline energy consumption characterized, we next performed a series of 

measurements to examine the energy consumption attributed to the production of ice by 

the automatic ice maker. 

U5.2.1 Mid Setting ResultsU 

First, the thermostat was set to the median position for each compartment and the ice 

maker was rendered operative. Water was supplied to the unit at a temperature of 32.2 oC. 

Fig. 5.3 shows the power signature of this unit while making ice under these conditions. 
41

Figure 5.3 Refrigerator Power During Ice Production at Mid Setting for 

French Door Unit #1 

Starting from an empty ice bin condition, the ice maker operated for less than 14 000 

seconds before ceasing ice production by sensing a full bin. During this ice production 

period, the freezer temperature deviated from its measured temperature prior to the ice 

making period, shown in Fig. 5.4. It is noted that each ice making cycle appears to 

include two successive ejection heater operation periods followed by the solenoid 

operation. Each time the heater is powered on, it consumes 150 W for approximately 120 

seconds; the solenoid consumes 40 W for 4 seconds. It is also noted that the freezer 

compartment became slightly colder for a short period, which was followed by a period 

of increasing freezer temperature. This is documented merely to illustrate that the ice 

making period is a transient period during which time the unit tries to adjust operation to 

produce ice while maintaining the prescribed conditions. For this reason, it was generally 

not possible to acquire a data set that encompasses real steady state operation. 
42

Figure 5.4 Temperature Response During Ice Production at Mid Setting for 

French Door Unit #1 

Since this unit had a variable-speed compressor, it was not necessary to consider 

compressor cycles, and the ice making energy was calculated by the considering all of the 

ice making cycles. The test period is then described as starting from the ice maker being 

cued to produce ice until the moment that the ice maker halts production based on it 

sensing a full bin condition. This is clearly visible on the figure above showing the 

power signature. 

Operation time: 13920 seconds = 03:52:00 

Measured refrigerator compartment temperature: (6.2 ± 0.1) oC 

Measured freezer compartment temperature: (-18.8 ± 0.1) oC 

Mass of ice produced: (0.69 ± 0.005) kg 

Ice production rate: (178.5 ± 1.3) grams per hour 

Energy expended during the test period: (503.0 ± 2.5) watt-hours 

Alternatively, this can be expressed as an average steady state power of (130.1 ± 0.7) W 

at the measured temperatures; or (1139.5 ± 5.7) kWh/yr. 

Single Data Point Method for Measuring Ice Maker Energy Consumption 

The first proposed approach to quantifying the ice making energy is to compare the 

steady state energy consumption with both thermostats set to their median setting for both 

ice producing and ice maker inoperative conditions. In this case, the differential power 

consumption was: 
43

(130.1 ± 0.7) W – (56.42 ± 0.28) W = (73.7 ± 0.8) W 

The energy used to produce ice in this configuration can then be calculated from the mass 

of ice, the differential power and the test period time. 

( )( )

( )

0.0737 kW 3.867 h

0.69 kg = 0.413 kWh/kg (± 0.005 kWh/kg)

In this case, the measured temperature in both compartments was colder than the 

measured temperature during the first test (same setting but no ice production), therefore 

the results of this ice-making test also include the additional load of maintaining slightly 

colder temperatures. 

U5.2.2 Cold Setting ResultsU 

The next approach was to attempt to use an interpolated energy (and ice production rate) 

using this and two other sets of measurements. We set the thermostats of each 

compartment to the coldest setting for the second ice making data set. The figure below 

shows the power signature recorded during this measurement. 

Figure 5.5 Power Response During Ice Production at Cold Setting for 

French Door Unit #1 

Under these test conditions, the unit does not abruptly shift back to the steady operation 

after it ceased ice production due to the full bin condition as was the case with the 
44

previous measurement. Rather it gradually reduced the compressor power until reaching 

the steady operation condition. The figure shows that ice production ceases at 

approximately 10 000 seconds, but the unit does not recover from the ice production until 

approximately 20 000 seconds. If we take the test period to be that of the ice production 

period only, we obtain the following values: 

Operation time: 9906 seconds = 02:45:06 

Measured refrigerator compartment temperature: (3.5 ± 0.1) oC 

Measured freezer compartment temperature: (-22.3 ± 0.1) oC 

Mass of ice produced: (0.55 ± 0.005) kg 

Ice production rate: (199.9 ± 1.8) grams per hour 

Energy expended during the test period: (436.0 ± 2.2) watt-hours 

Alternatively, this can be expressed as an average steady state power of (158.5 ± 0.8) W 

at the measured temperatures; or (1388.1 ± 6.9) kWh/yr. 

If the recovery period is included as part of the analysis, the following values are 

obtained: 

Operation time: 20000 seconds = 05:33:20 

Measured refrigerator compartment temperature: (3.4 ± 0.1) oC 

Measured freezer compartment temperature: (-23.0 ± 0.1) oC 

Mass of ice produced: (0.55 ± 0.005) kg 

Ice production rate: (99.0 ± 0.9) grams per hour 

Energy expended during the test period: (747.0 ± 3.7) watt-hours 

Alternatively, this can be expressed as an average steady state power of (134.5 ± 0.7) W 

at the measured temperatures; or (1177.8 ± 5.9) kWh/yr. 

Inclusion of the recovery period resulted in a colder freezer temperature and lower energy 

consumption, but also lower ice production rate. 

U5.2.3 Mixed Setting ResultsU 

The final measurement for this unit was performed with the freezer set to the warmest 

setting and the refrigerator set to the coldest setting. This data set, like that from the 

mid/mid setting with ice production, showed an abrupt reduction in compressor speed at 

the time that it ceased ice production. The following data were obtained from these 

measurements: 

Operation time: 11039 seconds = 03:03:59 

Measured refrigerator compartment temperature: (2.9 ± 0.1) oC 

Measured freezer compartment temperature: (-17.1 ± 0.1) oC 

Mass of ice produced: (0.56 ± 0.005) kg 

Ice production rate: (182.6 ± 1.6) grams per hour 

Energy expended during the test period: (395.2 ± 2.0) watt-hours 
45

Alternatively, this can be expressed as an average steady state power of (128.9 ± 0.6) W 

at the measured temperatures; or (1128.9 ± 5.6) kWh/yr. 

Three Data Point Method for Measuring Ice Maker Energy Consumption 

By applying the three point interpolation method for both the energy consumption and 

the ice production rate, the following values were obtained for conditions representative 

of 3.9 oC in the refrigerator and -17.8 oC in the freezer. The following data shows the 

overall results of the calculations using both possible ice production periods from the 

cold/cold setting data (i.e., with and without the recovery period). 

