7-i

CHAPTER 7. ENERGY AND WATER USE ANALYSIS

TABLE OF CONTENTS

7.1 INTRODUCTION....................................................................................................... 7-1

7.2 PER-CYCLE ENERGY AND WATER CONSUMPTION BY EFFICIENCY 

LEVEL........................................................................................................................ 7-1

7.3 AVERAGE ANNUAL ENERGY AND WATER CONSUMPTION BY 

EFFICIENCY LEVEL................................................................................................. 7-4

7.4 VARIABILITY OF DISHWASHER USAGE ............................................................. 7-6

LIST OF TABLES

Table 7.2.1 Standard-Sized Dishwashers: Annual Energy Use, Per-Cycle Water Use, 

and Standby Power Use by Efficiency Level.................................................... 7-2

Table 7.2.2 Compact Dishwashers: Annual Energy Use, Per-Cycle Water Use, and 

Standby Power Use by Efficiency Level........................................................... 7-2

Table 7.2.3 Standard-Sized Dishwashers: Per-Cycle Energy Use by Efficiency Level ........ 7-3

Table 7.2.4 Compact Dishwashers: Per-Cycle Energy Use by Efficiency Level.................. 7-4

Table 7.3.1 Standard-Sized Dishwashers: Annual Energy and Water Use by Efficiency 

Level................................................................................................................ 7-5

Table 7.3.2 Compact Dishwashers: Annual Energy and Water Use by Efficiency Level ..... 7-6

Table 7.4.1 RECS 2005 Dishwasher Usage Data ................................................................ 7-6

LIST OF FIGURES

Figure 7.4.2 Distribution of Dishwasher Annual Usage (cycles per year) based on 2005 

RECS Usage Data............................................................................................ 7-7
7-1

CHAPTER 7. ENERGY AND WATER USE ANALYSIS

7.1 INTRODUCTION

To carry out the life-cycle cost (LCC) and payback period (PBP) calculations described 

in chapter 8, DOE needed to determine the savings in operating costs that consumers would 

derive from more efficient products. DOE used consumer energy and water use data, along with 

energy and water prices, to develop the most significant component of consumer operating costs

in the LCC and PBP analyses. (Maintenance and repair costs are the other contributors to 

operating cost.) This chapter describes how DOE determined the annual energy and water 

consumption of residential dishwashers.

7.2 PER-CYCLE ENERGY AND WATER CONSUMPTION BY EFFICIENCY 

LEVEL

Dishwasher per-cycle energy consumption has three components: energy used for heating 

water, operating the machine, and drying the dishes. The energy used to operate a machine 

powers the motor (to pump water and dispose of food) and booster water heater. The DOE test 

procedure provides the following equations to calculate the total per-cycle energy consumption

of dishwashers:

DWCYCLE =WH + M + D

M D

e

K DWCYCLE =V ×T × + +

Where:

DWCYCLE = dishwasher per-cycle energy consumption,

V = water consumption in gallons per cycle,

T = nominal water temperature rise = 70 oF (assuming a nominal inlet 

water temperature of 120 oF),

K = specific heat of water in kWh per gallon per degree Fahrenheit 

(0.0024), or Btus per gallon per degree Fahrenheit (8.2),

e = efficiency of electric water heater (100 percent) or gas water heater (75

percent),

WH = V•T•K/e = water-heating per-cycle energy consumption,

M = machine per-cycle energy consumption, and

D = drying per-cycle energy consumption.

The largest component of dishwasher energy consumption is water-heating energy use. 

Water-heating energy use is directly dependent on water use. 
7-2

To develop per-cycle energy use values, DOE used data developed by the engineering 

analysis. As discussed in chapter 5, DOE analyzed specific efficiency levels for standard-sized 

and compact dishwashers. Table 7.2.1 and Table 7.2.2 provide the annual energy use, per-cycle 

water use, and standby power for standard-sized and compact dishwashers, respectively. Also 

included in the tables is the standby power consumption corresponding to each efficiency level. 

Table 7.2.1 Standard-Sized Dishwashers: Annual Energy Use, Per-Cycle Water Use, and 

Standby Power Use by Efficiency Level

Level Annual Energy Use Water Use

kWh/yr

Standby Power

gal/cyc W

Baseline 355 6.50 0

1 324 5.80 2.3

2 307 5.00 1.7

3 295 4.25 1.7

4 234 3.80 1.7

5 180 1.60 1.7

Table 7.2.2 Compact Dishwashers: Annual Energy Use, Per-Cycle Water Use, and 

Standby Power Use by Efficiency Level

Level

Annual Energy Use Water Use

kWh/yr

Standby Power

gal/cyc W

Baseline 260 4.50 2.3

1 222 3.50 2.3

2 154 2.10 1.7

Based on the data in Table 7.2.1 and Table 7.2.2, DOE derived per-cycle energy use 

estimates using equations and assumption in the DOE test procedure. 

