Most of the energy you put into a gasoline car is wasted; this is not the case for electric cars

Put 100 units of energy into a petrol car and you only get 20 units of motion out.

The internal combustion engine is shockingly inefficient.

For every dollar of petrol you put, you get just 20 cents’ worth of driving motion. The other 80 cents is wasted along the way – most of it as heat from the engine.

Electric cars are much better at converting energy into motion. For every dollar of electricity you put in, you get 67 cents of driving motion plus another 22 cents of energy that’s recovered from regenerative braking. That means you get 89 cents’ worth out.

These flows of energy in the combustion engine versus the electric car are shown in the chart below. The underlying numbers come from the US Environment Protection Agency (US EPA).

Here I’ve taken the average for each. Of course, some cars are more efficient than others. The average petrol car has an efficiency of 20%, but this can range from around 16% to 25%. Diesel cars tend to have a slightly higher efficiency of around one-third.

How much energy is recovered from regenerative braking in EVs will also vary: you’ll get more if you’re driving in the city versus the highway.

But slightly different numbers won’t change the main conclusion. Most of the energy inputs in a gasoline car are wasted, and electric cars are much more efficient.

Another way to draw this is as a bar chart. I’ve done this below.

Again: in a petrol car, only 20% of the energy inputs are given to the wheels to drive.

In an electric car, this is 67%, with another 22% that is recovered from regenerative braking.

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The efficiency gains of electric cars matter a lot for the energy transition

The massive gains in efficiency from electric cars matter for the consumer: more of the fuel they pay for is actually used to take them from A to B.¹ But it also matters for our broader energy transition.

When people compare how much gasoline we burn to how much low-carbon energy we generate, decarbonising transport seems almost impossible. They assume that we would need to produce the electricity equivalent of our global gasoline consumption.

But this is not true. With a switch to electric vehicles, we get rid of most of the ‘wasted’ energy. That means we need to produce much less electricity.

Let’s take an example. Say a country burned the equivalent of 1,000 terawatt-hours (TWh) of gasoline for road transport [all of these numbers are hypothetical]. You might imagine that we’d need to produce 1,000 TWh of low-carbon electricity to replace gasoline cars with electric ones. 

But that’s not true. You might need as little as 224 TWh of low-carbon electricity.² Four to five times less.

That makes a massive difference to how achievable this transition looks.

We would also need to factor in losses within a power plant itself; if you’re producing electricity from coal or gas, you will also have large heat losses in converting the raw fuel into electric power. For renewables, these losses are much smaller.

I’ll dig into this misperception more in a future post, with real examples of how the energy demand of transport will look a lot different in an electrified world.

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1. The relevance of this to the consumer also depends on the price of petrol compared to electricity. That would let us compare the running costs of an electric car versus a conventional one.

2. If you burn 1,000 TWh of petrol in cars, only around 200 TWh is actually going into driving motion. The other 800 TWh is wasted as heat.

That means the ‘useful’ energy demand of our car fleet is 200 TWh. To get that from electric cars, you'd need around 224 TWh. That's because they have around 89% efficiency. 200 TWh would go into driving, and 24 TWh would be wasted through inefficiencies.