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kWh and kW Explained

Have you come across the confusing jargon of kWh and kW when reading about electric cars? Wondering what they mean? Read our easy to understand guide to these perplexing terms.

Table of Contents

  • kWh Meaning
  • What Has kWh Got To Do With Electric Cars?
  • kW Meaning
  • How Does kW Relate To Electric Cars?
  • kW and BHP
  • kWh and kW – Maths Test!
  • What About The kW Rating Of Rapid Chargers?
  • Which Electric Cars Have The Biggest Batteries in kWh?
  • Summary

kWh Meaning

Let’s start with the easier one: kWh.

kWh stands for ‘kilowatt-hour’. And what is a kilowatt-hour? It’s a unit of electricity. So 1 kWh = 1 unit of electricity.

Your house has an electricity meter. It records how much electricity you use. You know that number on the meter that keeps going up? That number tells you the kilowatt-hours (kWh) – or units of electricity – you have consumed.

What Has kWh Got To Do With Electric Cars?

An all-electric car has a battery which powers an electric motor (or motors) which in turn makes the wheels go round.

That car battery stores units of electricity. It stores kWh.

Let’s consider the Renault Zoe. It has a 52 kWh battery. What does that mean? When fully charged, the Zoe can store 52 units of electricity.

As you drive the car, you use up electricity from the battery. It will go down from 52 kWh to 51, 50, 49 and so on.

Renault Zoe Example

Imagine you leave home in the Zoe on a full battery and then return at the end of the day having consumed 15 kWh of electricity (that equates to roughly 60 miles of driving). The battery will now have 37 kWh remaining in it (52 minus 15).

You plug the car in to start charging its battery. Assuming you have had a dedicated EV charging point installed, it will take a couple of hours to charge the battery back up to 52 kWh. That’s 1 kWh of electricity into your Zoe’s battery every 8 minutes, approximately.

kW Meaning

OK, now for the harder definition of kW.

kW is short for kilowatt. What’s a kilowatt? It’s a measure of how much electrical power a device needs to operate.

Think of a kettle. Many kettles are rated at 2 kW. That means the kettle needs 2 kW of power to boil your water.

Power, or kW, is like a flow of electricity from the socket, down the cable, into the kettle itself.

How Does kW Relate To Electric Cars?

Remember that dedicated charging point we talked about above when we charged the Zoe at home? It has a power rating of 7.4 kW.

A lot more electricity can flow through the 7.4 kW charging point at any given moment than can flow into the 2 kW kettle. The charging point can handle 3.7 times more electricity (7.4 divided by 2).

A Water Hose – Really?!

It’s like having a larger diameter water hose – you can get a higher volume of water through it every second. In this case, we are talking about a 'higher volume' of electricity flowing when you have a higher kW rating.

How is the car's on-board limit relevant?

So the Zoe plugged into a 7.4 kW charging point has its battery charged at the same rate of 7.4 kW. The electricity flows from the charging point, down the cable, into the car, without restriction.

Note, there are some electric cars that have an on-board charging limit lower than 7.4 kW, e.g. 6.6 kW for the Nissan Leaf. Your charging speed is therefore limited to 6.6 kW for the Leaf, even if it's plugged into a 7.4 kW charger.

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Just to confuse the issue a little…

kW and BHP

The power of traditional petrol and diesel car engines is measured in ‘BHP’ or ‘Brake Horse Power’.

The metric equivalent of BHP is – you guessed it – kW. So you will see either BHP or kW, or both, when you read about how powerful a car’s international combustion engine or electric motor(s) are. 

For example, the all-wheel-drive version of the Hyundai Ioniq 5 has a combined electric motor power rating of 305 BHP or 225 kW.

kWh and kW – Maths Test!

Ready for the final reveal? Here’s your question:

“How many units of electricity, or kWh, can you get into the Zoe’s battery after 1 hour of charging on a 7.4 kW charging point?”

We know, it’s like GCSE maths all over again!

Actually it’s fairly logical. A 7.4 kW charger charging for 1 hour delivers 7.4 kWh of electricity to the car. Here’s the maths: 7.4 kW x 1 hour = 7.4 kWh. Easy! 

So 2 hours of charging the Zoe puts roughly another 15 kWh of electricity back into the battery (7.4 x 2 = 14.8).

What About The kW Rating Of Rapid Chargers?

You’re on a trip from Southampton to Newcastle which is 320 miles. Your all-electric Polestar 2 Standard Range has a maximum battery range of 273 miles, so you can’t get all the way to Newcastle on one charge of the battery.

You stop at a motorway service station after 220 miles to top up your battery on the easy-to-use InstaVolt network.

The kind of chargers you find at service stations are known as ‘rapid’ chargers. They can get electricity into your car much more quickly than a home charging point.

Rapid charger ratings typically vary from 50 kW to 350 kW. Most of the newer ones being installed at the moment are between 120 kW and 150 kW.

Let’s imagine this motorway rapid charger is rated at 120 kW. The Long Range version of the Polestar 2 has a maximum on-board rapid charging rate of 125 kW, so you can take the full 120 kW from the charging point without any power limitations.

You’re in a rush and only want to stop and charge for 20 minutes? Charging at 120 kW will add roughly 40 kWh in that time, the equivalent of about 145 miles of range (assuming 3.67 miles per kWh efficiency – read our guide on EV Efficiency if this is all gobbledegook to you). 20 minutes later, you're on your way again to Newcastle.

Please note: electric cars don't always charge at a constant speed. Especially on rapid chargers, you'll notice the charge rate decreases while it's charging. This is to protect the battery cells from over-heating. The upshot? Charging often takes longer than you think and the timings above are therefore estimates.

Which Electric Cars Have The Biggest Batteries In kWh?

Here are the top 10 cars in terms of kWh battery capacity:

  1. Mercedes EQS, 108 kWh 
  2. BMW iX, 105 kWh 
  3. Tesla Model S, 90 kWh 
  4. Tesla Model X, 90 kWh 
  5. Ford Mustang Mach-E, Extended Range, 88 kWh 
  6. Audi e-tron, 86 kWh 
  7. Jaguar I-Pace, 85 kWh 
  8. Porsche Taycan, 84 kWh 
  9. Audi e-tron GT, 84 kWh 
  10. BMW i4, 81 kWh 

All battery sizes are based on useable capacity and are approximate.

Summary

If your mind is well and truly scrambled after reading this guide, here's a convenient summary:

  • The term ‘kWh’, or ‘kilowatt-hour’, signifies a ‘unit of electricity’.
  • Electric car batteries store units of electricity, or kWh, and as you drive they get used up.
  • The term ‘kW’, or ‘kilowatt’, refers to the power rating of charging points. 7.4 kW is normal for a home charger. Public chargers vary from 50 kW to 350 kW.
  • kW is also the metric equivalent of BHP or brake horse power, basically how powerful the engine or electric motor(s) are.

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