The difference in kW vs kWh is the difference between a rate and an amount. A kilowatt (kW) measures power, which is how fast energy is used or produced at a moment. A kilowatt hour (kWh) measures energy, which is the amount used or produced over time. Your bill charges for kWh, while appliances, chargers and solar systems are often described in kW, so you need both.
In this guide
The car analogy
Think of a car. Speed is power, and distance is energy. A car travelling at 100 km/h covers 100 km in one hour. A device drawing 1 kW uses 1 kWh in one hour.
A kettle rated at 2 kW uses 2 kWh if it ran for a whole hour. It boils water in three minutes, which is 0.05 hours, so one boil uses about 0.1 kWh. A powerful kettle is not a costly one, because it runs for a short time.
Examples to anchor the idea
A 2 kW heater running for 3 hours uses 6 kWh. At 0.28 per kWh, that costs 1.68.
A 100 W television running for 4 hours uses 0.4 kWh. Divide watts by 1,000 to get kW first, so 100 W is 0.1 kW, and 0.1 × 4 gives 0.4 kWh.
A 7.4 kW home EV charger running for one hour delivers up to 7.4 kWh. A car that needs 42.7 kWh from the socket takes about 5.8 hours. Our guide to Schuko sockets and wallboxes goes through charging times.
Prefixes and units
A watt (W) is the base unit of power. A kilowatt is 1,000 watts. A megawatt is 1,000 kilowatts. The same prefixes apply to energy. A kilowatt hour is 1,000 watt hours, and a megawatt hour (MWh) is 1,000 kWh.
| To convert | Do this |
|---|---|
| Watts to kilowatts | Divide by 1,000 |
| kW × hours | Gives kWh |
| kWh ÷ hours | Gives average kW |
| MJ to kWh | Divide by 3.6 |
Where each unit appears
Appliances list power in watts or kilowatts, because that is the rate at which they work. Chargers, heat pumps and solar inverters list kW. Batteries list capacity in kWh, because they store an amount. Bills show kWh used and the price per kWh. Gas meters show volume, which suppliers convert to kWh. A heat pump with an output of 8 kW can deliver 8 kWh of heat per hour at full power.
Solar panels use kWp and kWh
A solar system is sized in kWp, kilowatts peak, the power it produces in ideal test conditions. What matters for your bill is kWh produced over a year. A 6 kWp system in central Europe may produce around 5,400 kWh a year, which is 900 kWh per kWp, though the real figure depends on location and shading. The solar panel payback calculator takes both numbers.
Batteries: capacity and power
A home battery has two ratings. Capacity in kWh says how much energy it stores. Power in kW says how fast it can charge or discharge. A 10 kWh battery with a 5 kW rating can deliver 5 kW for two hours. A large capacity with a low power rating can keep the lights on but struggle to run an oven and a heater at the same time.
An electric car battery follows the same logic. A 64 kWh battery stores 64 kWh. A 7.4 kW charger fills it at 7.4 kW at most, and a 150 kW fast charger fills it faster, though the rate falls as the battery fills.
Common mistakes
Writing kw or KW is a small point, but the right form is kW, with a lowercase k and uppercase W. The more costly mistake is mixing up the units. A product described as a 3 kWh heater is not meaningful, because heaters are rated in kW. A battery described as 5 kW capacity does not say how much it stores. When you read a specification, check that kW is used for rates and kWh for amounts.
Another error is to compare a battery's kWh with an appliance's kW without a time. A 5 kWh battery runs a 1 kW load for five hours, and a 5 kW load for one hour.
Frequently asked questions
Is kWh per hour a valid unit?
No. A kWh is already a quantity of energy. Power is kW, and kW means kWh per hour.
How many kWh does a typical home use?
It varies widely with the country, the size of the home and the heating type. Check your own bills for the most reliable figure, using the electricity bill calculator.
Why does my provider charge for kWh and not kW?
You pay for the energy you use, not how fast you used it. Some commercial tariffs also charge for peak kW demand, which reflects the cost of providing the network capacity.