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I was reading in the New Zealand Herald at the weekend about a curious problem relating to electric cars. But it’s not a technological problem – it’s one of language.

My car runs on petrol. It’s fuel economy is pretty easy to measure. Start with a full tank, take a note of the odometer, run it until the gauge gets low, fill it with fuel, noting carefully how much goes in, take another note of the odometer, and then divide your kilometres by number of litres for kilometres per litre, or divide your litres by your kilometres and multiply by 100 to get litres per hundred kilometres. (Or work out miles per gallon if that’s what you want.)

It’s easy to compare one petrol car to another like that. It works with hybrids too, because, fundamentally, a hybrid still runs on petrol. Yes – it uses a combination of an electric and petrol engine to gain its efficiency (an electric engine is far better at accelerating from rest than a petrol one) – but the electrical energy stored in the battery comes from energy that has been converted from burning the petrol. The hybrid engine just allows the car to use this energy in a more efficient way. So a litres per 100 km value is reasonable to quote here.

But what about all-electric cars, where you get energy by plugging into your domestic (or other) electricity supply – or hybrids that can be ‘topped up’ by plugging-in? Here, litres per 100 km is clearly inappropriate – in the case of the former, it is zero, in the case of the latter, it is misleading, since petrol is not the only energy source.

Maybe it is necessary to quote both values – e.g. “X litres or Y kilowatt hours per hundred kilometres” (or some combination of the two). But is that confusing to a customer? How can he compare that with a purely petrol car? Maybe simply quoting cost is better – so many dollars in fuel (be it liquid or electrical) per hundred kilometres. But the problem with that is that fuel prices change.

Measuring carbon-dioxide emissions doesn’t work either – how much your electric car emits depends on the source of your electrical energy – in some places (those drawing on non fossil fuel sources) they might be much more ‘efficient’ than others. There’s a problem, and it’s not entirely obvious where to go.

Incidently, we call this kind of thing a ‘metric’ – the ‘standard’ way by which something is measured. There is in fact a whole branch of physics devoted to it – handily called metrology.