DOE developed the per-cycle dishwasher energy use by first subtracting from the total

annual dishwasher energy use the standby power energy use. The result is the annual dishwasher 

energy use, i.e., the energy dedicated to dishwashing only. The per-cycle dishwasher energy use 

is simply the annual dishwasher energy use divided by the average-use cycles per year.1 Arthur 

D. Little (ADL) conducted a comprehensive analysis of dishwasher usage in 2001 that revealed 

that dishwashers are used, on average, 215 cycles per year.2 

The following equation from the DOE test procedure demonstrates how the per-cycle 

dishwasher energy use is determined. Below is the equation for total annual energy use:
7-3

( )

1000

_ H N L DW DW N S ANNUAL CYCLE m

× = × + ×

Where:

DWANNUAL = total annual dishwasher energy consumption,

DWCYCLE = per-cycle dishwasher energy consumption,

N = representative average dishwasher use of 215 cycles per year,

Sm = average standby power in Watts,

H = total number of hours per year = 8,766, and

L = average duration of dishwasher cycle.

Because both the total annual dishwasher energy use and standby power consumption are 

known, the per-cycle dishwasher energy consumption simply is:

( )

N

H N L DW S

DW

ANNUAL m

CYCLE

1000

_ × − ×

=

Per-cycle dishwasher energy use is disaggregated into two general categories: (1) water 

heating; and (2) machine (e.g., motor energy for pumping) and dish drying from an electrical 

heating element. DOE determined the per-cycle water heating consumption by assuming the use 

of an electric water heater and multiplying the per-cycle water consumption by an assumed 

temperature rise of 70 oF (21 oC) and a specific heat of 0.0024 kWh/gal-oF (4.186 joule/gram-

°C). DOE determined the per-cycle machine and drying energy by subtracting the per-cycle 

water heating energy consumption from the per-cycle dishwasher energy consumption. Table 

7.2.3 and Table 7.2.4 show energy use and components for standard-sized and compact 

dishwashers, respectively.

Table 7.2.3 Standard-Sized Dishwashers: Per-Cycle Energy Use by Efficiency Level

Level

Energy Use Water Use

kWh/yr

Standby 

Power

gal/cyc W

Per-Cycle Energy Use Components

Total*

Water 

Heating**

gal/cyc

Machine + 

Drying

gal/cyc

Baseline

gal/cyc

355 6.50 0 1.65 1.09 0.56

1 324 5.80 2.3 1.42 0.97 0.44

2 307 5.00 1.7 1.36 0.84 0.52

3 295 4.25 1.7 1.30 0.71 0.59

4 234 3.80 1.7 1.02 0.64 0.38

5 180 1.60 1.7 0.77 0.27 0.50

* Standby annual energy use based on an assumed dishwasher cycle of one hour and 215 cycles per year; 

Standby hours = 8,766 hours – 215 * 1 hour = 8,551 hours.

** Based on the use of an electric water heater at 100% efficiency.
7-4

Table 7.2.4 Compact Dishwashers: Per-Cycle Energy Use by Efficiency Level

Level

Energy Use Water Use

kWh/yr

Standby 

Power

gal/cyc W

Per-Cycle Energy Use Components

Total*

Water 

Heating**

gal/cyc

Machine + 

Drying

gal/cyc

Baseline

gal/cyc

260 4.50 0.0 1.12 0.76 0.36

1 222 3.50 2.3 0.94 0.59 0.35

2 154 2.10 1.7 0.65 0.35 0.30

* Standby annual energy use based on an assumed dishwasher cycle of one hour and 215 cycles per year; 

Standby hours = 8,766 hours – 215 * 1 hour = 8,551 hours.

** Based on the use of an electric water heater at 100% efficiency.

7.3 AVERAGE ANNUAL ENERGY AND WATER CONSUMPTION BY 

EFFICIENCY LEVEL

DOE determined the average annual energy and water consumption by multiplying the 

per-cycle energy and water consumption by the number of cycles per year. 

In 2003, DOE revised its test procedure for dishwashers to more accurately establish their 

efficiency and energy and water use. The 2003 test procedure amendments included a reduction 

in the average-use cycles per year. As discussed above, ADL conducted a comprehensive 

analysis of dishwasher usage in 2001 in 26,000 households that revealed that dishwashers are 

used, on average, 215 cycles per year.2 The 2005 RECS provides data indicating the annual 

usage of households with dishwashers. Of the almost 4400 households in RECS, 2480 have 

dishwashers. The average-use value for dishwashers in the 2005 RECS households is 174 cycles 

per year. But because the ADL survey is a much more comprehensive and larger survey than the 

survey performed for RECS, DOE chose an average usage of 215 cycles per year as the most 

representative value for average dishwasher use. 

DOE calculated dishwasher annual energy consumption from the per-cycle values 

reported in Table 7.2.3 and Table 7.2.4 by multiplying them by the average-use cycles as shown 

below with the following equations: 

DWWH −ANN =WH × N

DWMACH −ANN = M × N

DWDRY −ANN = D × N
7-5

Where:

DWWH-ANN = total annual dishwasher energy consumption,

DWMACH-ANN = total per-cycle dishwasher energy consumption, and

DWDRY-ANN = total per-cycle dishwasher energy consumption.

