The short answer
A kilowatt (kW) measures power — how fast something uses electricity at a given moment. A kilowatt-hour (kWh) measures energy — how much electricity has been used over time. You're billed for kWh, not kW.
A simple way to think about it
kW is like the speed of a car; kWh is the distance travelled. A 2 kW heater is the 'speed'. Run it for 3 hours and it 'travels' 6 kWh.
The formula
kWh = kW × hours. If your appliance shows watts, divide by 1,000 first: 2,000 W = 2 kW.
Worked examples (example rate of 25p per kWh)
These use an example rate only. Check the unit rate on your own bill — it varies by supplier, tariff and region.
- A 1 kW heater for 1 hour uses 1 kWh — about 25p.
- A 2 kW heater for 1 hour uses 2 kWh — about 50p. For 4 hours: 8 kWh, about £2.00.
- A 3 kW kettle boiling for 3 minutes (0.05 hours) uses 0.15 kWh — under 4p.
- A 9.5 kW electric shower for 8 minutes uses about 1.27 kWh — around 32p.
Why a '3 kW' appliance doesn't always use 3 kWh an hour
The rating is the maximum power. Thermostats, eco modes and cycles mean many appliances — heaters, ovens, fridges, washing machines — draw full power only part of the time. Your real use is often lower than rating × hours.
Where you'll see each unit
- kW or W: on the appliance's rating plate, box or manual.
- kWh: on your bill, smart meter display and energy labels (often as kWh per year or per cycle).
Related: What is a kWh? · How to calculate appliance running costs