Memorandum 

Date: December 31, 2013 

To: Charlie Stephens, NEEA 

From: Jeffrey Uslan, Ben Larson, Bob Davis, Lucinda Gilman, Ecotope, Inc. 

Re: Icemaker Field Study Analysis 

In 2012, NEEA asked Ecotope to study refrigerator icemaker usage. The current Department of 

Energy test procedure for refrigerators does not take into account the energy use of icemakers; 

the use per duty cycle is known from laboratory testing, but real-life use patterns are not. The 

goal of this study was to measure a refrigerator’s icemaker usage in the field. 

To measure usage, a small logger was placed around the power wire of either the icemaker water 

valve or the icemaker itself. Each time the circuit was activated, an event was registered. It is 

worth noting that Ecotope measured not the number of times someone extracted ice from the 

icemaker, but the number of times the icemaker itself made more ice. Icemakers typically make 

five to eight cubes at a time. Kitchen temperature was also logged hourly with a separate logger 

located away from the refrigerator, range and other heat-generating appliances. Outside 

temperatures were gathered from local weather stations. 

Ninety sites were metered in Washington, Idaho and Oregon. Half of the sites metered had 

through-door icemakers and half had in-freezer icemakers. Refrigerators in the study were 

manufactured between 2001 and 2011. Fieldwork began in March 2012; sites were removed 

from the field in August 2012. Usable data were gathered from eighty-four sites. 

The results of our analysis were somewhat surprising. Icemaker use did not correlate with many 

of the factors that one would initially postulate. For example, higher icemaker usage did not 

correlate with outdoor temperature, the number of people in a house, or even the number of 

adults in a house. However, there were correlations with cooling zone, kitchen temperature, and 

the type of icemaker (through-door vs. in-freezer). These results indicate to us that behavior –

how often a household chooses, for whatever reason, to use ice – is an important driver of 

icemaker usage. 

Throughout the analysis, we generally present the results in terms of annual icemaker cycles, 

daily cycles, annual energy use, or daily energy use. The field metering period lasted roughly 

four months at each site but never covered an entire year. To calculate annual cycles, we scaled 

up the number of cycles observed in the measured period by the number of days not observed. 
The method relies on the simple assumption that the daily average of icemaker usage over the 

course of the year will be the same as it was over the time period studied. Having not observed, 

by definition, the unmetered months, we have no way of testing this assertion. For the 

information presented on the daily cycles, we do not scale the data, instead presenting only the 

data observed during the metered period. Throughout the memo, results are expressed as means; 

graphs show the mean result with a 90% confidence interval overlaid. By that we mean the 

interval within which we are 90% confident the true value will fall inside. 

Number of Annual Cycles, Average, and Range 

On average, icemakers cycle 1102 times per year or 3 times per day. Icemaker cycles ranged 

from a minimum of 60 cycles per year to a maximum of 3441; at the 25th percentile the icemaker 

cycled 470 times per year and at the 75th percentile, 1507 times per year. The top three sites 

show an unusually large number of cycles per year; it is possible that there is a problem with the 

icemaker at these sites. 

Figure 1. Number of Annual Cycles 

7

16

13

11

8

4

5

4 4

2

1

3

1

2

0 5

10 15

Occurence

0 500 1000 1500 2000 2500 3000 3500

Annual Cycles

Number of Annual Cycles
We investigated additional possible explanations for the number of cycles. There were no 

correlations to manufacturer, volume or year of manufacture. The number of annual cycles does 

not seem to trend upward or downward according to manufacture date. More icemaker cycles 

are seen in refrigerators manufactured between 2004-2007, the middle of our range, but there is 

no obvious explanation for the pattern. The number of annual cycles does not seem to trend 

upward or downward according to refrigerator volume or make. 