UIce production period only 

Energy consumption at Tfreeze = -17.8 oC and Tfridge = 3.9 oC 

(1142 ± 7) kWh/yr = (130.4 ± 0.8) W 

Ice production rate at Tfreeze = -17.8 oC and Tfridge = 3.9 oC 

 (182 ± 1) grams per hour 

UIce production and recovery periods 

Energy consumption at Tfreeze = -17.8 oC and Tfridge = 3.9 oC 

 (1134 ± 4) kWh/yr = (129.5 ± 0.5) W 

Ice production rate at Tfreeze = -17.8 oC and Tfridge = 3.9 oC 

(179 ± 2) grams per hour 

The results differ by 0.7 % on energy and 1.7 % on ice production rate. These 

calculations show that the result is largely unaffected by the inclusion of the recovery 

period for this unit; however, this may not hold true for a unit with a more aggressive 

recovery schedule. 

The following results were obtained by comparing the triangularly interpolated results 

from the tests with and without ice production. 

(130.4 ± 0.8) W – (62.1 ± 1.7) W = (68.3 ± 1.9) W 

The energy used to produce ice in this configuration can then be calculated from the mass 

of ice, the differential power and the test period time. 

(0.0683 kW) = kg 0.182

h

⎛ ⎞ ⎜ ⎟ ⎝ ⎠

 (0.375 ± 0.011) kWh/kg 

Next, it is also interesting to calculate the ice making energy by triangular interpolation 

of the non-ice making data to the temperatures realized during the ice making tests. First, 

the ice making test data at the mid/mid setting resulted in temperatures of 6.2 oC in the 

refrigerator and -18.8 oC in the freezer. Triangularly interpolating the non-ice making 

data points to these temperatures, we calculate a value of (59.48 ± 0.35) W, which is 

(70.6 ± 0.8) W less than the average power seen during the mid/mid test with ice 

production. Dividing this differential power by the ice production rate, the ice making 
46

energy at TB

fridgeB=6.2 oC and TBfreezeB=-18.8 oC is (0.396 ± 0.005) kWh/kg. Similarly, the ice 

making energy was calculated at the coldest and mixed settings as well. The coldest 

setting, at TB

fridgeB=3.5 oC and TBfreezeB=-22.3 oC is (0.436 ± 0.020) kWh/kg; the mixed setting, 

at TB

fridgeB=2.9 oC and TBfreezeB=-17.1 oC is (0.363 ± 0.005) kWh/kg. These results illustrate 

the dependence of the ice-making energy on the compartment temperatures. It is for this 

reason that a fixed set of temperature conditions should be used in order to form a 

repeatable basis for an ice-making energy test. 

5.3 Calculation Based Method 

Lastly, the theoretical ice-making energy was calculated using the simplified calculation 

approach. Each data set taken with the ice maker operative was started with the ice bin 

empty and logged until the bin reached its shut off limit. The first data set showed 

0.69 kg of ice produced through 10 cycles, the second showed 0.55 kg in 8 cycles, and 

the third showed 0.56 kg in 8 cycles. On average, this unit produces 69.2 grams of ice 

during each cycle. 

The solenoid valve draws 40 W for a period of 4 seconds to deliver water to the molds, 

which results in 160 Joules during each ice-making cycle, or 2.31 kJ/kg of ice produced. 

The ice ejection heater draws 150 watt for two 120 second periods to free the ice from the 

molds which results in 36,000 Joules per ice making cycle, or 520.2 kJ/kg of ice 

produced. The total energy consumed by these components is therefore 522.5 kJ/kg of 

ice produced. Also, the energy consumed by the ice ejection heater is ultimately 

dissipated as heat into the ice-making compartment; therefore it must be considered an 

additional heat load to be processed by the refrigeration system. 

The energy required to cool 32.2 oC water to freezing accounts for 505 kJ/kg. The 

refrigeration system must therefore meet the additional demand to remove 1025 kJ of 

heat per kg of ice produced. Using the assumed COP of 1, the additional compressor 

duty to remove this heat will total 1025 kJ/kg. Adding in the energy consumed by the 

solenoid and ejection heater brings the total load to 1547.5 kJ/kg; or in alternative units 

0.303 kWh/kg. 

5.4 Summary 

The ice-making energy was calculated by three methods as shown in Table 5.1. Also 

shown in the table is the energy impact of producing 0.816 kg of ice per day. 

The first method was to compare the difference in energy consumed with the thermostats 

set to their median positions. This method resulted in a temporary, relatively small 

departure from the non-ice producing freezer temperature because of the control system 

used in this cabinet. The second method attempted to characterize the ice making energy 

by using the three-point interpolation method and various temperature conditions. The 

final method was a simple calculation method based on the amount of energy required to 

process the ice making load, and to power the auxiliary devices required to make and 

store ice. This method relies heavily on an assumed COP value of unity. The values in 

the table below are shown in comparison to the baseline energy of 574 kWh/yr. 

 
47

Table 5.1 Ice making energy by various methods – french door unit #1 

Method Ice making energy 

kWh/kg kWh/yr % of baseline energy 

1 Median position only 0.413 123.0 21.4 % 

2 Triangular Interpolation 

(3.9 oC/-17.8 oC) 

0.375 112.0 19.5 % 

Triangulation (M/M) (6.2/-18.8) 0.396 118.3 20.6 % 

Triangulation (C/C) 

(3.5/-22.3) 

0.436 129.9 22.6 % 

Triangulation (C/W) 

(2.9/-17.1) 

0.363 108.4 18.9 % 

3 Calculation 0.430 128.0 22.3 % 

Error Associated with Incomplete Ice Maker Cycling 

This product has a variable speed compressor which greatly simplifies the recognition of 

energy associated with the ice maker. In this case, the compressor power maintains a 

steady part-load condition throughout its operation, and there is no compressor cycling. 

Since there is no compressor cycling, the analysis was conducted using a whole number 

of ice making cycles and there is no error associated with incomplete cycling in this 

manner. 

There is, however, a certain level of error due to the first batch of ice harvested upon the 

initiation of ice production. Once initiated, the ice maker begins by harvesting a batch of 

cubes, but the energy used to cool and freeze water into this set of cubes was expended 

long before the ice maker was rendered operative. Therefore, the energy recorded for the 

first batch of ice cubes is merely that of the ejection heater. This should have a much 

smaller impact on the end result than the error associated with the units employing 

cycling compressors, particularly since the ejection heaters are such a large portion of the 

total energy used by the ice maker. However, this particular unit does not take many ice 

making cycles to fill the ice bin, therefore this aspect is significant here. 

