A wonderful thing about electric motors is that they are extremely efficient compared to internal combustion engines. While the absolute best diesel car engines are approaching 40 per cent efficient, most are considerably less.
A typical petrol engine only uses 12-30 per cent of the energy that goes into it.
Or to flip that around, 68 -70 per cent does nothing more than generate heat and noise. The modern engine is extremely complicated, with many precision parts whirring and clunking.
It's also helped by the fact that an electric car doesn't need to run while you're waiting at the traffic lights.
An electric engine is very simple, and runs at about 77 per cent efficiency. That means they travel twice as far for the same amount of energy.
It also kills one of the myths put out by those wanting to undermine electric cars: even if the electric cars are charged from coal-fired power stations, they emit about 25 per cent less carbon than their fossil-fuelled equivalents.
While that's a big tick for electric cars, their real weakness is batteries.
Again, it comes down to energy.
A litre of petrol weighs about a one kilogram and contains roughly 10,000 watt hours of energy.
Meanwhile, an equivalent-weight commercial lithium battery only contains about 130 watt hours of energy.
In other words, the same weight of petrol contains roughly 77 times more energy.
To overcome this, electric cars pull a trick that petrol cars cannot do - they recover energy that would otherwise be lost when braking.
Most conventional brakes work by grabbing a disk to slow the wheels. That energy (kinetic) is wasted as heat and sound.
Regenerative braking mostly avoids this by using the car's engine in "reverse". Instead of using electricity to turn the wheels, they become generators.
That electricity is fed back to recharge the battery, thus improving mileage.
This story gives a clue to just how much energy a car uses.
Batteries for a Nissan Leaf or Tesla Model S might be able to store 80 to 100 kilowatt hours of electricity.
That would be enough to supply the entire energy needs of a typical home for a couple of days.
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