7-i

CHAPTER 7. ENERGY USE ANALYSIS

TABLE OF CONTENTS

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

7.2 CONSUMER SAMPLE................................................................................................... 7-1

7.3 STANDBY POWER CONSUMPTION.......................................................................... 7-2

7.3.1 Annual Energy Consumption by Efficiency Level.......................................................... 7-3

7.3.2 Variability of Annual Energy Consumption.................................................................... 7-4

LIST OF TABLES

Table 7.2.1 RECS 2005 Sample Statistics.............................................................................. 7-1

Table 7.3.1 Number of Meals Cooked per Day Based on RECS Sample .............................. 7-3

Table 7.3.2 RECS Microwave Oven Use Percentage............................................................. 7-3

Table 7.3.3 Microwave Ovens and Countertop Combination Ovens: Annual Energy 

Consumption by Standby Power Level................................................................ 7-4

Table 7.3.4 Over the Range Combination Ovens: Annual Energy Consumption by 

Standby Power Level ........................................................................................... 7-4

LIST OF FIGURE

Figure 7.3.2 Range of Annual Energy Use for Microwave Ovens and Countertop 

Combination Ovens ............................................................................................. 7-5

Figure 7.3.3 Range of Annual Energy Use for Over the Range Combination Ovens.............. 7-5
7-1

CHAPTER 7. ENERGY USE ANALYSIS

7.1 INTRODUCTION

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

chapter 8, DOE must determine the energy savings to consumers from more efficient products. 

This chapter describes how DOE determined the annual standby energy consumption of 

microwave ovens for use in the LCC and PBP analyses.

The engineering analysis described in chapter 5 reports energy use based on the DOE test 

procedure. This test provides standardized results that serve as the basis for comparing the 

performance of different appliances used under the same conditions. Actual usage in the field 

varies depending on the conditions in which the appliances are operated. 

To establish a reasonable range of energy consumption of microwave ovens in the field, 

DOE primarily used data from the Energy Information Administration (EIA)’s 2005 Residential 

Energy Consumption Survey (RECS 2005).

1 RECS is a national sample survey of housing units 

that collects statistical information on the consumption of and expenditures for energy in housing 

units along with data on energy-related characteristics of the housing units and occupants. RECS 

2005 collected data on 4,381 housing units and was constructed by EIA to be a national 

representation of the household population in the United States.

7.2 CONSUMER SAMPLE

The subset of RECS 2005 records used for the microwave oven sample met the following 

criteria:

• The household had a microwave

Because RECS 2005 provides no means of determining the type of microwave present in 

a given household, DOE used the same sample for both product classes. The RECS reported 

saturation of microwave ovens in 2005 was 88 percent.

Table 7.2.1 RECS 2005 Sample Statistics

Product Class Selection Criterion No. of 

Records

No. of U.S. 

Households 

Represented 

(million)

All Has Microwave = Yes 3835 97.7
7-2

7.3 STANDBY POWER CONSUMPTION

To estimate the annual energy use associated with standby power (ESP) for both product 

classes, DOE used the following expression where the standby power is multiplied by the 

number of hours in a year the microwave oven is in standby mode:

ESP = PSP × HoursSP

Where:

PSP = standby power (watts), and

HoursSP = hours in a year the microwave oven is in standby mode (kWh/yr). 

The annual standby hours equal the total hours in a year less the number of hours the 

microwave oven is in active operation. DOE determined the hours of active operation 

(HoursACTIVE) with the following expression:

HoursACTIVE = HoursNOPR ×UseRECS

Where:

HoursNOPR = average microwave operating hours from the NOPR, and

UseRECS = relative usage factor.

In the previous NOPR for cooking products, DOE determined the average hours of 

operation for microwaves (HoursNOPR) to be 71 hours per year.2 DOE has no reason to believe 

that this number has changed. 

For each household, RECS 2005 provides information on the frequency of microwave 

oven usage and the number of hot meals prepared per day. DOE calculated the RECS usage 

factor (UseRECS) for each household in the sample using the following expression:

( ) UseRECS MealsRECS FreqRECS UseAVG = × /

Where:

MealsRECS = RECS reported household hot meals per day,

FreqRECS = RECS reported microwave use frequency, and

UseAVG = Weighted average usage factor.