The heaters for this unit use a total of 36 kJ to harvest each batch of ice, which weighed 

approximately 69 grams. Therefore, in each measurement the energy recorded for the 

first batch of ice was only 0.149 kJ/kg. If the mass of the first batch of ice is subtracted 

from each data set along with the ejection heater power, the following results shown in 

Table 5.2 are obtained, which are slightly higher than those shown in Table 5.1. 
48

Table 5.2 Energy consumption error associated with first batch 

french door unit #1 

Method 

kWh/kg 

Measured 

Value 

Value Excluding 

First Batch 

Difference 

1 Median position 

only 

0.413 0.443 7.3 % 

2 Triangular 

Interpolation 

0.374 0.403 7.8 % 

Although it is not difficult to correct for the first batch of ice in this case, it may not be 

possible for every unit because it may not always be possible to determine the number of 

cubes harvested in each cycle, etc. as was seen with the other test subjects. There is, 

however, a simple approach to avoid this type of error entirely at the onset of testing. If 

the water supplied to the cabinet is interrupted, the ice molds would be emptied prior to 

the beginning of the data collection period. Therefore, one could begin the test by 

opening the water supply and then rendering the ice maker operative. After rendering the 

ice maker operative, the ejection heaters would operate and then the solenoid valve would 

fill the molds with water for the first batch of ice. The test period would then begin at the 

first solenoid event and end after the last batch of ice is harvested, thereby consisting of a 

whole number of ice-making cycles.
49

6: Experimental Results for French Door Refrigerator Freezer #2 

This unit, shown in Fig. 6.1, is a 765 liter (27 cubic foot) French door, energy star rated 

model with through the door ice and water service. It has variable defrost control, 

electronic thermostat control, and a variable-speed compressor. The unit has a factoryinstalled ice maker which is located in the upper portion of the refrigerator compartment. 

It is connected to an external water supply and produces ice in the ceiling of the 

refrigerator compartment and stores it in a bin located in the door of the refrigerator 

compartment. This unit freezes ice using a set of small ducts inside the walls of the 

cabinets to bring cold air from the freezer compartment located beneath the refrigerator 

compartment. The compartment temperatures used for the calculations that follow are 

the average of the time averaged values reported from each thermocouple during the test 

duration. 

Figure 6.1 French Door Unit #2 

The minimum and maximum time between defrost for this unit are 8 and 96 hours of 

compressor run time, respectively. According to the method outlined in the current DOE 
50

energy test procedure, these values result in a calculated average time between defrost of 

30 hours of compressor run time. 

This unit also has an automatically controlled anti-sweat heater, which responds to the 

ambient humidity condition. The manufacturer provided the control algorithm for this 

feature, and we maintained a dew point below 8 oC for all tests in order to ensure that this 

feature would not consume electricity outside the scope of this set of tests. 

6.1 French Door Refrigerator Freezer #2 with No Ice Production 

U6.1.1 Mid Setting ResultsU 

During the first test, the energy consumption was measured at with the thermostat set to 

the median setting for each compartment. The following results were obtained: 

Steady state operation time: 13497 seconds = 03:44:57 

Measured refrigerator compartment temperature: (3.7 ± 0.1) oC 

Measured freezer compartment temperature: (-16.8 ± 0.1) oC 

Energy expended during the test period: (237.2 ± 1.2) watt-hours 

Alternatively, this can be expressed as an average steady state power of (63.28 ± 0.32) W 

at the measured temperatures. In alternative units, (554.3 ± 2.8) kWh/yr. 

Measurements during the defrost sequence yielded the following results: 

Defrost time: 8701 seconds = 02:25:01 

Energy expended during defrost period: (303.3 ± 1.5) watt-hours 

Combining the results of these two portions of the test, assuming that the average time 

between defrost is 30 hours, we arrive at the total energy consumption of 

(576.3 ± 2.9) kWh/yr at the measured temperatures. 

U6.1.2 Cold Setting ResultsU 

The second set of tests were conducted with the thermostat for each compartment set to 

the coldest setting. During this test, the compressor did not cycle on and off, but did 

exhibit temperature and power oscillations due to the part load compressor speed and the 

periodic cooling load shifts between the refrigerator and freezer compartment. In order to 

ensure that the most repeatable result was captured, the test period used consisted of a 

whole number of these oscillations. The following results were obtained: 

Steady state operation time: 13622 seconds = 03:47:02 

Measured refrigerator compartment temperature: (-1.2 ± 0.1) oC 

Measured freezer compartment temperature: (-20.0 ± 0.1) oC 

Energy expended during the test period: (287.7 ± 1.4) watt-hours 

Alternatively, this can be expressed as an average steady state power of (76.04 ± 0.38) W 

at the measured temperatures, or (666.1 ± 3.3) kWh/yr. 
51

Measurements during the defrost sequence yielded the following results: 

Defrost time: 14400 seconds = 04:00:00 (no cycling condition) 

Energy expended during defrost period: (450.9 ± 2.3) watt-hours 

Incorporating the defrost energy results in an overall energy consumption value of 

(687.6 ± 3.4) kWh/yr at these temperatures. 

U6.1.3 Mixed Setting ResultsU 

The thermostat for this unit was programmed in order to optimize performance while 

simultaneously maintaining temperatures of 7.2 oC and -15.0 oC in the refrigerator and 

freezer, respectively. Therefore, a third data set was taken with the refrigerator set to the 

warmest setting and the freezer set to the coldest setting in order to appropriate data for 

the three point interpolation method. These settings resulted in the compressor operating 

at part load conditions, but with periodic cycling. Because of this, the data was reduced 

using a whole number of complete compressor cycles. The third data set resulted in the 

following values: 

Steady state operation time: 33243 seconds = 03:44:57 

Measured refrigerator compartment temperature: (7.2 ± 0.1) oC 

Measured freezer compartment temperature: (-20.3 ± 0.1) oC 

Energy expended during the test period: (640.4 ± 3.2) watt-hours 

Alternatively, this can be expressed as an average steady state power of (69.35 ± 0.35) W 

at the measured temperatures, or (607.5 ± 3.0) kWh/yr. Measurements during the defrost 

sequence yielded the following results: 

Defrost time: 23818 seconds = 06:36:58 

Energy expended during defrost period: (615.0 ± 3.1) watt-hours 

Incorporating the defrost energy results in an overall energy consumption value of 

(630.3 ± 3.2) kWh/yr at these temperatures. 

