kW vs kWh: what is the difference?
kW and kWh are the two numbers you see most when buying or charging an electric car, and they measure different things.
kW (kilowatt) is power: how fast energy is flowing right now. kWh (kilowatt-hour) is energy: how much has flowed over time. Think of a bucket and a tap: the bucket's size is kWh, and the tap's flow rate is kW.
Power x time = energy
Energy / power = time
- Battery = kWh (size).
- Charger = kW (speed).
- Efficiency = miles per kWh.
How it works
kWh = kW x hours
1 kW for 1 hour is 1 kWh. Rearranged, hours = kWh / kW, which is how charging time is estimated. 1 kWh equals 3.6 megajoules.
Worked example
7 kW for 8 hours = 56 kWh. A 60 kWh battery on a 7 kW charger takes about 8 h 34 min from empty, before losses.
Frequently asked questions
Is kW or kWh more important for an electric car?
Both. kWh tells you how far the car can go (battery size), and kW tells you how quickly you can charge it (charger power and the car's maximum rate).
How many kWh is 1 kW?
They are different units, so you cannot convert directly. Multiply kW by hours to get kWh: 1 kW for 1 hour is 1 kWh.
How much does a 7 kW charger add per hour?
About 7 kWh drawn from the grid, or roughly 6.3 kWh into the battery after charging losses. At 3.5 to 4 miles per kWh that is roughly 22 to 25 miles of range per hour.
Why is my electricity bill in kWh?
Because it bills energy used. 1 kWh is the energy a 1 kW device uses in one hour, for example a 1 kW heater running for an hour.