Power versus energy
Kilowatts and kilowatt-hours sound alike but measure different things. A kilowatt is a rate — how fast a device uses energy at any moment. A kilowatt-hour is a quantity — the total energy used, which is what a utility meter counts and what you pay for. A 3 kW load isn’t 3 kWh of anything until you say how long it runs.
This conversion is the everyday tool for turning an appliance’s power rating and its hours of use into an energy figure, and from there into a cost.
The kW to kWh formula
Worked example
The calculator’s defaults are 3 kW running for 4 hours.
Twelve kilowatt-hours. At an electricity rate of, say, 16 cents per kWh, that session costs about 1.92 dollars. Run the same load 4 hours a day for a month and it’s roughly 360 kWh, close to 58 dollars.
kW to kWh at a glance
| Power | 1 hr | 4 hr | 8 hr | 24 hr |
|---|---|---|---|---|
| 0.5 kW | 0.5 kWh | 2 kWh | 4 kWh | 12 kWh |
| 1.5 kW | 1.5 kWh | 6 kWh | 12 kWh | 36 kWh |
| 3 kW | 3 kWh | 12 kWh | 24 kWh | 72 kWh |
| 5 kW | 5 kWh | 20 kWh | 40 kWh | 120 kWh |
When the load isn’t constant
The multiplication assumes steady power for the whole interval. A resistance heater running flat-out fits that. A refrigerator, a thermostat-controlled space heater, or a variable-speed pump does not — they draw full power only part of the time. For those, multiply the rated kW by the duty cycle (the fraction of the interval the device actually runs) or split the time into stretches where the draw is roughly constant and add the results.
Common mistakes
- Using rated kW for a cycling appliance, which overstates the energy.
- Mixing minutes and hours. The time must be in hours; 90 minutes is 1.5 hr.
- Confusing the kWh result with kW. One is the total; the other is the rate.
For the reverse, use the kWh to kW calculator. To start from a wattage instead, see watts to kWh.