Table 1. Annual Cycles by Manufacture Date 

Manufacture Date Bin Annual Cycles

Mean EB n

2000-2003 1097 189 19

2004-2007 1197 160 33

2008-2012 948 139 29

Table 2. Annual Cycles by Volume 

Volume (ft3)

Annual Cycles

Mean EB n

18-19 878 126 9

20-21 1061 196 25

22-23 776 189 5

24-25 1172 142 35

26+ 1215 364 6

Table 3. Annual Cycles by Make 

Refrigerator Make Annual Cycles

Mean EB n

Amana 1260 440 5

Frigidaire 660 137 6

General Electric 1538 275 13

Kenmore 1285 275 23

Kitchen Aid 862 237 6

LG 658 321 2

Maytag 629 216 8

Samsung 1221 277 4

Whirlpool 978 249 11

Icemaker Cycle Impact on Energy Use

There is a fairly strong correlation between refrigerator energy usage and the number of annual 

cycles (see Figure 2). This relationship is clear for all refrigerators taken together, and for 

through-door icemakers, but it is not as strong for in-freezer icemakers (see Table 2). This 

muddiness may be due to a low number of high energy users in the latter group; the high energy 
users have fewer icemaker cycles per year, but there are only 4 refrigerators in this group. A 

regression analysis for this group showed that there was still a significant relationship between 

energy use and icemaker cycles.

Figure 2. Annual Cycles by Total Refrigerator Energy Usage 

Table 2. Annual Cycles by Total Refrigerator Energy Usage 

Annual Kwh Annual Cycles

Mean EB n

400 884 124 19

500 1056 156 27

600 1235 244 14

Annual Kwh Annual Cycles for In Freezer Ice Dispensers

Mean EB n

400 794 139 13

500 815 145 12

600 676 239 4

Annual Kwh Annual Cycles for Through Door Ice Dispensers

Mean EB n

400 1079 249 6

500 1250 250 15

600 1458 306 10

0

500

1000 1500 2000

Annual Cycles

400 500 600

Annual Energy (kWh)

Annual Cycles 90% Confidence Interval

Annual Cycles by Energy Usage
Certainly, the number of icemaker cycles contributes to energy consumption, but the extent to 

which it contributes to the energy consumption of the entire refrigerator in this study is not clear. 

There are other factors involved, such as overall refrigerator/freezer efficiency and individual 

household behavior. 

Regional Impact 

We assessed the data for any variations based on geography or climate and found correlations 

with cooling zone. 

Icemaker cycles show no statistical differences by state. Table 3 shows the difference in cycles 

by state. Washington has slightly fewer cycles on average but is within the error bounds of the 

other two states’ averages. The sites in Washington were generally in cooler climate zones 

which could explain the slightly lower mean value. Nevertheless, the differences are not 

significant. 

Table 3. Annual Cycles by State 

State Annual Cycle

Mean EB n

ID 1358 176 7

OR 1097 156 27

WA 1067 132 47

Icemaker cycles are strongly dependent on heating and cooling zones. This is intuitively apparent 

for cooling zones, but given that our measurements were taken primarily in summer months we 

believe the dependence on heating zone is merely a product of the correlation between heating 

zones and cooling zones- the correlation is 0.65 which is to say that most of the cooling zone 2 

sites are also located in heating zone 2 instead of heating zone 1. 
Figure 3. Annual Cycles by Cooling Zone 

Table 4. Annual Cycles by Cooling Zone 

Cooling 

Climate 

Zones

Annual Cycles

Mean EB n

1 920 90 61

2 1655 223 20

0

500

1000 1500 2000

Annual Cycles

1 2

Cooling Zone

Annual Cycles 90% Confidence Interval

Annual Cycles by Cooling Zone
Figure 4. Annual Energy by Cooling Zone 

Table 5. Annual Energy by Cooling Zone 

Cooling 

Climate 

Zones

Annual kWh

Mean EB n

1 577 19 58

2 641 55 18

0

250 500 750

1000

Annual Energy (kWh)

