kW vs kWh in EV Charging and Why the Difference Matters

You plug into a 150 kW charger, the screen says 42 kWh delivered, and the receipt charges per kWh while the sign out front advertised kW. Most new EV owners hit this moment and quietly guess. Understanding kW vs kWh takes about five minutes, and once it clicks you can predict charging times, compare chargers, and spot a bad deal before you plug in.

I spend a lot of time at charging sites talking to drivers, and confusion between these two units is behind more frustration than faulty hardware. Here is the plain version, with real numbers.

The simple difference between kW and kWh

kW (kilowatt) is a rate. It tells you how fast energy is flowing at a given moment, the way litres per minute describes a tap.

kWh (kilowatt-hour) is an amount. It tells you how much energy has been moved or stored in total, the way litres describes how much water is in the bucket.

The link between them is time. If energy flows at 10 kW for one hour, you get 10 kWh. At 50 kW for 30 minutes, you get 25 kWh. The formula is:

kWh = kW x hours

That one line explains almost everything on a charger screen, a car spec sheet, and an electricity bill.

Where you see each unit on an EV

Both units show up constantly, and knowing which is which keeps the numbers straight.

Motor output is also often listed in kW, as a measure of power. A car with a 150 kW motor has roughly 200 horsepower. That is a separate figure from charging speed, and the two are unrelated.

Working out charging time from kW vs kWh

Once you know the formula, charging time is simple division: energy needed in kWh, divided by charging rate in kW.

Home charging example

Say your car has a 64 kWh usable battery and you arrive home at 20 percent. To reach 80 percent you need 60 percent of 64 kWh, which is about 38 kWh. On a 7 kW home charger, that is 38 divided by 7, or about 5.5 hours. Add roughly 10 percent for charging losses and you are looking at around 6 hours, comfortably overnight.

Rapid charging example

The same 38 kWh on a 150 kW charger would, in theory, take about 15 minutes. In practice it usually takes 25 to 30. The car's own limit, battery temperature, and the charging curve all slow things down, which we explain in why EV fast charging slows after 80 percent.

The useful habit is to divide by the lower of two numbers: the charger's rating or the car's maximum. A car that tops out at 50 kW gains nothing from a 350 kW charger.

Why a bigger kW number is not always better

Here is where I push back on the usual advice. Plenty of forums suggest buying the most powerful home charger you can, often 22 kW. For most drivers in most homes, that is wasted money.

First, many homes simply cannot supply it. A 22 kW charger needs a three-phase supply, which most houses in the UK and much of Europe do not have. On single-phase power, the charger will run at 7 kW no matter what the box says.

Second, most cars cannot accept it. Many EVs have an onboard AC charger limited to 7 kW or 11 kW. Plug a 22 kW wallbox into an 11 kW car and you charge at 11 kW.

Third, you do not need the speed. The average UK driver covers around 20 to 25 miles a day. At 3.5 miles per kWh, that is about 7 kWh, which a 7 kW charger replaces in an hour. Even a weekly top-up of 50 kWh fits easily into one night. A 7 kW unit costs less to install, works on standard supply, and covers nearly every need. Our comparison of a home charger versus a wall plug goes further into which setup fits which driver.

Reading the price per kWh

Because you pay for energy, not speed, the per kWh price is what matters for cost. Some quick comparisons using typical UK figures:

• Home on a standard tariff at around 25p per kWh: 38 kWh costs about £9.50.
• Home on an off-peak EV tariff at around 7p to 9p per kWh: the same 38 kWh costs about £2.70 to £3.40.
• Public rapid charger at around 75p to 85p per kWh: the same 38 kWh costs about £28 to £32.

To turn that into a per mile cost, divide the price per kWh by your efficiency. At 25p per kWh and 3.5 miles per kWh, each mile costs about 7p. At 80p per kWh, the same mile costs nearly 23p.

Some networks also add session fees, idle fees, or time-based charges on top. The breakdown in how public fast chargers price a session shows how to work out the real cost per kWh when those extras apply.

Common mix-ups to avoid

• Saying "kW per hour". kW is already a rate. Energy over time is kWh, not kW per hour.
• Treating the battery size as fully usable. A battery listed at 82 kWh may have around 77 kWh usable. Use the usable figure for your calculations.
• Ignoring losses. On AC home charging, roughly 8 to 12 percent of the energy drawn from the wall does not reach the battery. Your bill counts what left the wall.
• Expecting peak kW all session. The headline rate is a peak, not an average. Average speed over a 10 to 80 percent session is often half the peak figure.

Frequently asked questions

How many kWh does it take to fully charge an electric car?

It depends on the usable battery size, which ranges from about 40 kWh in small city cars to over 100 kWh in large SUVs. Add around 10 percent for charging losses on AC charging. A 60 kWh battery from empty to full usually draws about 66 kWh from the wall.

Is a 7 kW charger fast enough for an EV?

For most drivers, yes. A 7 kW charger adds roughly 25 to 30 miles of range per hour, so it can replace a typical day's driving in about an hour and refill most batteries overnight.

Why does my car charge slower than the charger's kW rating?

The charger rating is a maximum. Your car's own charging limit, the battery's temperature and state of charge, and whether the site is sharing power between several cars all reduce the actual rate. Charging is usually fastest between 10 and 50 percent.