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Batteries & energy · Explainer

kW and kWh measure different things, and the confusion is expensive

One is a rate and one is a quantity. Mixing them up is why people expect a 250 kW charger to add 250 kWh, and why a big battery and a fast charge are unrelated claims.

By Michael Westergaard · Updated 17 Sept 2026 · 2 min

The two numbers look like variants of each other and are not. Almost every confusing claim about electric cars comes from treating them as interchangeable.

A useful way in: a tap and a bucket. The rate water leaves the tap is power, in kilowatts. The amount of water in the bucket is energy, in kilowatt-hours. A wider tap fills the bucket faster, but it does not tell you how big the bucket is, and a big bucket tells you nothing about how fast it fills.

Which number is which on a car

The battery is a bucket, so it is quoted in kilowatt-hours. A pack described as 75 kWh holds 75 kilowatt-hours of usable energy, give or take the buffer the maker reserves at each end to protect the cells.

The motor is a tap, so its output is in kilowatts. So is the charger. A 250 kW charging peak is a rate, and it says nothing at all about how much the car holds.

Consumption ties the two together. An efficient saloon uses roughly 15 to 20 kWh per 100 km in mixed driving, so range is division: a 75 kWh pack at 18 kWh per 100 km gets you a little over 400 km. That is arithmetic anyone can do, and it is a better guide than a headline range figure measured on a laboratory cycle, for the reasons set out in the piece on test cycles.

Why charging time is not capacity divided by charger power

The obvious sum, 75 kWh into a 250 kW charger, gives eighteen minutes. Nobody sees eighteen minutes, because the peak is a peak.

Charging power is high when the battery is nearly empty and falls away as it fills, following a curve the maker sets to protect the cells. The last twenty per cent is always slow. It also falls when the pack is cold, when the pack is hot, and when the charger is sharing its cabinet with another car.

This is why a peak kW figure on a spec sheet is close to meaningless on its own, and why the honest number is the 10 to 80 per cent time.

C-rate, the figure that connects them

Divide charging power by pack capacity and you get the C-rate. Two hundred kilowatts into a 100 kWh pack is 2C; the same 200 kW into a 50 kWh pack is 4C, which is a far harder ask of the same chemistry.

C-rate is why a smaller battery advertising the same peak kW as a larger one is making the more impressive claim, and why comparing peak charging power between cars with different pack sizes flatters the big one.

The same distinction bites in a smaller device, where the advertised unit is charge rather than energy: mAh, Wh and why a phone battery cannot be compared on the number on the box.

Sources

  1. US Department of Energy, Alternative Fuels Data Center: electricity basics, read 17 September 2026
  2. BIPM, The International System of Units, on the watt and the watt-hour, read 17 September 2026

Terms defined here

Kilowatt (kW)
A unit of power: the rate at which energy moves, at one instant. A motor's output and a charger's speed are both rates, so both are quoted in kW.
Kilowatt-hour (kWh)
A unit of energy: one kilowatt sustained for one hour. A battery's capacity and a car's consumption are amounts, so both are quoted in kWh.
C-rate
Charge or discharge power divided by pack capacity. A 100 kWh pack taking 200 kW is charging at 2C, which says more about the stress on the cells than the raw kW figure does.

Northline Post publishes information and opinion, not investment advice. Nothing here is a recommendation to buy or sell any security. The author may hold positions in companies covered.

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