1 2

Cooling Zone

Annual Energy 90% Confidence Interval

Refrigerator-Freezer Energy Use by Cooling Zone
Figure 5. Annual Cycles by Heating Zone 

Table 6. Annual Cycles by Heating Zone 

Heating 

Climate Zone

Annual Cycles

Mean EB n

1 981 93 69

2 1893 264 11

Inhabitant Demographics 

A regression analysis on the number of occupants showed that neither the number of occupants 

nor the number of adults (age 19+) in a household had any significant impact on the number of 

annual cycles observed. There was a slight correlation with the age of the occupants – children 

seem to contribute to fewer ice making cycles than adults. In the regression analysis on number 

of occupants, the constant term dominates the model. Additional linear regression analyses, with 

and without a constant, show that the statistically significant age groups are ages 19 to 64 and 

ages 65+. 

0

500

1000 1500 2000 2500

Annual Cycles

1 2

Heating Zone

Annual Cycles 90% Confidence Interval

Annual Cycles by Heating Zone
Table 7. Annual Cycles by Number of Occupants

Occupants Annual Cycles

Mean EB n

1 660 139 12

2 1105 127 38

3 1059 249 15

4 1603 303 11

Table 8. Annual Cycles by Number of Adults 

Adults Annual cycles

Mean EB n

1 640 119 14

2 1224 117 54

3 1070 325 10

Seasonal Impact 

There is a seasonal impact on icemaker use. Use was higher in July and August by 3-8 ice cubes 

per day. This probably represents 1-2 additional iced drinks. Use was also somewhat higher in 

April, but we can find no explanation for this. These findings are robust even when outliers are 

excluded. 

Figure 6. Daily Cycles by Month 

0 2 4 6 8

Daily Cycles

Apr May Jun Jul Aug

Months

Daily Cycles 90% Confidence Interval

Daily Cycles by Month
Table 9. Daily Cycles by Month 

Month Daily Cycles

Mean EB n

Apr 6.01 0.17 827

May 5.37 0.12 1137

Jun 5.59 0.14 1103

Jul 6.67 0.15 1260

Aug 6.61 0.23 582

Temperature 

There does not seem to be a relationship between icemaker cycles and outdoor temperature. 

Neither daily maximum nor daily average outdoor temperatures drive icemaker usage; there is a 

weak relationship with minimum outdoor temperature, but that may be spurious. A better 

predictor is indoor (kitchen) temperature. 

Table 10. Daily Cycles by Outdoor Temperature Bin 

Temperature 

Bins

Daily Cycles

Mean EB n

40 6.9 0.9 43

45 6.1 0.3 226

50 5.1 0.2 564

55 5.5 0.2 606

60 5.7 0.2 522

65 5.4 0.3 237

70 4.9 0.4 81
Figure 7. Daily Cycles by Indoor Temperature Bin 

Table 11. Daily Cycles by Indoor Temperature Bin 

Indoor 

Temperature

Daily Cycles

Mean EB n

62 4.1 0.4 32

64 5.1 0.4 98

66 5.4 0.3 228

68 5.4 0.2 695

70 5.6 0.1 1060

72 6.2 0.2 983

74 6.5 0.2 836

76 6.4 0.2 528

78 6.6 0.4 250

80 6.5 0.6 110

82 7.3 0.8 54

84 9.1 1.7 21

Through-Door vs. In-Freezer Ice Dispensers 

There is a difference between the annual ice making cycles of refrigerators with and without 

through-door dispensers. Through-door ice dispensers have more annual ice making cycles than 

in-freezer dispensers. It is certainly easier to obtain ice from a through-door ice dispenser; 

5

5.5

6

6.5

7

7.5

Daily Cycles

65 70 75 80

Temperature Bin (F)

Daily Cycles 90% Confidence Interval

Daily Cycles by Indoor Temperature
however, whether the ease of access drives ice use or whether people who have a tendency to use 

ice preferentially select through-door icemakers is unknown. Figure 8 and Table 12 show the 

average number of cycles; Figure 9 and Table 13 show median cycles. 