U6.1.4 Combined Test Results for French Door Refrigerator Freezer #2 with No Ice 

Production 

The steady operation values from these three data sets were used to calculate the energy 

consumption at -17.8 oC and 3.9 oC using the three point interpolation method which 

resulted in an energy consumption value of (576 ± 3) kWh/yr. The total energy 

consumption, including the defrost energy, was (598 ± 3) kWh/yr. 

6.2 French Door Refrigerator Freezer #2 with Ice Production 

With the baseline energy consumption characterized, we next performed a series of 

measurements to examine the energy consumption attributed to the production of ice by 

the automatic ice maker. 

U
52

6.2.1 Mid Setting ResultsU 

The thermostat was set to the median position for each compartment and the ice maker 

was rendered operative. Water was supplied to the unit at a temperature of 32.2 oC. Fig. 

6.2 shows the power signature of this unit while making ice under these conditions. 

Figure 6.2 Refrigerator Power for Ice Production at Mid Setting for 

French Door Unit #2 

Starting from an empty ice bin condition, the ice maker operated for less than 44 000 

seconds before undergoing a defrost period. This unit continued to produce ice during 

the defrost and recovery period, however, only the data leading up to the defrost was used 

for this analysis in order to separate out the effects of the defrost cycle. In total, 2.37 kg 

of ice was produced during 40 ice making cycles; 26 of these cycles occurred before the 

defrost; therefore the mass of 1.54 kg was used for this analysis. Each time the heater is 

powered on, it consumes 165 W for approximately 70 seconds, and the solenoid 

consumes 50 W for 4 seconds. 

It is also noted that the freezer compartment became slightly warmer for a short period 

near the beginning of the data set, when the ice maker was switched on, and gradually 

decreased over the duration of the test period. This temperature increase was in response 

to the additional load realized by the freezer compartment as cold air was bled off to 

produce ice in the ice maker. It is also interesting to note that the temperature in the 

refrigerator compartment became slightly colder when the ice maker was rendered 

operative. This is because the ice maker is located in the refrigerator compartment and 
53

the additional supply of cold air needed to freeze ice has a cooling effect on the entire 

compartment. 

Figure 6.3 Temperature Response to Ice Production at Mid Setting for 

French Door Unit #2 

Since the compressor maintained a part load condition throughout the data set and did not 

switch on and off, it was not necessary to consider compressor cycles and the ice-making 

energy was calculated by considering all of the ice making cycles prior to the defrost. 

The test period is then described as starting from the ice maker being cued to produce ice 

until the moment that the defrost heater was activated. This is clearly visible on Fig. 6.3. 

Operation time: 41273 seconds = 11:27:53 

Measured refrigerator compartment temperature: (3.2 ± 0.1) oC 

Measured freezer compartment temperature: (-16.8 ± 0.1) oC 

Mass of ice produced: (1.54 ± 0.005) kg 

Ice production rate: (134.37 ± 0.44) grams per hour 

Energy expended during the test period: (1052.0 ± 5.3) watt-hours 

Alternatively, this can be expressed as an average steady state power of (91.79 ± 0.46) W 

at the measured temperatures; or (804.1 ± 4.0) kWh/yr. 

Single Data Point Method for Measuring Ice-Maker Energy Consumption 

The first proposed approach to quantifying the ice making energy is to compare the 

steady state energy consumption with both thermostats set to their median setting for both 

ice producing and ice maker inoperative conditions. In this case, the differential power 

consumption was: 
54

(91.79 ± 0.46) W – (63.28 ± 0.32) W = (28.51 ± 0.56) W 

The energy used to produce ice in this configuration can then be calculated from the mass 

of ice, the differential power and the test period time. 

( )( )

( )

0.02851 kW 11.46 h = 1.54 kg (0.212 ± 0.004) kWh/kg 

Comparing the temperature results of the tests at the median setting shows that this 

particular unit maintained fairly consistent compartment temperatures whether or not the 

unit was producing ice. Although there was some transient behavior at the onset of ice 

making, the time averaged values showed that the freezer compartment temperature 

remained nearly identical temperatures, but the refrigerator compartment temperature was 

approximately 0.5 oC colder while making ice. 

U6.2.2 Cold Setting ResultsU 

The next approach was to attempt to use an interpolated energy (and ice production rate) 

using three sets of measurements. We set the thermostats of each compartment to the 

coldest setting for the second ice making data set. Fig. 6.4 shows the power signature 

recorded during this measurement. This figure shows that the compressor power 

fluctuates under these conditions; this is because this unit constantly adjusts the 

compressor speed in order to maintain constant temperatures in the compartments. We 

averaged the data over the entire data set in order to dampen out these effects. 

Figure 6.4 Power for Ice Production at Cold Setting 

French Door Unit #2 
55

Operation time: 54928 seconds = 15:15:28 

Measured refrigerator compartment temperature: (-0.1 ± 0.1) oC 

Measured freezer compartment temperature: (-18.2 ± 0.1) oC 

Mass of ice produced: (1.800 ± 0.005) kg 

Ice production rate: (117.97 ± 0.33) grams per hour 

Energy expended during the test period: (1541.9 ± 7.7) watt-hours 

Alternatively, this can be expressed as an average steady state power of 

(101.06 ± 0.51) W at the measured temperatures; or (885.3 ± 4.4) kWh/yr. 

U6.2.3 Mixed Setting ResultsU 

The final measurement for this unit was performed with the freezer set to the coldest 

setting and the refrigerator set to the warmest setting. This data set showed a gradual 

change in temperatures during the initiation and termination of ice production. It also 

displayed an abrupt reduction in compressor speed and a return to periodic oscillation at 

the time that it ceased ice production, shown in Fig. 6.5. 

Figure 6.5 Refrigerator Power with Ice Production at Mixed Setting for 

French Door Unit #2 
56

Figure 6.6 Refrigerator and Freezer Compartment Temperatures with Ice Production at 

Warm/Cold Setting for French Door Unit #2 

The following data were obtained from these measurements: 

Operation time: 48398 seconds = 13:26:38 

Measured refrigerator compartment temperature: (5.4 ± 0.1) oC 

Measured freezer compartment temperature: (-18.4 ± 0.1) oC 

Mass of ice produced: (1.57 ± 0.005) kg 

Ice production rate: (116.9 ± 0.4) grams per hour 

Energy expended during the test period: (1324.1 ± 6.6) watt-hours 

Alternatively, this can be expressed as an average steady state power of (98.5 ± 0.5) W at 

the measured temperatures; or (862.8 ± 4.3) kWh/yr. 

Three Data Point Method for Measuring Ice-Maker Energy Consumption 

By applying the three point interpolation method for both the energy consumption and 

the ice production rate, the following values were obtained for conditions representative 

of 3.9 oC in the refrigerator and -17.8 oC in the freezer. 

