As electric cars become more common, we’re having to get familiar with a whole new set of terminology that describes them and one of the most important terms is kWh, which stands for kilowatt hours. In EVs, kWh is used in reference to its battery.
But what does kWh actually mean in practice, and why do you need to understand what it’s telling you? In this guide, we’re going to explain everything you need to know about kWh and what it means for you.
What is a kWh?
Kilowatt hour is a measure of energy, not to be confused with the kilowatt (kW) which is a measure of power. There are various complex ways of describing what a single kWh is. For instance, it’s equivalent to 3.6 megajoules, a joule being a measure of the work done when a force of one Newton moves a mass through a distance of one metre.
If that’s too much like a physics lesson, here’s another way of thinking of a kWh. Your electricity provider charges you per kWh of electricity you use. Let’s say you pay 34p per kWh and your TV uses electricity at a rate of 1kW per hour. Your electricity provider supplied 1kWh of energy for that hour of watching TV and charges you 34p for it.
How does kWh relate to electric cars?
We’ve established that kWh is a unit of energy. In EVs, it’s used to describe the amount of energy stored in the car’s battery pack – the battery’s capacity, in other words. The higher the number, the greater the capacity. So, the Kia EV6’s 77.4kWh battery stores more energy than the Nissan Leaf’s 40kWh battery.
Note that kW – kilowatts – is used to describe the power of an EV’s electric motor.
It’s not too wide of the mark to think of an EV’s battery like a petrol or diesel car’s fuel tank. The fuel tank has a fixed capacity, as the battery does. When the battery runs down, you refill it with electricity in much the same way you’d refill a tank with fuel.
There are benefits and costs to getting a car with a big fuel tank, or one with a big battery. We’ll go over what those are later.
What does mpkWh mean?
You’re probably familiar with the term mpg – miles per gallon – the unit we use to measure fuel economy in the UK. The figure attached to the term tells you how quickly you’re burning through the fuel in your car’s tank. If your car does 40mpg and has a 10 gallon (45.86 litre) fuel tank, it has a theoretical range of 400 miles.
mpkWh – miles per kilowatt hour – is the equivalent measure of efficiency for an electric car. If you achieve 3.5mpkWh and your car has a 50kWh battery, you should be able to travel 175 miles before the battery runs flat.
Does more kWh mean more range?
Up to a point, yes. For instance, the BMW i4 eDrive35 has a 70.2kWh battery and a WLTP range of 299 miles. The eDrive40 version of the i4 has an 83.9kWh battery and a range of up to 365 miles.
However, it’s not quite as cut-and-dry as that. An EV’s range is affected by the same factors that influence a petrol or diesel car’s mpg. The car’s size and weight play a role, as does the power of the electric motor. Your own driving style can be a big factor, as well.
So, if you had two EVs with exactly the same battery capacity and power, but one was a small hatchback and the other was a large SUV, the SUV would have less range. That’s because the SUV’s greater size and weight make it inherently less efficient, therefore it’s simply not capable of providing the same range as the hatchback.
Does kWh tell me how long an EV takes to charge?
You can certainly make a decent estimate. For instance, a battery with a capacity of 77kWh can take roughly 10 hours to completely recharge using a 7.7kW wallbox charger of the sort you might have installed at home. You can also extrapolate how much the recharge is likely to cost.
Inevitably, there are many more factors that affect how quickly an EV’s battery recharges, like the power rating of its on-board charger. Many electric cars are optimised to charge up to 80% of the battery’s capacity very quickly using a public rapid charger, as well.
Should I get a car with high kWh?
Cars with a high number of kWh – a big battery – usually have considerably more range than cars with low kWh. For example, the Mercedes-Benz EQS 450+ has a vast 107.8kWh battery and a massive range of 452 miles on the WLTP cycle – the longest of any EV currently available in the UK. Contrast that with the MINI Electric, which has a tiny 32.6kWh battery and a range of just 143 miles.
However, large batteries are expensive and heavy – the EQS costs £105,000 and weighs 2,500kg. they also take a long time to recharge, which can cost quite a lot of money. The MINI Electric costs £32,000, weighs 1,400kg and takes only a few hours to recharge at much lower cost.
That’s not to say you always need to fork out tons of money to get an EV with a big battery and long WLTP range, though. The Extended Range version of the MG4 EV has a 77kWh battery and 323 mile range, yet it costs less than £40,000.
Should I get a car with low kWh?
In general, batteries with lower kWh tend to be fitted to smaller cars. For instance, you’ll find a 50kWh battery in the Peugeot e-208. There are plenty of mid-size family cars available with smaller batteries, too, like the 40kWh version of the Nissan Leaf.
Smaller batteries have the advantage of being cheaper and lighter, and they recharge relatively quickly at low cost. However, range will be on the low side.
