What do kWh and kW mean for a car?
On the motorway slip road, the driver accelerates firmly. For a moment the display shows 120 kW. A minute later, at a steady speed, it settles at about 20 kW.
The battery still holds about 54 kWh.
The questionWhy does a car need two units, kW and kWh, and which one says what?
Guess firstHow long does it take to add 42 kWh at a home wallbox with 11 kW?RevealHide
About 3.8 hours: 42 kWh divided by 11 kW. Energy divided by power gives time.
the energy it stores
the most power it can deliver
the limit of its onboard charger
the peak its battery accepts
A composed example close to a typical new electric car. Its battery matches the EU average of 2025.
Amount and rate
- kWh, kilowatt-hours, is an amount of energy: what the battery holds and what a trip uses.
- kW, kilowatts, is a rate: how fast energy flows, into the battery or out of it.
The two connect through time. Energy equals power multiplied by time, so 11 kW for one hour is 11 kWh, and time equals energy divided by power.
Computed at constant power. Doubling the power halves the time, so the gain shrinks at the top. A real fast-charging session follows the battery's charging curve and takes longer than at constant peak power.
Where each limit sits
At an AC wallbox, the car converts the current itself with its , so the onboard charger sets the limit. At a DC fast charger, the charger converts the current and the battery sets the limit.
Consumption joins the two units: kWh per 100 km. At 17 kWh per 100 km, 70 kWh last a little over 400 km. Stop 06 shows why that figure moves.
Check your understanding
3 questions. Answer all of them to complete this stop. Each answer explains itself.
A car adds 22 kWh at a constant 11 kW. How long does that take?
Time equals energy divided by power: 22 kWh divided by 11 kW is 2 hours.
What limits the power at an AC wallbox?
With AC, the conversion happens in the car. The onboard charger's rating is the ceiling, often 11 kW.
Which figure describes the size of a battery?
Size is an amount of energy, so it is given in kWh. kW figures describe rates, kWh per 100 km describes consumption.
- Onboard charger The converter in the car that turns AC from a wallbox or public AC charger into DC for the battery; its rating limits AC charging power. Glossary
- IEA: Global EV Outlook 2026: Electric vehicle batteries. https://www.iea.org/reports/global-ev-outlook-2026/electric-vehicle-batteries. Checked 09 Oct 2026. Licence: CC BY 4.0.
- U.S. Department of Energy, Alternative Fuels Data Center: How do all-electric cars work?. https://afdc.energy.gov/vehicles/how-do-all-electric-cars-work. Checked 09 Oct 2026.