Miles per kWh: EV efficiency explained

We are all used to gauging the fuel efficiency of petrol and diesel cars by simply looking at the mpg – or miles per gallon – figures.

Even though we have bought fuel in metric litres for the past three decades, we still use the imperial measurements for benchmarking a car’s efficiency simply because it is what we are all used to. We expect a small hatchback or diesel to get around 50mpg and a big SUV or supercar to return 20mpg – or even less.

But now we are gradually moving to electric cars, we will have to get used to new figures and benchmarks to help us choose the right model. While electricity is usually cheaper than fuel, the costs of an inefficient electric car will soon add up, especially if you need to charge away from home – public chargers tend to cost more than your domestic electricity supply.

Using more energy will also mean you have to charge more often, which could be a serious inconvenience.

In the UK the accepted ‘norm’ for measuring the efficiency of EVs is miles per kWh, but what do the figures mean and what is considered good?

What does miles per kWh mean?

Rather than burning fuel, electric vehicles use the energy in the battery to turn the motors and propel the car. The amount of electrical power stored in the cells is measured in kilowatt hours – or kWh for short. More efficient electric cars will be able to go further on each kWh, in the same way that an economical petrol car will be able to go further on a gallon of fuel than an uneconomical one.

How can I calculate the miles per kWh?

If you look at the specifications of any electric car in a Carwow review it will tell you battery size in kWh. Something like a Vauxhall Corsa Electric will have a 51Wh pack while a large SUV such as a BMW iX can have a massive 105kWh battery. These are the ‘net’ capacity figures as car makers keep a little bit of battery power in reserve to protect the cells, as they can be damaged if they are left completely discharged. Before you do the sums it’s important to make sure this is the figure you are using rather than the bigger ‘gross’ number.

In the specifications you will also see a range figure, which usually has the letters WLTP after it – this means it is the official and independent result. If there is more than one listed, look for the ‘Combined’ figure. Some car makers try to make the result look better by quoting the ‘City’ range, which is more impressive but not realistic unless all your driving will be in urban areas.

If you divide the combined range figure by the usable (net) battery size, you’ll get the miles per kWh. In the case of the Corsa this is 248 miles divided by 51kWh, giving 4.86 miles/kWh. The BMW iX can go 382 miles divided by 105kWh, which equals 3.63 miles/kWh.

Bear in mind that these are results from laboratory tests and should only be used as an indicator of efficiency rather than relied upon if you are making a journey. You may be able to improve on them in the real world but it’s more likely that you will see worse results or wide variations depending on the weather, type of roads and your driving style.

What is considered ‘good’ miles per kWh?

Electric cars are becoming more efficient as the technology improves and this means that the figures are climbing upwards all the time. Generally the smaller, lighter cars are going to be able to squeeze more miles out of each kWh for the same reason they are more economical with petrol or diesel. With less weight to move around, the motors don’t have to work as hard. But it’s not that clear cut.

With electric cars it is often the case that larger, more expensive models can beat the small and cheaper cars as they have more sophisticated engineering and better aerodynamics.

For example, the most efficient car in Carwow’s listings is currently the Tesla Model 3 with a figure of 5.08 miles/kWh. This compares with a 4.59 mile/kWh for the much smaller and lighter Fiat 500e, for example.

Most new electric cars will have a figure of more than 4 miles per kWh, while older heavy electric SUVs such as a Mercedes EQC will only manage 2.89.

There are even significant variations within the same model ranges. Models such as the Peugeot e-208, Vauxhall Corsa Electric and Mokka Electric have the choice of an older (but cheaper) powertrain with a 50kWh battery or a newer upgrade with a 51kWh pack. The 2% upgrade in battery capacity gives you an extra 12% range due to the more efficient chemistry and newer motors.

That means it’s important to check the figures in the Carwow reviews before you choose your new electric car.

How can you improve an EV’s miles per kWh?

Car makers are constantly looking for ways to improve efficiency as it gives them a competitive edge. Besides the developments in battery chemistry, software and motor efficiency, they will also try to reduce weight and improve aerodynamics. Even the tyres fitted can make a big difference, with special rubber and tread patterns designed specifically to reduce the rolling resistance.

It’s also worth looking out for a car which has a heat pump fitted as these are a far more efficient way of warming the inside of a car than a less sophisticated resistance heater.

As a driver there will be ways you can improve the efficiency too, so you spend less time and money on topping up the battery. For example, setting a timer or using an app to precondition the interior will mean you can use power taken from your charger to warm (or cool) the car rather than using the battery’s energy. It will also mean you won’t have to wait or use de-icer on the coldest days.

The biggest difference you will be able to make is by modifying the way you drive. Constant harsh braking and acceleration will hit the range figures hard, so try to be smoother and look ahead to anticipate the road ahead. Select the regenerative braking programme so the energy you use is not wasted when you slow down.

If you are really concerned about stretching the longest range in your electric car, try choosing a more economical route for longer journeys. Navigation apps including Google will usually offer an ‘Eco’ option which may take a few minutes longer but will be shorter and involve driving at lower speeds. The extra time could well be offset by not having to stop for a top up charge.

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