How to work out what an appliance costs to run

Three numbers are enough:

watts ÷ 1,000 × hours of use × price per kWh = cost

Dividing by 1,000 turns watts into kilowatts. Kilowatts times hours gives kilowatt-hours (kWh), which is the unit your electricity is sold in.

Look for the label or nameplate on the appliance, usually on the back or underside, or check the manual. It gives the power in watts (W).

If the label lists only amps, multiply by the voltage. A standard U.S. outlet supplies 120 volts, so an appliance drawing 10 amps uses about 1,200 watts. Large appliances such as electric dryers and ranges run on 240 volts.

The label shows the most the appliance can draw. Many appliances do not run flat out the whole time they are switched on. A refrigerator or an air conditioner cycles on and off, and a heater with a thermostat stops once the room is warm. For those, the hours that count are the hours it is actually running, which is fewer than the hours it is plugged in.

Your electric bill states the price in cents per kWh. Use that if you can, because rates differ a great deal from one area to another. If you do not have a bill to hand, the electricity price page shows the current U.S. average.

Say a 1,500-watt heater runs for 8 hours and your rate is 15¢ per kWh. These numbers are only an illustration.

1,500 ÷ 1,000 × 8 × $0.15 = $1.80 a day

The calculator on the homepage does this sum with your own numbers.

The answer is an estimate, not a meter reading. It does not include the fixed monthly charges on a bill, and it assumes one price per kWh. If your plan charges different rates at different times of day, the true cost depends on when the appliance runs.