Petrol is better: do the maths

Whether the source be petrol, a new-fangled hydrogen fuel cell or lithium-ion batteries, it takes energy to move a car. Why is one better than the other?

It's all in the numbers. Whether the source be petrol, a new-fangled hydrogen fuel cell or lithium-ion batteries, it takes energy to move a car. The more energy you can carry with you, the further you can travel before stopping. But it's really about energy density, a measure of how much energy you can carry conveniently. More specifically, you want to package your onboard fuel as densely (energy per kilogram) and compactly (energy per litre) as possible, the first so that the car doesn't have to haul around fuel-sapping extra weight and the second so that more of the car's volume can be devoted to passengers and their cargo and less to its fuel tank.

And this, folks, is where the current state of battery technology fails. Petrol's energy density is amazingly potent; there are about 13,500 watt-hours (the ability to produce 13,500 watts for one hour or one watt for 13,500 hours) of energy per kilogram of petrol. The most optimistic numbers I've seen for lithium-ion batteries, on the other hand, are 250 watt-hours per kilogram. Yes, those numbers are right. That means good old-fashioned petrol punches 54 times as hard for the same amount of weight, the fundamental reason electric cars' range is so pitiful compared with those fossil-fuelled.

In the EVs' defence, electric motors more efficiently transmit energy to the road, some electric motors boasting 90 per cent efficiency while internal combustion engines can translate as little as 15 per cent of their energy into vehicular motivation. However, even being generous, that means EVs face a nine times deficit versus traditional cars. Easier to understand is the comparison between the V8-powered SLS and the E-Cell version (the first time we have had such a direct comparison between an EV and its petrol-fed counterpart). The basic SLS is rated at 10.3L/100km on the highway. With its onboard 85L petrol tank, it could - assuming your right foot were virtuous - travel more than 800km before emptying. Said petrol tank, by the way, weighs about 60kg when full.

The E-Cell, on the other hand, has a claimed range of 150km, though I managed only 120. With a battery weighing 450kg and rated at 48 kilowatt-hours, the EV version has to carry seven times as much fuel onboard to go a sixth the distance. These are not numbers that are going to reverse overnight no matter how much research is done. And that's not mentioning the fact that the petrol-fed SLS could be completely refuelled in less than five minutes; the E-Cell will take close to 24 hours on your home's 110-volt circuit.