How to calculate power usage on appliances

Written by josh fredman
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There are a number of ways you can calculate power usage on your home appliances. However, they are all surprisingly complicated. Most appliances do not use their maximum amount of power continuously while in operation, which means that it would be inaccurate to base usage on the rated wattage. You could measure the current, and there are ammeters that can do this without interrupting a circuit, but the hot and neutral wires would need to be physically separate, which usually isn't the case. The simplest solution, then, is to isolate the appliance and read your electric meter.

Skill level:
Moderate

Things you need

• Flashlight
• Pencil and paper
• Calculator
• Watch or timer

Measuring Power Usage

1. 1

De-energise all circuits at the breaker, except for the one powering the appliance you want to test. Unplug everything else from that circuit. It would make sense to do this during the daytime. If your water heater is electrically powered, and your appliance uses hot water, keep the water heater circuit energised.

2. 2

Go to your electric meter and take a reading. If you're in an apartment building, you may need to ask your property manager for access to the utility room. The fastest-moving dial on the meter (usually on the far right) measures watt-hours. The next one over measures 10 watt-hours, and so forth. (Some meters also have a 1/10 watt-hour dial.) Seattle.gov offers help on how to read an electric meter.

3. 3

Make a note of the time, and run the appliance. If it uses power at a continuous rate, run it for an arbitrary amount of time---say, 20 minutes. If it has multiple cycles that use power at different rates depending on the cycle, such as a dishwasher or dryer, then run the appliance all the way through from start to finish. If hot water has been used, allow the tank to fully reheat.

4. 4

Return to the electric meter and record the new reading. Subtract the initial reading from the new reading. This is how much power, in watt-hours, your appliance has used for the duration (including water heating, if applicable). Divide the number by 1,000 to convert watt-hours into kilowatt-hours.

5. 5

Note the time and count the number of minutes that passed between the time you took your first and second readings of the meter. Convert this number to hours by dividing it by 60. This conversion is necessary because your electric meter measures kilowatt-hours, and so the time units need to be in hours.

6. 6

Take the kilowatt-hour energy usage value that you calculated between meter readings, and divide this number by the hour value that you just converted. This gives you the power, in kilowatts, used by the appliance (plus the water heater if applicable), during the time it ran, presented as an average. You can convert the number to watts by multiplying by 1,000.

7. 7

Repeat the experiment as necessary if your appliance has different levels of operation. For instance, an electric stove has hundreds of possible settings, depending on the temperature of the oven and of each specific element on the range. You don't have to measure all of these, but you can measure a few of them to get a general idea of how much power is used at different levels.

Estimating Usage Costs

1. 1

Estimate the amount of time, in hours, that you use your appliance over a given period of time, such as a week or a month. You also can measure this number directly by counting the minutes during which your appliance is in use over this time period, and converting it to hours.

2. 2

Multiply the average power in watts of your appliance by the amount of time in hours that your appliance was in use during the given time span. Divide this number by 1,000. This will tell you how many kilowatt-hours of electricity your appliance used during the period.

3. 3

Multiply this kilowatt-hour value by the kilowatt-hour rate as supplied on your electric bill. This will tell you, in dollars, how much money your appliance costs to run during the given time period.

4. 4

Improve the accuracy of your cost estimate by measuring usage at various points in the year and averaging out the estimates.

5. 5

Estimate yearly average cost by multiplying accordingly. For example, if you estimated costs for one week, then multiply the cost by 52. If you estimated costs for one month, multiply the cost by 12.

Tips and warnings

• If your appliance has good documentation in the user's manual about how much power it uses in its various modes of operation, you can consult these numbers and perhaps save yourself some work.
• If you have key equipment that absolutely cannot be unplugged, then get a reading on it separately by turning off everything else and measuring how much power the key equipment uses in a given amount of time. Then subtract this value from your appliance power usage figures.

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