On a charger, in your app and on your bill, two units appear that look alike: kW and kWh. They don’t mean the same thing. Once you know the difference, you can work out how long a charge takes and roughly what it costs.
kW is power
Kilowatt (kW) tells you how fast energy flows through the cable. Think of how far a tap is open. An 11 kW charger delivers more per hour than a 3.7 kW one.
kWh is quantity
Kilowatt-hour (kWh) is the energy itself, the water in the bucket. Your car’s battery has a capacity in kWh, and at the charger you usually pay per kWh.
The two are linked through time. An 11 kW charger delivering full power for an hour gives you 11 kWh. After half an hour that is 5.5 kWh.
Working out charging time
The formula: charging time = energy ÷ power.
Say you want to add 30 kWh and your car can handle the charger’s power:
- At 3.7 kW: 30 ÷ 3.7 = just over 8 hours.
- At 11 kW: 30 ÷ 11 = 2.7 hours.
- At 100 kW: 30 ÷ 100 = 0.3 hours, so 18 minutes.
Watch the decimal. 2.7 hours is not 2 hours and 7 minutes. Multiply the part after the decimal point by 60: 0.73 × 60 = 44 minutes, so about 2 hours 45 minutes.
How much range is that?
A car’s consumption is given in kWh per 100 km. If yours uses 18 kWh per 100 km, 1 kWh takes you about 5.5 km. Your 30 kWh therefore give around 165 km. The real figure depends on speed, temperature and driving style.
That also shows how much range a charger adds per hour:
- 3.7 kW gives about 20 km per hour.
- 11 kW gives about 60 km per hour.
- 100 kW gives about 550 km per hour, on paper.
You rarely get that last one. More on that below.
What will you pay?
If you pay per kWh, it is simple: kWh × price per kWh. With an example price of 0.40 euro per kWh (purely for the sum, not a real tariff), 30 kWh costs 12 euro. The real price differs per charger, operator and charging card. Check the tariff in the app or on the charger before you start.
Some chargers bill per minute or per hour instead of per kWh. Then your charging speed does affect the price: a car that charges slowly gets more expensive at such a tariff. Also, what you pay for is measured at the charger. A small part of that energy is lost as heat along the way, so slightly less ends up in your battery than you pay for.
Why you rarely get full power
The power on the charger is a maximum. What you actually get depends on several things:
- Your car may accept less than the charger offers. On AC, the charger inside your car sets the ceiling. On DC, the battery does.
- Power drops as the battery fills up. At a fast charger, the second part of the battery charges noticeably slower than the first.
- A cold battery charges more slowly.
- If several cars share the same charging station, the power is sometimes split.
An example with an assumed 60 kWh battery. Going from 20% to 80% is 36 kWh. At a constant 100 kW that would take 36 minutes. If the power drops along the way and averages 60 kW, it becomes 36 ÷ 60 = 0.6 hours, which is also 36 minutes. If it averages 40 kW, you are there for 54 minutes. So calculate with the average, not with the number on the charger.
How to go about it
- Find your car’s maximum charging power on AC and on DC in the manual or specifications.
- Check what the charger delivers, on the charger itself or in your charging card’s app. The map helps you find chargers nearby.
- Take the lower of the two. That is your ceiling.
- Divide the energy you need by that power.
- Add a margin, especially if you want to charge above 80% or if it is cold.
Common mistakes
The first: mixing up kW and kWh. “My battery is 60 kW” is wrong, because a battery has a capacity in kWh. It is an easy way to compare specifications and tariffs incorrectly.
The second: looking for a more powerful charger when your car can’t use it. If the charger offers more than your car accepts, the car doesn’t charge faster. You might still pay a higher tariff for power you don’t use.
To see why charging slows down, read about the charging curve. The difference between alternating and direct current is in AC vs DC charging. For an estimate for your car and tariff, use the charging calculator.