# Car radios and fuel expenditure

Bearing in mind that energy can be neither created or destroyed I assume that if I’m driving along with my radio on (and this goes for all the other electrical components that can be switched on and off) the energy to run it comes from the cars alternator, which in turn is powered by the fuel in my car.
Therefore am I using more fuel when I have the radio on then when it is off? The energy has to come from somewhere.

Sure you are, but the amount is very small. One full throttle start from a stop light would wipe out the fuel savings from a whole lot of not listening to the radio.

Yes. Your logic is correct. The amount of additional fuel is insignificant, of course.

One horsepower is 746 watt. If your radio uses 10 W, your engine needs to generate an extra 0.02 horsepowers when the radio is on. (Assuming roughly 70% efficiency for the alternator.) The headlights use a bit more power.

Horsepowers?

Horse’spower.

At the current exchange rate, .02 horsepowers = 0.43 ponies.

What a bunch of nags.

But isn’t the alternator going to be doing the same amount of work charging the battery whether or not the radio is on? Its not like it shuts down when the battery is full.

Nags? Nags? Where do you get off calling us a bunch of equines? You don’t know me! How impolite! Why I oughta strap your ass to a saddle and, and…what?

Never mind.

No, the alternator is running under heavier load when there is a large electical demand. This requirers more power from the engine to turn the belt.

No, a generator becomes harder to turn when it is generating more power. When you turn on all your electrical accessories the alternator outputs more current, and to do that the alternator draws more power from the engine.

p.s. Thanks QED, I do appreciate corrections of my English. It’s far from perfect as you all know by now…

This is incorrect. The output of the alternator is a directly relational to the power requirements of the engine. When the battery is full the alternator actaully turns on and off many times a second. When a load is placed on the engine the alternator on time in increased, the off time is decreased.
If the alternator charged at the same rate all the time you would boil the battery dry in just a few hours.

Piffle! It’s better than mine half the time.

er…yeah…it was a pun on your posts…and…stuff.