It’s not so long ago that automobile clubs and the media spread an absurd figure for increased gas consumption when driving with your lights on. Apparently the difference was half a liter per one hundred kilometers, or 470 mpg. That was grist for the mill of opponents of daytime running lights, with some being very quick in converting that figure to megatons of CO₂. But does driving without lights really save you anything?

Yes, it does. But only in theory and in purely mathematical terms. Of course lights need energy, and that doesn’t come free. The opposite assumption that the generator always runs alongside the lights anyway and that any additional power consumption makes no difference is just as wrong, by the way. The generator takes very different amounts of power from the engine.

A maximum of 150 watts

But however much it uses for the lights – it’s not very much. A few figures will help clarify this. Two halogen headlights need 110 watts. Add to that 30 to 40 watts for parking lights, but only if the car is still equipped with conventional incandescent lamps and not with LEDs. Xenon lights need less because each headlight only consumes 40 watts. LEDs would be in the same region, but only in theory. After all, cars with LEDs usually have daytime running lights, the consumption of which is below 20 watts.

But let’s use a generous 150 watts. There’s hardly an engine these days that still delivers less than 50 kilowatts, that’s 50,000 watts. At 50 kilometers an hour on a flat road at constant speed, a car in the Golf class needs approximately five kilowatts, or 5,000 watts. So 150 is basically neither here nor there. And of course the five kilowatts isn’t the end of the matter. Roads are not always even and you can basically never go at constant speed. Not to mention unnecessarily rapid acceleration and high speeds.

Other loads need a lot more

Add to that the fact that apart from the energy guzzlers that are required to simply move forward, there are a whole lot of others too. No, it’s not the frequently criticized stereo system either. Its consumption is comparable to that of the lights. Drivers who switch on the air-conditioning to bring down summer temperatures – which are just as high as gas prices at the moment – to bearable levels use between two and five kilowatts. In other words way over 10 times what you need for the lights. The engine cooling system with its electric fan accounts for between 500 and 800 watts in this summer heat. The cold also doesn’t save that much, because then the rear window heater and fan heater often need a kilowatt together.

Reliable calculations assume that even full high beam with halogen headlamps costs no more than 0.15 liters per 100 kilometers, or 1,500 mpg. Generally that’s below the noticeable limit. Something that has a far greater impact is the correct air pressure in the tires. If it’s too low, the rolling resistance increases significantly.

So the lights can stay on even during the day if the car doesn’t switch them on and off automatically.

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