There is also a difference between the annual energy use of refrigerators with and without 

through-door dispensers. Through-door ice dispenser refrigerators use more energy annually. 

Unlike duty cycles, energy use has been annualized using a trigonometric fit method (discussed 

in Appendix A below). 

Figure 8. Average Annual Cycles by Ice Dispenser Type 

Table 12. Average Annual Cycles by Ice Dispenser Type 

Ice Dispenser Annual Cycles

Mean EB n

In Freezer 866 112 39

Through-door 1321 140 42

0

500

1000 1500 2000

Annual Cycles

In Freezer Through door

Ice Dispenser

Annual Cycles 90% Confidence Interval

Annual Cycles by Ice Dispenser
Figure 9. Median Annual Cycles by Ice Dispenser Type

Table 13. Median Annual Cycles by Ice Dispenser Type 

Ice 

Dispenser

Annual Cycles

25th 

Quartile Median 75th Quartile n

In Freezer 397 610 1179 39

Through

Door 618 1009 2023 42

0

1,000 2,000 3,000 4,000

Annual Cycles

In Freezer Through Door

Ice Dispenser

excludes outside values

Annual Cycles by Ice Dispenser
Figure 10. Refrigerator Energy Use by Ice Dispenser Type 

Table 14. Refrigerator Energy Use by Ice Dispenser Type 

Ice Dispenser Annual kWh

Mean EB n

In Freezer 535 20 36

Through-door 642 30 40

Ice Dispenser and Cooling Zone 

The dependence on delivery method and cooling zone suggests taking a deeper look at the 

combination of the two factors. Below is an investigation of each factor when the other is held 

constant. A visual and tabular inspection shows that both energy consumption and ice making 

cycles are dependent on either cooling zone or delivery system even when the other factor is held 

constant, the only exception being energy consumption of in-freezer ice makers among cooling 

zones.

0

100 200 300 400 500 600 700

Annual Energy (kWh)

In Freezer Through door

Ice Dispenser

Annual Energy 90% Confidence Interval

Refrigerator-Freezer Energy Use by Ice Dispenser Type
Figure 11. Annual Cycles by Ice Dispenser and Cooling Zone 

Table 15. Annual Cycles by Ice Dispenser and Cooling Zone 

Annual Cycles

Cooling Zone

Ice Dispenser 1 2 n

In Freezer Mean 734 1375 39

EB 103 331

Through Door Mean 1113 1843 42

EB 141 299

0

1000 2000 3000 4000

Annual Cycles

In Freezer Through Door

excludes outside values

Annual Cycles by Ice Dispenser

Cooling Zone 1 Cooling Zone 2

Overall
Figure 12. Annual Energy by Ice Dispenser and Cooling Zone 

Table 16. Annual Energy by Ice Dispenser and Cooling Zone 

Annual Energy (kWh)

Cooling Zone

Ice Dispenser 1 2 n

In Freezer Mean 537 529 36

EB 25 28

Through Door Mean 616 712 40

EB 27 83

Annual Refrigerator Energy Use

Table 17 shows four different quantifications of annual refrigerator energy use. The table 

contains the rated energy from DOE testing; the annual use, which is based on the measured data 

linearly extrapolated to the full year; the hypothetical annual use if the indoor temperature were 

always within the 69°F to 71°F range; and the trigonometric fit average annual use, our best 

estimate of actual energy, as described in Appendix A. 

0

200 400 600 800

1000

Annual Energy (kWh)

In Freezer Through Door

excludes outside values

Annual Energy by Ice Dispenser

Cooling Zone 1 Cooling Zone 2

Overall
Table 17. Rated and Measured Energy Use 

Annual kWh

Mean EB n

Rated annual kWh 546 9 89

Annual kWh 592 20 76

Annual kWh Within Temp Range 581 18 75

Trig Fit Average kWh 543 19 82

Using an analysis of variance test, we compared rated annual kWh and raw observed annual 

kWh. Our analysis of variance produced a p-value of 0.029; this indicates that the rated energy 

use of refrigerators and observed annual energy use are not significantly related. 