Energy consumption: (845 ± 4) kWh/yr ((96.4 ± 0.5) W) 

Ice production rate: (2.96 ± 0.02) kg/day ((123.3 ± 0.9) grams per hour) 

The following results were obtained by comparing the triangularly interpolated results 

from the tests with and without ice production. 
57

(96.4 ± 0.5) W – (65.8 ± 0.3) W = (30.6 ± 0.6) W 

The energy used to produce ice in this configuration can then be calculated from the mass 

of ice, the differential power and the test period time. 

(0.0306 kW) = kg 0.1233

h

⎛ ⎞ ⎜ ⎟ ⎝ ⎠

 (0.248 ± 0.005) kWh/kg

Next, it is of interest to calculate the ice-making energy by triangular interpolation of the 

non ice-making data to the temperatures realized during the ice-making tests. First, the 

ice making test data at the mid/mid setting resulted in temperatures of 3.2 oC in the 

refrigerator and -16.8 oC in the freezer. Triangularly interpolating the non-ice making 

data points to these temperatures, we calculate a value of (63.7 ± 0.5) W, which is 

(28.1 ± 0.7) W less than the average power seen during the mid/mid test with ice 

production. Dividing this differential power by the ice production rate, the ice making 

energy at TB

fridgeB=3.2 oC and TBfreezeB=-16.8 oC is (0.209 ± 0.005) kWh/kg. Similarly, the 

ice-making energy was calculated at the coldest and mixed settings as well. The coldest 

setting, at TB

fridgeB=-0.1 oC and TBfreezeB=-18.2 oC is (0.260 ± 0.009) kWh/kg; the mixed 

setting, at TB

fridgeB=5.4 oC and TBfreezeB=-18.4 oC is (0.278 ± 0.005) kWh/kg. These results 

confirm the observation from the previous unit that the ice-making energy is fairly 

dependent on the refrigerator and freezer compartment temperatures. 

6.3 Calculation Based Method 

Lastly, the theoretical ice-making energy was calculated using the simplified calculation 

approach. Each data set taken with the ice maker operative was started with the ice bin 

empty and logged until the bin reached its shut off limit. The first data set showed 

1.54 kg of ice produced through 26 cycles, the second showed 1.8 kg in 36 cycles, and 

the third showed 1.57 kg in 32 cycles. On average, this unit produces 52.2 grams of ice 

during each cycle. 

The solenoid valve draws 50 W for a period of 4 seconds to deliver water to the molds 

which results in 200 Joules, or 3.83 kJ/kg of ice produced. The ice ejection heater draws 

165 W for 70 second periods to free the ice from the molds which results in 11,550 Joules, 

or 221.26 kJ/kg of ice produced. The total energy consumed by these components is 

therefore 225.10 kJ/kg of ice produced. Also, the energy consumed by the ice ejection 

heater is ultimately dissipated as heat into the ice-making compartment; therefore it must 

be considered an additional heat load to be processed by the refrigeration system. 

Considering the energy required to cool 32.2 oC water to freezing and the additional load 

of the heaters, the refrigeration system must therefore meet the additional demand of to 

remove 726 kJ of heat per kg of ice produced. Using the assumed COP of 1, the 

additional compressor duty to remove this heat will total 726 kJ/kg. Adding in the energy 

consumed by the solenoid and ejection heater brings the total load to 951.1 kJ/kg; or in 

alternative units 0.264 kWh/kg. 
58

6.4 Summary 

The ice making energy was calculated by 4 methods as shown in Table 6.1. the table also 

shows the energy impact of producing 0.816 kg of ice per day. 

The first method was to compare the difference in energy consumed with the thermostats 

set to their median positions. This method resulted in a temporary departure from the 

refrigerator cabinet temperature because of the control system used in this cabinet. The 

second method attempts to characterize the ice making energy by using the various threepoint interpolation methods. The final method was a simple calculation method based on 

the amount of energy required to process the ice making load, and to power the auxiliary 

devices required to make and store ice. The values in Table 6.1 are shown in comparison 

to the baseline energy of 598 kWh/yr. 

 

Table 6.1 Ice making energy by various methods – french door unit #2 

Method Ice making energy 

kWh/kg kWh/yr % of baseline energy 

1 Median position only 0.212 63.1 10.6 % 

2 Triangulation 

(3.9/-17.8) 

0.248 74.1 12.4 % 

Triangulation (M/M) (3.2/-16.8) 0.209 62.2 10.4 % 

Triangulation (C/C) 

(-0.1/-18.2) 

0.260 78.0 13.0 % 

Triangulation (W/C) 

(5.4/-18.4) 

0.278 82.8 13.8 % 

3 Calculation 0.264 78.7 13.2 % 

Error Associated with Incomplete Ice-Maker Cycling 

This product, like the other French door unit, has a variable-speed compressor which 

simplified the recognition of energy associated with the ice maker because the 

compressor never cycled off during ice production. Since there was no compressor 

cycling during the ice-making tests, the analysis was conducted using a whole number of 

ice-making cycles and there is no error associated with incomplete cycling. 

As was the case with the French Door Refrigerator Freezer Unit #1, the level of error due 

to the first batch of ice harvested upon the initiation of ice production is examined in this 

section. Performing a similar analysis yields significant results. As mentioned in the 

previous section, however, the influence of this first batch of ice on the energy 

consumption can be easily eliminated by using the valve approach described in the 

previous section of this report. 
59

Table 6.2 Energy consumption error associated with first batch 

french door unit #2 

Method 

kWh/kg 

Measured 

Value 

Value Excluding 

First Batch 

Difference 

1 Median position only 0.212 0.219 3.3 % 

2 Triangular Interpolation 0.248 0.257 3.6 % 
60

7: Observations 

Four refrigerators with automatic ice makers were examined in this study to evaluate test 

procedure options for determining energy consumption due to ice production by 

automatic ice makers. The units varied in configuration, technological sophistication, 

and most importantly ice-making energy consumption. This study centered on an 

approach that was based on using the longest attainable period of test data that included 

steady state or cyclic operation. The length of the test period for each data set was 

limited by either a complete filling of the ice storage bin or by an interruption of steady 

operation due to a defrost event. 

The first two units analyzed employed a single-speed compressor that maintained 

temperature by cycling on and off. For these units, the ice-making cycles did not 

typically coincide with the compressor cycles, which compromised the reproducibility of 

the test data. However, the reproducibility of the end result is strictly a function of the 

number of undisturbed sequential cycles that can be found within a data set. A unit with 

a very large ice storage bin operating in such a way that it shows a long time between 

defrost events will be simpler to evaluate than one with a small storage bin or a short time 

between defrost events. 

