Electric car efficiency explained: miles per kWh

Think fuel economy figures will soon be a thing of the past with electric vehicles? Think again, electric cars need to be efficient, too

James Wilson Craig Hale
Aug 10, 2021

If you own an electric car, you'll be familiar with people asking you how far it will travel before the battery dies, and how long it'll take to charge back up. EV ownership doesn't mean saying goodbye to efficiency figures, though; yes, they can be significantly cheaper to run, but there is still some cost involved, so efficiency figures remain a vital part of motoring. After all, there is no point in having an electric car capable of driving 400 miles on a single charge if it uses the same amount of energy as Poland over the entirety of winter to do so.

Electric cars are supposed to be efficient, so with that in mind, it makes sense they are as efficient as possible when it comes to energy consumption. Just because there are no fumes coming out of the back of electric cars doesn't mean there is no environmental impact in driving them. For now, at least, the burning of fossil fuels generates most of the power used to charge these electric cars in the UK.

In years to come, a comparison between the efficiency of one electric car against another will have the same gravity as current comparisons between petrol-powered cars. Ultimately, they all follow a similar track. Smaller electric cars with more modest performance tend to have the best economy – think of cars such as the Nissan Leaf and Renault Zoe. Meanwhile, larger, more luxurious models which come with greater performance such as the Mercedes EQC and Jaguar I-Pace fare much worse when it comes to economy.

Be careful not to be suckered in by an impressive looking range figure, though, when considering electric car economy. It may use a large battery, while a car with a slightly shorter range could use a significantly smaller battery, making it more efficient.

Electric car efficiency

The efficiency of an electric car is commonly defined by how much electricity a car uses to travel a set distance. It will look something like this: 34 kWh per 100 miles or 340 Wh per mile. It is worth noting, that when it comes to units of measurement ‘per’ is often shown as a “/”, so the above could look like this: 34 kWh/100 miles or 340 Wh/mile. In the case of these units, the lower the figure, the more efficient the car is, as it uses less energy per mile.

To help keep things confusing, some manufacturers present electric car economy as how far a car can travel on a set amount of electricity. That means it would look something like this: 3 miles/kWh. Also, depending on the car manufacturer, economy may be quoted in terms of kilometres rather than miles. When this type of unit is used, the more efficient a car is, the higher the figure, as it travels further per unit of energy. A measurement that goes by miles/kWh is becoming the most common, just like miles per gallon, rather than litres per 100km which has been more widely adopted in Europe. 

kWh and Wh refer to kilowatt-hour and watt-hour respectively; batteries are generally measured in kWh, but some manufacturers insist on using Ah (amp-hour) like BMW with its i3. For a full breakdown of electric car jargon, take a read of our dedicated electric car jargon explainer article.

Calculating miles per kWh

In theory, electric car economy can be calculated by using a car’s battery capacity and its official range. For example, an electric car with a 40 kWh battery pack and a 100-mile range would have an economy/consumption figure of 2.5 miles/kWh. However, using this simplistic approach is more of an approximate guide, as for one reason or another, not all the battery capacity can be used to power the wheels. A mid-range Volkswagen ID.3 (pictured above), for example, has a 62kWh battery, but only 58kWh is used to drive the car.

So, in reality, the best way to calculate economy is to fully charge an electric car, drive it until it is flat then measure how much electricity it takes to fully recharge it. This is how official economy figures are calculated, although these are under lab conditions, so you can expect the figures to be a little optimistic compared with driving on real roads packed with stop-start traffic, hills and changeable weather.

Electric car economy, much like petrol and diesel cars, changes all the time depending on things such as road gradient and how fast you are driving. A battery's efficiency is also hugely dependent on its environment: outside temperature, wind speed and rainfall/humidity play a substantial role in how many miles per kWh you can get.

Up until now, these figures haven't always been readily available. However, as demand for electric cars grows and carmakers gain more understanding of what information the electric car market needs, economy figures should become easier to track down.

Good and bad electric car economy

It's still very early days for electric cars, but we are beginning to get an idea, with the current crop of cars at least, what constitutes a good level of efficiency. It is worth noting that the values below are based on official WLTP range figures, i.e. those found under test conditions.

In reality, cars will typically achieve lower economy figures than those found under WLTP testing procedures, but the only way to have economy figures that you can compare is by having a test which is the same for all - i.e. in a controlled environment, so at least they will all be slightly ambitious.

ScoreEconomy figures
Wh/mileWh/kmkWh/100 mileskWh/100 kmMiles/kWh
Excellent190-225118-14019-2312-145.0+
Good226-260141-16124-2615-164.0-4.9
So so261-295162-18327-3017-183.0-3.9
Might as well burn fuel296+184+31+19+0-2.9

So should you buy an electric car with the highest economy?

Not always, no. While the Volkswagen e-Up is clearly very efficient, its claimed range is only just north of 80 miles, which just isn’t good enough for many drivers. At the same time, the Tesla Model S Performance has a range of 367 miles but is one of the least efficient cars as per the table above.

It all boils down to that sweet spot of which is just right for you. For many, the best mix of economy, range, price and practicality will likely be cars such as the Nissan Leaf and Hyundai Ioniq Electric.

 

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