How it works
Electricity is billed by the kilowatt-hour (kWh): 1,000 watts running for one hour. Multiply an appliance’s wattage by its hours of use, divide by 1,000, and multiply by your rate.
The wattage on a label is usually the maximum. Devices that cycle on and off, like refrigerators, or that vary their power, like computers, use less on average. A plug-in power meter gives the most accurate reading.
For your rate, divide the total on your electric bill by the kWh used. That captures delivery fees and taxes along with the energy charge.
Formula
kWh = watts × hours ÷ 1000; cost = kWh × rate
Questions people ask
What is the average electricity rate in the US?
The US residential average has been in the range of 16 to 18 cents per kWh in recent years, with large differences by state. Use the rate from your own bill for an accurate number.
How much does it cost to leave a device on standby?
A device drawing 5 W on standby around the clock uses about 44 kWh a year, roughly $7 to $8 at 17¢/kWh.
My appliance lists amps, not watts. What do I do?
Multiply amps by volts. A 10 amp appliance on a 120 V US outlet draws up to 1,200 watts.