However, when observed annual energy use is restricted to times when the kitchen temperature 

is between 69°F and 71°F, we find a p-value of 0.079. This indicates that the rated annual energy 

use is significantly related to the actual energy use. Kitchen temperature is within 69°F and 71°F, 

23% of the time averaged across all sites. This is within one degree of the DOE test condition –

refrigerators are tested at 90°F and no door openings and closings, with the assumption that that 

corresponds to an actual kitchen temperature of 70°F with regular door openings and closings. 

Figure 14. Rated vs. Actual Energy Use 

200 400 600 800

1,000

Annual kWh

Energy Estimates

excludes outside values

Rated vs. Actual Energy Use

Rated kWh Average kWh

kWh, IDT within 69-71 Degrees Trig Fit Average kWh
When we perform an analysis of variance test comparing the rated energy kWh to the 

trigonometric fitted prediction, we find a p-value of 0.87. This indicates that the rated annual 

energy use is significantly related to the actual energy use. 

Looking at these comparisons, we see that rated energy use is approximately 35 kWh lower than 

annual energy use when the refrigerators are operating at a kitchen temperature between 69-

71°F. It is 3 kWh higher than the annualized kWh ratings across all temperatures. Thus, it is a 

good estimate of energy use compared to the Department of Energy standard, and very close to 

real-world observations. 

Conclusions 

Icemaker use is not driven by as many factors as we believed at the onset of the study. 

Teenagers are not a driver of icemaker use; inhabitant demographics generally do not appear to 

be a driver of icemaker use; outdoor temperature is not a driver of icemaker use. The primary 

factors are indoor temperature, the ease of access to the ice – whether it is through-door or in the 

freezer – and climate zone. The fact that climate zone is a driver of icemaker use while outdoor 

temperature is not as curious. Icemaker use also has a correlation with overall refrigerator 

energy use. 

The study had certain limitations. We did not study the energy use of the icemaker 

independently of the refrigerator as a whole due to budgetary constraints. We had relatively few 

homes with high occupancy rates (>3 people per home). Our only stratification was on the type 

of refrigerator – through-door vs. in-freezer. However, our data are robust enough to support the 

conclusions given. 
Appendix A. Energy Use: Annualized Projections 

Our data are predominantly taken from summer months. Given the observed dependence on 

months we would expect to see a seasonal change in energy use from the data if observations 

spanned the entire year. In order to come up with a fair annual estimate accounting for the colder 

months of the year we have implemented a linear regression on trigonometric terms. This 

method has been used in other Northwest studies1 (and elsewhere). Our fit methodology was 

based on an article by Nicholas Cox published in the Stata journal2. Scaling the entire year as a 

unit we fit sine and cosine terms. 

 

  

This allows us to approximate the energy use of the entire year. An initial fit yielded two annual 

peaks so the fit was reproduced to halve the period, forcing a single annual peak. As expected, 

the fit finds the peak during the summer. The trigonometric fit captures the seasonal variation in 

kitchen temperature, placing more use in the summer when the kitchen is warmer and less in the 

winter when it is cooler. 

Figure 15. Example Trigonometric Fit of Refrigerator Power 

0

100 200 300 400

Watts

0 1 2 3 4 5 6 7 8 9 10 11 12

Month

Watts Predicted Watts

Fitted Values

Trigonometric Fit for Site 11452
1Roos, C. and D. Baylon. May, 1993. Non-Space Heating Electrical Consumption in Manufactured 

Homes. Residential Construction Demonstration Project, Cycle II. Prepared for Bonneville Power 

Administration under Contract No. DE-AM79-91BP13330. 

2Cox, N. “Speaking Stata: In Praise of Trigonometric Predictors,” Stata Journal 6 (2006) 561-579.