The second two units studied were French door units. These units were particularly 

interesting because they produce ice inside the refrigerator compartment rather than the 

freezer compartment, which was expected to result in less efficient ice production. 

However, these are high-end units that employ sophisticated features. They both use 

inverter-driven variable-speed compressors, which are more efficient than single-speed 

compressors. These units ultimately showed similar ice-making energy consumption 

values when compared to the units that made ice in the freezer. One particularly 

interesting aspect of the units with variable-speed compressors is that the ice-making 

cycles can be easily separated from the data because the compressors do not cycle on and 

off. This greatly simplifies the determination of ice making energy. 

This study also considered a purely calculation-based approach to estimate the ice-maker 

energy consumption. This method proved to be undesirable because the ice making 

energy consumption must be based on the actual mass of ice produced, which must be a 

measured quantity. Also, this approach is based on an assumed COP of the system, 

which is not a traceable or reliable quantity. 

The data showed that the compartment temperatures often deviate from their set points 

when the ice maker is rendered operative. Furthermore, the ice-making energy 

consumption generally varies with the compartment temperatures. For this reason, it is 

recommended that the ice-making energy be determined at a specified standard set of 

cabinet temperatures in order to have a good basis for comparison of products. It is also 

suggested that a three point triangular interpolation method could be used to determine 

the ice-making energy consumption if two point interpolation does not closely 

approximate the target refrigerator and freezer temperatures simultaneously. 
61

The most reliable and cross-comparative results were those obtained using the 

interpolation methods and the target temperatures of 3.9 oC in the refrigerator 

and -17.8 oC in the freezer. Examination of these results shows that the measured ice 

maker energy consumption ranges from 0. 375 kWh/kg down to 0.248 kWh/kg. In 

general, roughly quarter of the energy expended during the ice production process was 

due to the operation of electric resistance heaters, and it is estimated that approximately 

one third of the remaining energy was expended to remove the heat added by these 

heaters. In other words, approximately half of the total energy used to make ice is 

actually used to cool and freeze water into ice, the other half is used to remove it from the 

molds. This indicates that there are substantial opportunities for efficiency improvements 

merely by optimizing the operation of the heaters associated with the ice makers or by 

eliminating them altogether and using an alternative method to free the ice from the 

molds. 

It should also be noted that the theoretical limit of ice-making energy is merely the 

difference between the inlet water’s enthalpy and that of the ice in its final state, divided 

by the vapor compression system’s COP. The enthalpy difference is a firm parameter of 

505 kJ/kg for water entering the ice maker at 32.2 oC (90 oF) and being brought to a final 

temperature of -17.8 oC (0 oF). The COP is limited by the Carnot efficiency of a vapor 

compression cycle operating between a condensing temperature of 32.2 oC and an 

evaporating temperature of -17.8 oC, at these conditions COPB

CarnotB is 5.1. Therefore, the 

theoretical limit of ice making energy consumption is 99 kJ/kg or 0.028 kWh/kg. 

Although this is an unattainable limit in practice, it provides insight to the fact that the 

best technology evaluated in this study is an order of magnitude less efficient than the 

theoretical limit. 
62

8: Recommendations for Ice-Making Energy Consumption Test

Through this study, several key aspects of a repeatable, reproducible ice-making energy 

consumption test have been determined. In order to minimize overall test burden, the 

several parameters of an ice-making energy consumption test can be aligned with the 

basic energy consumption test for domestic refrigerating appliances including ambient 

temperature in test chamber of 32.2 oC (90 oF) and water inlet temperature of 32.2 oC 

(90 oF), since it is not practical to deliver water to the test unit at a temperature different 

from the ambient condition. 

The ice-making energy consumption should be determined by measuring the energy 

consumption of the unit while actively producing ice and subtracting the energy 

consumption of the unit while not actively producing ice. Interpolation of the results of 

individual measurements to a set of compartment temperatures, specifically 3.9 oC in the 

refrigerator compartment and -17.8 oC in the freezer compartment, could be used as a 

means to unify the conditions at which the test results are based. The ice production rate 

should be interpolated to these temperatures by the same means. 

The result of the ice-making energy consumption measurement is then defined as the icemaking energy consumption at the specified target temperatures divided by the ice 

production rate at the specified target temperatures. 

The test period for the ice-making energy consumption test must include enough data to 

ensure a repeatable and reproducible measurement. This is a difficult parameter to define 

for units that use a single-speed compressor that maintains conditions in the 

compartments by switching on and off. The recommendation for this type of unit is to 

perform the energy consumption test over a whole number of compressor cycles and to 

calculate the cumulative average of the power draw and the ice production rate. The data 

set is considered sufficiently long if this parameter does not change significantly when 

another compressor cycle is included in the analysis. 

If a defrost event interrupts the data set, only the portion of the data collected during the 

steady operation period should be used. The mass of ice produced during the steady 

portion may be estimated by multiplying the total mass of ice produced by the proportion 

of ice-making cycles that occurred during this period. 

For the case of a variable-speed compressor, the test is greatly simplified because the test 

period is not affected by compressor cycles. In this case, the water supply line should be 

outfitted with a valve that can stop the supply of water to the cabinet. With the valve shut, 

the ice maker can be operated until the ice molds are emptied. The ice-making energy 

consumption test can begin by opening the water supply valve and simultaneously 

rendering the ice maker operative. The test cabinet will then begin operation of the ice 

maker with an empty bin condition, and the test period can be taken from the first 

solenoid opening to the last ice ejection heater operation; a whole number of ice-making 

cycles. 
63

9: Open Items for Future Consideration 

Through this study, several issues became apparent for consideration of future 

investigation. The first step is to validate the recommended methodology through 

additional testing. We will consider the developed measurement techniques as an 

adequate tool for rating these products if the results of multiple measurements of several 

automatic ice makers can be reproduced at different laboratories. 

Another issue is that the measurement techniques must be validated for use with other 

styles of automatic ice makers. Four styles of automatic ice makers were examined in the 

present study, the energy use of each was heavily influenced by electric resistance heaters 

used to free frozen ice from the ice maker. Other technologies currently exist for ejecting 

ice from the ice makers, and it is of interest to examine test data for such units to ensure 

that such a method is applicable for these technologies. 

