Energy use
kWh Calculator
Kilowatt-hours are power times time. Enter watts or kilowatts and how long the load runs. Optionally price the energy with your own rate.
Numbers stay in this browser. EnergyToolMage does not upload your inputs or require an account.
Formula
kWh = watts × hours ÷ 1000 or kWh = kW × hours
Assumptions
- Power is constant over the hours you entered.
- 1 kW = 1000 W exactly.
- Optional cost uses your rate, or no cost if you leave rate blank.
How it works
Energy is accumulated in milliwatt-hours (watts × hours × 1000) and then divided by 1,000,000 to get kWh. That avoids casual floating-point drift on ordinary household numbers.
Inputs and results
Inputs: power (W or kW), hours, optional USD/kWh. Outputs: kWh and optional USD.
Worked examples
1500 W space heater for 4 hours
1500 × 4 ÷ 1000 = 6 kWh. At an example $0.16/kWh that block would be $0.96.
2.4 kW charger for 3 hours
2.4 × 3 = 7.2 kWh drawn at the wall if you entered charger power, not battery-delivered energy.
How to use this calculator
- Enter power and choose W or kW.
- Enter hours the load runs.
- Optionally enter USD/kWh to see a dollar estimate.
- Read kWh first. Cost is secondary and uses only the rate you typed.
Limitations
- Does not convert BTU, therms, or horsepower. Those are unit-table jobs unless you already converted to watts.
- Does not include charging losses. Use the EV charging calculator for loss-adjusted grid energy.
Sources and freshness
- Every rate, wattage, hour, and loss percentage is a user input or a labeled example. Nothing here is a live utility tariff.
- The $0.16/kWh figure that appears in examples is an illustrative default, not a U.S. average and not current.
- Annualization uses 365.25 days per year and 365.25 ÷ 12 days per month. Your billing cycle will differ.
- The 1000 W = 1 kW factor is the SI prefix, not a measured dataset.
Questions
What is the difference between W, kW, Wh, and kWh?
W and kW are power (how hard a load draws). 1000 W = 1 kW. Wh and kWh are energy (power × time). 1000 W for 1 hour is 1 kWh. This page always multiplies by hours; it is not a bare W↔kW table.
Can I enter kilowatts instead of watts?
Yes. Choose kW as the power unit. Then kWh = kW × hours.
Why might this disagree with my meter?
Nameplate watts are not always the running draw. Motors, heat pumps, and chargers vary. The calculator uses the number you typed.
Related energy tools
- Electricity costMultiply energy you already know in kilowatt-hours by the dollars-per-kWh on your bill. Use this when a meter, EnergyGuide label, or another calculator already gave you kWh.
- Appliance costUse this when you know an appliance’s watts and how often you run it. The page annualizes with a disclosed calendar convention and your bill rate.
- Battery runtimeRuntime is usable energy divided by load. Enter the kilowatt-hours you can actually draw and the watts or kilowatts the load uses. This is not a home-battery installation price.