Do headlights use a lot of petrol?

Leo de Vos has heard that driving with your lights on during the day consumes a significant amount of petrol. You do use measurably more fuel running the lights but whether that’s significant depends on how you look at it.

We could get the numbers for this by going out with a fleet of cars, carefully road testing them and collating the results in a shiny binder, but I’m already sitting down (and I don’t see you getting your keys) so I’m going with a quick calculation on the back of an envelope instead.

Lets say that the headlights have a power rating of about 200 watts. Power is energy used per second, and energy is measured in joules. If we ran them for an hour we’d use a bit over 700 thousand joules. That comes from petrol which is burnt in the engine, that drives the alternator, that produces the electricity that Jack, I mean that drives the lights. You lose a lot in all that, so lets say that around four million joules worth of petrol is burnt to drive the lights for an hour.

That sounds like quite a lot of energy but lets see how it stacks up against total fuel used. A modern, efficient car might get around five litres per hundred kilometres. A litre of petrol contains about 35 million joules of energy, so an hour of open-road driving requires about 165 million joules worth. The headlights consume less than 2.5% more petrol than that.

It’s only a rough figure, but one that errs on the high side. A typical car won’t get five litres per hundred kilometres, and the engine-alternator-headlights system is probably less wasteful than I’ve assumed.

You won’t notice the difference driving and probably not even at the pump. But if you’re the type who thinks mother earth sheds a tear for every joule that’s wasted you’ll be horrified at my cavalier attitude. In the end personal taste has a lot to say about whether you think headlights during daylight is a bright idea or just a bit dippy.

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