The weighted average usage factor was calculated using the following expression:
7-3

∑

∑ × × =

RECS

AVG AVG RECS

AVG Weight

Meals Freq Weight Use

Where:

MealsAVG = Average household hot meals per day,

FreqAVG = Average microwave use frequency, and

Weight = RECS household weight.

The responses in RECS for MealsRECS and FreqRECS fall into one of the bins shown in 

Tables 7.3.1 and 7.3.2, respectively. For each question, DOE assumed a uniform or triangular 

distribution within the boundaries of each bin and randomly assigned a value from within the 

appropriate range to each sample household. For the calculation of the average usage factor 

(UseAVG), DOE used the average values from the respective bins.

Table 7.3.1 Number of Meals Cooked per Day Based on RECS Sample

Bin Description Meals per day

Min Max

0 Doesn't cook/Never cooks 0.0 0.0

1 Three or more times a day 3.0 4.0

2 two times a day 1.0 3.0

3 Once a day 0.5 1.5

4 A few times each week 0.2 0.8

5 About once a week 0.1 0.2

6 Less than once a week 0.0 0.1

Table 7.3.2 RECS Microwave Oven Use Percentage

Bin Amount of food cooked in 

the microwave

Use Percentage

Min Max

1 Most or all 90% 100%

2 About half 25% 75%

3 Some or very little 20% 30%

4

Used only for snacks, 

defrosting, or reheating 

food 

5% 25%

7.3.1 Annual Energy Consumption by Efficiency Level

Tables 7.3.3 and 7.3.4 present the average annual energy consumption by efficiency level 

for both product classes.
7-4

Table 7.3.3 Microwave Ovens and Countertop Combination Ovens: Annual Energy 

Consumption by Standby Power Level

Efficiency 

Level

Standby Energy Use

(W) (kWh/year)

Baseline 4.00 34.75

1 2.00 17.38

2 1.50 13.03

3 1.00 8.69

4 0.02 0.17

Table 7.3.4 Over the Range Combination Ovens: Annual Energy Consumption by 

Standby Power Level

Efficiency 

Level

Standby Energy Use

(W) (kWh/year)

Baseline 4.50 39.10

1 3.70 32.15

2 2.70 23.46

3 2.20 19.11

4 0.04 0.35

7.3.2 Variability of Annual Energy Consumption

Figures 7.3.1 and 7.3.2 show the range of average annual energy consumption by 

efficiency level for both product classes. Because variation among households in active use of 

microwave ovens has a small effect on the number of standby hours, the range in standby energy 

use is very small.
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0

5

10

15

20

25

30

35

40

Level 0 Level 1 Level 2 Level 3 Level 4

Annual Energy Use (kWh)

Efficiency Level

Microwave and MWO-Combo CT Average Annual Energy Use

Average; Median; Box 25%-75%; Whisker 5%-95%

median

average

Figure 7.3.2 Range of Annual Energy Use for Microwave Ovens 

and Countertop Combination Ovens

0

5

10

15

20

25

30

35

40

45

Level 0 Level 1 Level 2 Level 3 Level 4

Annual Energy Use (kWh)

Efficiency Level

Combination Oven - OTR Average Annual Energy Use

Average; Median; Box 25%-75%; Whisker 5%-95%

median

average

Figure 7.3.3 Range of Annual Energy Use for Over the Range 

Combination Ovens
7-6

REFERENCES

1. U.S. Department of Energy: Energy Information Administration, Residential Energy 

Consumption Survey: 2005 Public Use Data Files. 2008: Washington, DC. 

http://www.eia.doe.gov/emeu/recs/recspubuse05/pubuse05.html

2. U.S. Department of Energy - Energy Efficiency & Renewable Energy, Technical Support 

Document: Energy Efficiency Program for Consumer Products and Commercial and 

Industrial Equipment, Residential Dishwashers, Dehumidifiers, and Cooking Products, 

and Commercial Clothes Washers, 2008. Washington, DC.