Electric car efficiency explained

You’ve probably heard by now that running an electric car can save you a fortune in running costs, because electricity is generally cheaper than petrol or diesel. And, of course, it’s better for the environment when it comes to polluting exhaust emissions.

But are electric cars actually more efficient than a petrol or diesel car in the way they use their energy source? Here, we’ll explain how to calculate the efficiency of electric cars, and how they stack up against petrol and diesel cars.

What is a electric car’s efficiency?

For many, many years in the UK, we’ve measured a petrol or diesel car’s efficiency using Miles Per Gallon, or MPG. (In good old British style, we’ve held on to the now-outdated unit of gallons, despite the fact that fuel pumps have measured their output in litres for decades.)

This means that when you look at a car’s specifications, you’ll often see an official fuel economy – for example, 50mpg. This figure comes from standardised tests carried out before a new car is released, and it’s sometimes known as the combined WLTP fuel economy.

However, because electric cars don’t use gallons of electricity, we can’t attribute the same measurement to them. So how do we measure how much electricity is used by an electric car over distance?

To a certain extent, you can work out the theoretical efficiency by looking at the specification of the car. How much energy a battery can hold is measured in kilowatt hours (kWh). If you know that, and you know the car’s official range, you can calculate the efficiency by dividing the range by the kWh. This is measured in miles per kWh, while you can sometimes see this written as m/kWh.

For example, a car with an 82kWh battery and an official range of 300 miles has an efficiency of 3.66 miles per kWh. A car with a 50kWh battery and a range of 200 miles has an efficiency of 4kWh. The higher the number, the more efficient the car is.

In the same way that some countries use different measurements for fuel economy (such as litres/100km), some car manufacturers use measurements other than miles per kWh. You might see kWh/100km, which means how much energy is used over a 100km (62-mile) distance. You can convert between the two – 1 m/kWh is equal to 62.14 kWh/100km.

How accurate is an electric car’s official efficiency rating?

You’ve probably noticed that just because your petrol or diesel car promises 60mpg, that doesn’t mean you’re going to get it. The exact real-world efficiency of a car depends on a range of variables, and the same is true with electric cars.

The official figure comes from a standardised test that incorporates simulations of several different types of driving. The aim is to reflect a typical mix of driving, but in reality, everybody uses their car in different ways. However, because the test is the same for every type of car, it lets you get a good comparison between the efficiencies of different models.

What factors can affect my car’s efficiency rating?

Factors that can affect your electric car’s efficiency include how many people you’re carrying, how much luggage or other stuff is in the car and what kind of driving you do – journeys at low speeds around town can use far less electricity than long stretches at 70mph on the motorway.

If your tyres aren’t pumped up to the correct pressure your efficiency can drop, and how you drive can make a big impact too – do you flow with the traffic and avoid unnecessary braking and acceleration, or are you getting up to speed as fast as possible and braking hard at the last minute?

One factor particular to electric cars is ambient temperature. Batteries are far less efficient at low temperatures, and although modern cars are engineered to minimise the impact, cold weather can lower an electric car’s range by some margin.

How can I improve my electric car’s efficiency?

There are numerous steps you can take to eke out extra miles from your electric car’s battery charge. The simplest way is to reduce weight. The less weight your car’s motor has to pull, the less battery charge it uses. So, take out everything that you don’t need. All that clutter that’s been in the boot for months? Take it out

That roof rack that you haven’t used since your last holiday? Not only is it adding weight, it’s also affecting the car’s aerodynamics by adding drag, and the motor has to work harder to overcome that. Take it off until you need it.

Check your tyre pressures. Not only is this a safety tip – under- or over-inflated tyres won’t grip the road as well – it can also make a big difference to efficiency. Check your car’s recommended pressures and make sure the tyres match, as they’ll roll over the road surface more efficiently.

This one will depend on the time of year, but ask yourself – do you need the heater or air conditioning? These use battery energy to heat or cool your electric car, so if the ambient temperature allows, turn them off. It could get you further and save you pennies.

Think about how you drive, too. Try to accelerate only as much as you need to, as hard acceleration puts more of a drain on the battery. Watch the road further ahead so you can anticipate when you need to slow down – don’t wait until the last minute to brake before you get to traffic lights. If you can avoid stopping wherever possible, then do so – try to keep momentum up when you can, as it’s basically free speed!

What speed should I drive my electric car at for maximum efficiency?

For years, motorists have been advised to drive at around 50mph, as that’s the speed that’s most efficient for petrol and diesel cars. It’s the fine balance point between aerodynamic drag and the most efficient engine speed.

However, things are different with electric cars. Because they don’t use multi-speed gearboxes, EVs don’t need to find that specific speed that gives maximum efficiency. However, the impact of aerodynamic drag – fighting against the wind at higher speeds – still applies. That means that you’ll get more miles per kWh at lower speeds.

We’re not suggesting you crawl around at 15mph everywhere, but it’s a factor worth keeping in mind. Do you need to do 70mph on the motorway, or could you time your journey so that 60mph will be fine? Or better yet, could you pick an alternative route that doesn’t require you to go on high-speed roads?