kWh calculator

How to convert watts to kWh

To convert watts to kWh, multiply the power in watts by the hours of use and divide by 1,000. One kilowatt-hour is 1,000 watts for one hour, the unit on every U.S. electric bill. Then multiply kWh by your price per kWh for the cost.

• kWh per day = watts × hours per day ÷ 1,000
• kWh per month = kWh per day × days used
• Cost = kWh × price per kWh (cents ÷ 100)
• Watts from a nameplate in amps = amps × volts (120 V for most outlets, 240 V for dryers, ranges and EV chargers)

Worked example. A 32-amp Level 2 charger on 240 volts draws 32 × 240 = 7,680 watts. Charging 1.5 hours a day for 30 days uses 7.68 × 1.5 × 30 = 345.6 kWh, which costs $62.35 at 18.04¢ per kWh. One kWh also equals 3,412 Btu of heat, the conversion the heating calculators use.^[btu]

Cost to run common appliances

The table runs the calculator for six examples at the U.S. average price; the wattages and hours are typical label figures used as inputs, not measurements, so put your own device's numbers in the calculator.^[eia861m]

Heating and charging dominate: a single space heater on for evenings can add as much as the average home's whole lighting and electronics use. Electric heat for a whole house is compared with gas, propane and oil on the heat pump vs furnace cost page.

How much electricity does a space heater use?

A space heater labeled 1,500 watts uses 1.5 kWh for every hour it runs on high; on a lower setting or when its thermostat cycles it uses less. Six hours a day for a month is 270 kWh. Every electric resistance heater turns one kWh into 3,412 Btu of heat, while a heat pump rated at the federal minimum of HSPF2 7.5 delivers about 7,500 Btu per kWh over a heating season, a seasonal COP of about 2.2.^{[btu][cfr430]} The heat pump cost page prices that difference for each state.

Cost to run an appliance in your state

The same appliance costs more than twice as much to run in New England or California as in the cheapest states. 100 kWh a month costs $12.35 in North Dakota, $15.98 in Texas, $30.39 in Massachusetts and $44.27 in Hawaii at each state's 12-month average.^[eia861m] Pick your state in the calculator to load its price, or see the full electricity rates by state list. To turn your monthly total into a bill with a fixed charge, use the electricity cost calculator.

Common kWh mistakes

• Using the plug's circuit rating (15 or 20 amps) instead of the device's own nameplate.
• Assuming a fridge or AC runs at full watts all day; compressors cycle.
• Forgetting standby use: devices that stay "on" (consoles, set-top boxes) draw a few watts 24 hours a day.
• Reading kW (power) as kWh (energy). A 2 kW device for 3 hours is 6 kWh.