kW vs kWh: what's the difference?
Short answer: kW is how fast, kWh is how much. Multiply the power by the hours and you have the energy. Fill in any two of the three boxes below and the third one follows.
kW × hours = kWh · 2 kW for 3 hours = 6 kWh
Which is which?
A kilowatt (kW) measures power: how fast energy flows right now. A kilowatt-hour (kWh) measures energy: how much has flowed in total. It's the same difference as between km/h and km. Drive at 50 km/h for two hours and you have covered 100 km. Run 2 kW for three hours and you have used 6 kWh.
Your electricity bill mostly counts kWh. Some, like the capacity tariff in Flanders, also charge for your peak kW. The label on a kettle, a heater or a car charger gives kW.
On the napkin
A 2 kW kettle that boils for three minutes uses 2 × 0.05 = 0.1 kWh. A 10 W LED bulb, which is 0.01 kW, needs ten hours to use the same 0.1 kWh. The kettle draws 200 times as much power, and at the same price per kWh, the two cost exactly the same.
People sometimes say "kW per hour" when they mean kWh. A kW per hour would be power that keeps rising, like a car speeding up, and it hardly ever comes up at home.
Common values
| Appliance and power | Time | Energy |
|---|---|---|
| 10 W LED bulb | 10 hours | 0.1 kWh |
| 2 kW kettle | 3 minutes | 0.1 kWh |
| 1 kW heater | 1 hour | 1 kWh |
| 11 kW home car charger (three-phase, mainland Europe) | 4 hours | 44 kWh |
| 3 MW wind turbine at full power | 1 hour | 3,000 kWh |
Need another unit?
The full energy converter shows any amount in every energy and power unit at once, from joules to therms to cups of tea.
Want this in a spreadsheet?
I'll email you two free ones: the converter for every energy unit, and a heat pump versus gas boiler check using your own bill. One short email a month at most.
Send me bothSources: 1 kW = 1,000 joules per second and 1 kWh = 3.6 MJ by definition (BIPM, The International System of Units). Kettle: a typical appliance rating. Wind turbine size: LBNL Land-Based Wind Market Report and WindEurope, as in the full converter. Car charger: 3 × 230 V × 16 A = 11 kW; in the UK home chargers are usually 7 kW.
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