MPG to kWh – electric car efficiency explained

MPG to kWh – electric car efficiency explained

Posted by

Kevin Blackmore

March 2022

Electric vehicles are considerably more efficient than petrol or diesel models, but that doesn’t mean all EVs are highly efficient. Some get more out of each electron than others.

But how do you tell which model is more efficient?

Most drivers understand miles per gallon (mpg) as a measurement for fuel consumption, but do you understand miles per kWh?

Efficiency for electric cars

Since miles per gallon has been used for decades as a mark of how efficient a vehicle is, consumers and fleet drivers tend to know what a good figure looks like, or for that matter what a poor fuel figure looks like. And despite the fact that we buy fuel in litres, mpg has stuck around as the measure of fuel efficiency.

With a new ‘fuel type’ comes a new measure. However, like petrol and diesel, there are a few different ways to quote it – the market hasn’t properly settled on how to present the figure universally.

What is fixed is that ‘kWh’ is used rather than litres or gallons. Battery capacity, given in kilowatt hours, indicates how much charge is used over a set distance.

Imperial vs metric

There is already a split in how an electric vehicle’s efficiency is displayed for users – broadly divided into imperial and metric. Initially, the electric vehicle market quoted efficiency statistics in kWh per 100km (kWh/100km) as this is how the vehicles are tested to calculate their official economy scores.

In Europe, petrol and diesel models have efficiency calculated in l/100km – litres per 100km, and this was simply transferred to kWh per 100km when electric vehicles started to be tested. For those used to l/100km, it was a simple switch, and although the numbers themselves had changed, the thinking behind them hadn’t. The smaller number, the more efficient the vehicle, and fuel had been simply switched to electricity.

In the UK, few use l/100km and instead manufacturers, leasing providers, fleet management companies, dealerships, and drivers use a figure converted to mpg – here the larger the number, the more efficient the vehicle (the opposite of l/100km).

As such, there are already a large number of electric vehicles that have a converted ‘mpg’ figure. Instead of switching l/100km to l/62 miles – 100 km equates to a rounded figure of 62 miles – UK economy scores get converted one stage further to miles per kWh (m/kWh). Again, this is a simple switch of fuel, but there is no real consensus as to which figure is going to win out in the minds of drivers across the country (though things are switching towards m/kWh rather than moving away from it).

There are also Wh per mile, kWh per 100 miles, or Wh per km scores available on occasion, but these don’t fit with anything familiar to most drivers, and are unlikely to catch on in the long run.

Calculating efficiency

Most drivers will use the official WLTP derived economy score to help pick which car they drive, but few will then rely on that during usership. Instead, drivers often start getting their own feel of how far their vehicle will travel on a tank or on a battery charge.

This being the case, many EV drivers start working on a ‘miles per percent’ basis, should the EV being driven display a clear battery percentage figure in the instrument panel.

However, for a more familiar figure, calculating efficiency works in the same way as calculating how much fuel has been used over a set distance.

Manufacturers publish a battery capacity, and more importantly often give a net battery capacity since most EV batteries have a buffer held back at the top and bottom of the charge range to extend battery life.

Where it is given, drivers can simply reset the trip computer, charge their car to 100%, and see how far that charge will get them. Divide the total number of miles covered by the kWh used, and the real-world m/kWh score is calculated.

Where no net kWh battery capacity is given, by using a smart charge point, it’s possible to see how much charge has been added between certain percentage figures. For example, if a driver arrives at a charger on 50% and gets a full charge, adding 40 kW, the vehicle has a net 80 kWh battery with which to calculate the energy economy.

What’s a good figure?

Not many electric vehicles will routinely cover more than five miles per kWh, though it does depend greatly on the car itself, driving style, and where it is driven. Most EVs will cover between three and four miles per kWh, though larger, more powerful models will achieve lower levels of efficiency.

Although most electric cars have more than enough range to deal with daily driving needs for a great many drivers, the more efficient the vehicle, the less it will cost to run. So while efficiency may be a key attribute of petrol and diesel models, despite being largely disregarded in EVs at the moment, it will soon start coming to the fore as more reach UK roads.

That’s because the cheapest way to charge an EV remains at home; fast chargers will sometimes be a necessity, but they come at a cost that is far in excess of the official HMRC AER guideline of 5p per mile, with the potential to leave drivers out of pocket for the charging experience.

So understanding your kWh efficiency will not only take you further, it will save you money in the long run too.


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