The objective of an automatic ice maker is to produce a quantity of ice, not maintain a 

specific condition; therefore, it is not appropriate to measure the amount of energy that an 

automatic ice maker uses during a specified amount of time. Instead, such products must 

be rated on their ice-making energy consumption, which is the amount of energy 

expended normalized to the amount of ice produced. However, the value of ice-making 

energy consumption can be scaled to an appropriate annualized energy consumption 

using a usage factor that represents the amount of ice that the units actually produce in a 

typical residential setting. To date, there is limited information regarding consumer 

usage of automatic ice makers and it is recommended that studies be conducted to seek 

this information. 

Throughout this study, the effects of ice making on the contribution of energy related to 

periodic defrosting was not considered. Operation of automatic ice makers may, under 

certain conditions, strongly impact the defrost frequency of certain refrigerator designs. 

This is a secondary issue but is very complex and would be difficult to factor into a rated 

energy consumption value for an ice maker; however, the impact may be quite substantial 

as evidenced by some of the units examined in this study. 

Finally, this study relied on an assumption that the ice production rate at the specific 

target temperatures could be estimated by interpolating the results of the measurements in 

the same manner as energy consumption. Although this method is a good first 

approximation, it is necessary to either verify this method or to quantify the error 

associate with this approach. 
64

10: References 

AHAM, 1979. ANSI/AHAM HRF-1-1979, American National Standard for Household 

Refrigerators and Household Freezers. Chicago: Association of Home Appliance 

Manufacturers. 

AHAM, 2009. AHAM Update to DOE on Status of Ice Maker Energy Test Procedure. 

November 19, 2009. 

ASNZ, 2007. ASNZ 4474.1 Performance of household electrical appliances—

Refrigerating appliances. Part 1 Energy Consumption and Performance. 

Uniform Test Method for Measuring the Energy Consumption of Electric Refrigerators 

and Electric Refrigerator-Freezers, 10 Federal Register 430, Subpart B, Appendix A1 

(01JAN2010), pp. 159-167. 

Haider, I., Feng, H., and Radermacher, R., 1996. Experimental Results of a Household 

Automatic Icemaker in a Refrigerator/Freezer. ASHRAE Transactions: Symposia, SA96-7-3, pp. 541-545. 

Meier, A. and Martinez, M., 1996. Energy Use of Ice Making in Domestic Refrigerators. 

ASHRAE Transactions, Vol., 102, Pr. 2, pp. 1071-1076. 
65

Appendix: Uncertainty Analysis 

The equation used to calculate the energy consumption using 2 point interpolation is as 

follows: 

1 22 1 ( ) ( )

1 2

E T T ET T ref ref E

T T

−− − = −

Where E is the energy consumption (kWh/yr) and T is the compartment temperature (oC), 

the subscripts 1 and 2 refer to the data point and ‘ref’ refers to the standardized reference 

temperature at which the equation calculates the energy consumption. 

In order to calculate the propagation of measurement uncertainty, it is necessary to take 

the partial derivatives of energy with respect to each measured quantity: 

( )( )

( )

12 2

2

1 1 2

E E ET T ref

T T T

∂ − − = ∂ −

( )

( )

2

1 12

E T T ref

E T T

∂ − = ∂ −

( )( )

( )

12 1

2

2 1 2

E E ET T ref

T T T

∂ − − = ∂ −

( )

( )

1

2 12

E T Tref

E T T

∂ − = ∂ −

And the uncertainty of the calculated energy consumption is: 

1 12 2

222 2

112 2

ET E T E

EE E E UU U U U

TE T E

⎛ ⎞⎛ ⎞⎛ ⎞⎛ ⎞ ∂∂ ∂ ∂

= +++ ⎜ ⎟⎜ ⎟⎜ ⎟⎜ ⎟ ∂∂ ∂ ∂ ⎝ ⎠⎝ ⎠⎝ ⎠⎝ ⎠

Where U is the uncertainty of the value specified by the accompanying subscript, in the 

same units as the quantity. 
66

The uncertainty analysis for the triangular interpolation approach is much more 

complicated since; it incorporates the measurements from both compartments 

simultaneously. The subscripts ‘A’ and ‘B’ are used to denote Refrigerator and Freezer 

compartments. The first step is to calculate the location of a theoretical fourth point from 

the following relationship: 

 

( )

( )

( )

( )

( )

( )

( )

( )

1

2 13 1 3 1 2

4 1

2 31 31 2

xA B xB A B B B B B xB

A xB B

A xA A A A A A xA

TT T TT T T T T T

T T T

TT TT TT TT

−

⎛ ⎞ − − −− ⎛ ⎞

= − −+ ⎜ ⎟⎜ − ⎟ ⎜ ⎟ − − −− ⎝ ⎠⎝ ⎠

Or alternatively written: 

123 12 1 2 1 3 231 23 3 1 3 2

4

12 1 21 23 32 3 1 3

A A B A A xB A xA B A xA B A A B A A xB A xA B A xA B

A

A B A xB A B A B A B A xB xA B xA B

TT T TT T TTT TTT T TT T TT TTT TTT T

TT TT TT TT TT TT TT TT

−+−−+ +− = −−+ − ++−

The partial derivatives of TB

A4B with respect to each of the other measured temperatures 

are: 

( )

( )

( )( )

4 23 2 2 3

1 12 1 21 23 32 3 1 3

2 1 2 3 1 2 1 21 32 3 1 2 3 3 13 2

12 1 21

A A B A xB xA B xA B

A A B A xB A B A B A B A xB xA B xA B

B xB A A B A A xB A xA B A xA B A A B A A xB A xA B A xA B

A B A xB A B

T TT TT TT TT

T TT TT T T TT TT TT TT TT

T T TTT TTT TTT TTT TTT TTT TTT TTT

TT TT TT T

∂ −+− = ∂ −−+ − ++−

−− − + − − + + −

( ) −−+ 2

A B A B A xB xA B xA B 23 32 3 1 3 T TT TT TT TT − ++−

( )

( )

( )( )

4 13 1 31 3

2 12 1 21 23 32 3 1 3

1 3 123 12 1 2 1 3 231 23 3 1 3 2

12 1 21

A A B A xB A B A xB

A A B A xB A B A B A B A xB xA B xA B

B B A A B A A xB A xA B A xA B A A B A A xB A xA B A xA B

A B A xB A B

T TT TT TT TT

T TT TT T T T T TT TT TT TT

T T TTT TTT TTT TTT TTT TTT TTT TTT

TT TT TT

∂ −−+ = ∂ −−+ − ++−

−+ − + − − + + − − ( ) −−+ 2

TT TT TT TT TT A B A B A xB xA B xA B 23 32 3 1 3 − ++−

( )

( )

( )( )