DOE calculated dishwasher annual water consumption with the following equation: 

DWWATER − ANN = DWWATER −CYC × N

Where:

DWWATER-ANN = total annual dishwasher water consumption, and

DWWATER-CYC = total per-cycle dishwasher water consumption.

The annual energy and water consumption shown in Table 7.3.1 for standard dishwashers 

and in Table 7.3.2 for compact dishwashers reflect an annual usage of 215 cycles per year. The 

annual water-heating energy consumption reflects the use of an electric, gas, or oil water heater.

Table 7.3.1 Standard-Sized Dishwashers: Annual Energy and Water Use by Efficiency 

Level

Efficiency 

Level

Annual Energy Use Annual

Water

Total Use

gal/yr

Water Heating*

kWh/yr

Machine + 

Drying + Standby

kWh/yr

Electric

kWh/yr

Gas

kWh/yr

Oil

MMBtu/yr

Baseline

MMBtu/yr

355.0 234.8 1.068 1.068 120.2 0.0 1,398

1 324.0 209.5 0.953 0.953 94.8 19.7 1,247

2 307.0 180.6 0.822 0.822 111.9 14.5 1,075

3 295.0 153.5 0.698 0.698 127.0 14.5 914

4 234.0 137.3 0.624 0.624 82.2 14.5 817

5 180.0 57.8 0.263 0.263 107.7 14.5 344

* Electric, gas, and oil water heating based on water heater efficiencies: 100% for electric, 75% for gas, 75% for 

oil.
7-6

Table 7.3.2 Compact Dishwashers: Annual Energy and Water Use by Efficiency Level

Efficiency 

Level

Annual Energy Use Annual

Water

Total Use

gal/yr

Water Heating*

kWh/yr

Machine + 

Drying + Standby

kWh/yr

Electric

kWh/yr

Gas

kWh/yr

Oil

MMBtu/yr

Baseline

MMBtu/yr

260.0 162.5 0.739 0.739 77.8 19.7 968

1 222.0 126.4 0.575 0.575 75.9 19.7 753

2 154.0 75.9 0.345 0.345 63.6 14.5 452

* Electric, gas, and oil water heating based on water heater efficiencies: 100% for electric, 75% for gas, 75% for 

oil.

7.4 VARIABILITY OF DISHWASHER USAGE

The 2005 RECS indicates that 2,480 of the 4,382 households in the survey use 

dishwashers. For each household using a dishwasher, RECS provides data on the number of 

dishwasher cycles in the following bins: (1) less than once per week, (2) once per week, (3) 2–3 

times per week, (4) 4-6 times per week, (5) at least once per day. DOE converted the above to 

annual values and created a triangular or uniform distribution for each bin. Table 7.4.1 shows the 

share of households in each bin and the distribution used. DOE randomly assigned a specific 

numerical value from within the appropriate bin to each household in the dishwasher sample. 

The average number of cycles per year derived from the RECS 2005 data is 174. 

Table 7.4.1 RECS 2005 Dishwasher Usage Data

Bin

Share of RECS Households

Distribution Used Percentage* % Count

Less than once per week 17 402 Triangular, 1 to 52

Once per week 14 325 Uniform, 26 to 78

2–3 times per week 34 834 Uniform, 78 to 182

4–6 times per week 18 464 Uniform, 182 to 338

At least once per day 17 455 Triangular, 300/400/500

*Percentages represent weighted values.

To determine the variability of dishwasher usage, DOE normalized the household usage 

values from RECS so that the average-use value equaled 215 cycles per year rather than the 

weighted-average value of 174 cycles per year as indicated by RECS. DOE determined the usage 

for each RECS household with a dishwasher based on the following equation:

RECS AVG

DOE AVG

DW DW HH N

N

N N

_

_ = _ ×
7-7

Where:

NDW = modified dishwasher usage for specific RECS household,

NDW_HH = dishwasher usage for specific RECS household as specified by RECS,

NDOE_AVG = average dishwasher usage of 215 cycles per year as established in the DOE 

test procedure, and

NRECS_AVG = average dishwasher usage of 174 cycles per year as established by RECS.

With the normalized dishwasher usage known for each RECS household, DOE 

determined the corresponding annual energy and water consumption. For all RECS households, 

dishwasher use frequency varies from 1 to over 600 cycles per year. Figure 7.4.2 shows the 

probability distribution of the modified dishwasher usage that DOE determined for each RECS 

household.

Figure 7.4.2 Distribution of Dishwasher Annual Usage (cycles per year) Based 

on 2005 RECS Usage Data
7-8

REFERENCES

 

1 U.S. Department of Energy–Office of Energy Efficiency and Renewable Energy. Energy 

Conservation Program for Consumer Products: Test Procedure for Dishwashers, Final Rule. 

Federal Register. August 29, 2003. Vol. 68, no. 168: pp. 51887–51903.

2 Arthur D. Little. Review of Survey Data to Support Revisions to DOE’s Dishwasher Test 

Procedure, December 18, 2001. Prepared for the U.S. Department of Energy by Arthur D. 

Little: Cambridge, MA.