Electric cars have their own units of measurement. But they are not necessarily clear to everyone. So let's try to clarify the main ones. One question we often hear in the electric sector concerns the difference between kilowatts (kW) and kilowatt-hours (kWh).

These two terms sound similar, but are in fact used to designate very different concepts. What is a kW? What is it used for? And what is a kWh?

Power (kW)...

Apart from electric cars, the kilowatt, or rather the watt, which is a sub-multiple of the kilowatt, is the unit used to measure power, i.e. the maximum 'force' that a motor or other device can exert. The kW is used to indicate the power of many objects, from charging stations to combustion-powered cars.

The kW is directly related to the horsepower, with which it has only one difference: a multiplication coefficient of around 1.36. In short: 1 kW equals 1.36 PS (metric horsepower) and 100 kW equals 136 PS. And this applies regardless of the type of engine that delivers this power. Kilowatts and horsepower indicate the same thing, but with slightly different scales and values. For example, it's a bit like temperature, which can be indicated in degrees Celsius or Fahrenheit.

Mercedes Vision One-Eleven, il motore elettrico assiale di Yasa

The Yasa axial electric motor that will power Mercedes' high-performance cars.

To return to more rigorous definitions, a kilowatt is equal to 1,000 watts and 1 watt is equal to 1 joule (unit for measuring work and heat) per second (unit for measuring time). This is why, whether a car is electric or petrol-powered (or hydrogen-powered or equipped with another type of powertrain), the kW is always correct: it indicates the power available to the car to move.

... and energy (kWh)

The kilowatt-hour is another thing altogether: it's the unit for measuring energy. In an electric car, the energy is stored in the battery, whereas in a combustion engine car, it is released by the combustion of fuel in the combustion chamber. In short, these are very different things.

Renault 5, la batteria

The battery of the future Renault 5

But in a way, we can find a link. It's not entirely wrong to compare kWh to litres of petrol: in both cases, the more there is, the more energy there is to transmit to the engine(s). In conclusion, kWh gives a clear indication of the amount of energy contained in the battery. Or, more precisely, they indicate how many kW a battery can make available to the motor in one hour of use.

It's clear from this definition that this is an average value, because the amount of energy available to the engine in a given time interval is not constant, but varies according to speed, road and countless other parameters. However, in theoretical terms, a car with a battery containing 100 kWh of energy and a motor rated at 100 kW could be driven at maximum possible speed for exactly 60 minutes. Without taking into account friction and heat losses, which are physiological under normal conditions.

Gallery: Electric car, how long does the battery last