What are kiloWatt-hours and what is the cost per mile of an electric car?

When you’re thinking about buying an electric car, you’ll hear lots of technical terms bandied around. You may get told, for instance, that a car will cost a certain amount to run per kiloWatt-hour. This might not mean much to you or me! What does it mean in real terms?

What is a kWh?

A kiloWatt-hour is the most common unit for measuring electricity usage. Watts have been used for a long time to measure the electricity requirement of lights and appliances, and the kWh (1000Watts) is now used to measure the efficiency of electric car models. Roughly speaking, the cost per kWh in an electric car is the equivalent of what you pay per litre for petrol or diesel at the pumps with a ‘normal’ car. In practical terms, the way to explain the cost of kiloWatt-hours used by electric cars goes something like this:

  • The first important figure is how much you pay for one hour of electricity from the National Grid. The average price per unit (kWh) is 12.5p, but it depends on your tariff and provider.
  • The second is the size of your car’s battery in kWh. The larger the capacity of your electric car’s battery (in kWh), the further it will be able to go before recharging. But it will take longer to charge in the first place!

So how much does it cost to charge an electric car?

To work out the charging costs of an electric car, simply multiply the cost of electricity per unit (kWh) by the battery capacity of the model in kW.

Let’s take the example of a standard single-rate electricity tariff and a mid-range electric car (Renault Zoe):

12p x 41kWh = 492

This means a total home charging cost (to full battery capacity) of £4.92.

How does this cost per kWh compare to cost per litre for a normal petrol car?

In this example, we take the current average price of petrol per litre and multiply it by the size of the fuel tank – in this case, we have taken the example of a 45l Renault Clio.

£1.19 x 45l = £53.55

So a full tank of petrol will cost you £50, but a full charge of an electric battery will cost a fiver.

It’s worth saying at this point that you can’t really compare like-for-like, as the range (i.e. how far a car will get before requiring re-fuelling/recharging) is different for petrol and electric cars. The majority of petrol cars have a range of around 2-3 times that of an electric car. Costs per mile are difficult to estimate because of the wide number of variables that affect them. Clearly, how long exactly this 41kWh lasts/how far it gets you depends on your driving speed and the load of the car, among other things. So the ‘real-life range’ will be different from the one specified by the manufacturer. This aside, there is still a big difference in the cost of powering electric and petrol/diesel cars. Electric normally works out far cheaper per mile.

Cost per mile of electric cars

To work out the cost per mile to run an electric car, you divide the cost of charging per mile by the range of the car. If we round up the £4.92 charging cost for the Renault Zoe above, and use the manufacturer’s specified range (250 miles), we get:

5/250 = 2p per mile.

Compare this to the cost per mile of the petrol car above (more than £1) and you can see why electric cars are known for their efficiency!

As mentioned above, real costs would probably be slightly higher, because this range assumes full efficiency, but hopefully we’ve explained why electric cars are a cheap option, at least in terms of running costs. These could be reduced even further if the battery was charged at off-peak times with an Economy 7 or time of use tariff. The cheapest way to charge an electric car is at home, and the easiest way of doing this is with your own electric car charging point.

Installing an electric car charging point

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