4 21 2 1 2

3 12 1 21 23 32 3 1 3

2 1 2 3 1 2 1 21 32 3 1 2 3 3 13 2

12 1 21

A A B A xB xA B xA B

A A B A xB A B A B A B A xB xA B xA B

B xB A A B A A xB A xA B A xA B A A B A A xB A xA B A xA B

A B A xB A B

T TT TT TT TT

T TT TT TT TT TT TT TT TT

T T TTT TTT TTT TTT TTT TTT TTT TTT

TT TT TT

∂ −+ +− = ∂ −−+ − ++−

−+ − + − − + + − − ( ) − − 2

+ − ++− TT TT TT TT TT A B A B A xB xA B xA B 23 32 3 1 3

( )

( )

( )( )

4 23 3

1 12 1 21 23 32 3 1 3

2 123 12 1 2 1 3 231 23 3 1 3 2

12 1 21 23 32

A A A A xA

B A B A xB A B A B A B A xB xA B xA B

A xA A A B A A xB A xA B A xA B A A B A A xB A xA B A xA B

A B A xB A B A B A B

T TT TT

T TT TT T T T T TT TT TT TT

T T TT T TT T TTT TTT T TT T TT TTT TTT

TT TT TT TT TT

∂ − + = ∂ −−+ − ++−

−+ − + − − + + − − ( ) −−+ − 2

++− TT TT TT A xB xA B xA B 3 13
67

( )

( )

( )( )

4 1 3

2 12 1 21 23 32 3 1 3

1 3 123 12 1 2 1 3 231 23 3 1 3 2

12 1 21 23 32

A A xA A xA

B A B A xB A B A B A B A xB xA B xA B

A A A A B A A xB A xA B A xA B A A B A A xB A xA B A xA B

A B A xB A B A B A B

T TT TT

T TT TT TT T T TT TT TT TT

T T TTT TTT TTT TTT TTT TTT TTT TTT

TT TT TT T T TT T

∂ − = ∂ −−+ − ++−

− −+−−+ +− − ( ) −−+ − + 2

A xB xA B xA B 3 13 T TT TT + −

( )

( )

( )( )

4 12 1

3 12 1 21 23 32 3 1 3

2 123 12 1 2 1 3 231 23 3 1 3 2

12 1 21 23 32

A A A A xA

B A B A xB A B A B A B A xB xA B xA B

A xA A A B A A xB A xA B A xA B A A B A A xB A xA B A xA B

A B A xB A B A B A B

T TT TT

T TT TT T T TT TT TT TT TT

T T TTT TTT TTT TTT TTT TTT TTT TTT

TT TT TT TT TT T

∂ − = ∂ −−+ − ++−

− −+−−+ +− − ( ) −−+ − + 2

A xB xA B xA B 3 13 T TT TT + −

The uncertainty of the temperature in compartment A is: 

4 1 1

22 22 2 2

44 4 44 4

23 23

12 3 12 3

A

AA A AA A

T AA A BB B

AA A BB B

TT T TT T UUU U UU U

TT T TT T

⎛ ⎞⎛ ⎞ ⎛ ⎞⎛ ⎞ ∂∂ ∂ ∂∂ ∂ ⎛⎞ ⎛⎞

= + + ++ + ⎜ ⎟⎜ ⎟ ⎜ ⎟⎜ ⎟ ⎜ ⎟ ⎜ ⎟ ∂∂ ∂ ∂∂ ∂ ⎝ ⎠⎝ ⎠ ⎝ ⎠⎝ ⎠ ⎝ ⎠ ⎝ ⎠

Once the uncertainty of Temperature TB

A4B is defined, the uncertainty of the corresponding 

energy, EB

4B, can be calculated from the equation for EB4B. 

( )( )

( )

4 1

4 1 31

3 1

A A

A A

T T

EE EE

T T

⎡ ⎤ − =+ − ⎢ ⎥ − ⎣ ⎦

The partial derivatives of EB

4B with respect to each measured quantity are: 

( )

( ) 4 4 1

1 31

1 A A

A A

E T T

E T T

∂ − = − ∂ −

( )

( ) 4 4 1

3 31

A A

A A

E T T

E T T

∂ − = ∂ −

( ) ( )

( )

( )

4 4 1

3 1 2

1 31 3 1

1 A A

A AA A A

E T T

E E

T TT T T

∂ ⎛ ⎞ − − =− + ⎜ ⎟

∂ − − ⎝ ⎠

( )( )

( )

4 31 4 1

2

3 3 1

A A

A A A

E E ET T

T T T

∂ − − = − ∂ −
68

( )

( ) 4 3 1

A AA 4 31

E E E

T TT

∂ − = ∂ −

The uncertainty of EB

4B is therefore: 

41 3 1 3 4

22 2 2 2

44 4 4 4

13 1 3 4

EE E T T T AA A

AAA

EE E E E UUU U U U

EET T T

⎛⎞ ⎛ ⎞ ⎛ ⎞ ∂∂ ∂ ∂ ∂ ⎛⎞ ⎛ ⎞

= ++ + + ⎜⎟ ⎜ ⎟ ⎜ ⎟ ⎜⎟ ⎜ ⎟ ∂∂∂ ∂ ∂ ⎝⎠ ⎝ ⎠ ⎝ ⎠ ⎝⎠ ⎝ ⎠

Finally, the uncertainty of the overall energy consumption at the triangulated coordinates 

can be calculated from its equation: 

( ) ( )

( )

2

2 42

4 2

xA A

x

A A

T T

EE EE

T T

⎡ ⎤ − =+ − ⎢ ⎥ − ⎣ ⎦

The partial derivatives of EB

xB with respect to each measured quantity are: 

( )

( )

2

2 42

1 x xA A

A A

E T T

E TT

∂ − = − ∂ −

( )

( )

2

4 42

x xA A

A A

E T T

E T T

∂ − = ∂ −

( )

24 2 44 4

2

2 4 2

x A xA A xA

A A A

E ET ET ET ET

T T T

∂ −−+ = ∂ −

( )

2 22 42 4

2

4 4 2

x xA A A xA

A A A

E ET ET ET ET

T T T

∂ −+− = ∂ −

The uncertainty of EB

xB is therefore: 

24 2 4

22 2 2

24 2 4

x A A

xx x x

EE E T T

A A

EE E E UU U U U

EET T

⎛ ⎞⎛ ⎞⎛ ⎞⎛ ⎞ ∂∂ ∂ ∂

= ++ + ⎜ ⎟⎜ ⎟⎜ ⎟⎜ ⎟ ∂∂∂ ∂ ⎝ ⎠⎝ ⎠⎝ ⎠⎝ ⎠