Watts measure power
A watt is a unit of power. It describes the rate at which an appliance uses energy while operating. A 1,500-watt space heater draws power faster than a 100-watt device, but total energy use also depends on runtime.
Kilowatts are 1,000 watts
Divide watts by 1,000 to convert them to kilowatts. A 1,500-watt heater is a 1.5-kilowatt heater.
Kilowatts = watts ÷ 1,000
Kilowatt-hours measure energy
A kilowatt-hour, abbreviated kWh, is the energy used by one kilowatt of power for one hour. Running a 1.5-kilowatt heater at full power for two hours uses 3 kWh.
Energy in kWh = kilowatts × hours
At $0.17/kWh, those 3 kWh would cost an estimated $0.51.
Electricity cost = kWh × rate in $/kWh
Power rating is not always average power
A nameplate may show maximum or rated power rather than the device’s average draw. Thermostats cycle heating and cooling equipment. Motors change load. Electronics use different amounts in active, idle, and sleep modes. When safe and appropriate, a compatible energy monitor can provide a more representative input.
Annual kWh can be the better input
Products that cycle continuously, such as refrigerators, are difficult to estimate from maximum watts and hours. An EnergyGuide label’s annual kWh already incorporates standardized cycling, making it more useful for a planning estimate.
Try the numbers yourself
Use the appliance electricity cost calculator for watts and runtime, or the electricity bill estimator when you already know monthly household kWh.
Common input mistake
Do not enter cents directly into a field labeled $/kWh. Convert 17 cents to $0.17. Entering 17 instead of 0.17 would make the estimate one hundred times too high.