Car question -- does running "accessories" affect HP?

What effect, if any, does running the A/C, headlights, stereo system, etc., have on a car’s horsepower? I’m pretty sure (based solely on personal observation) that running the A/C causes my car to have less acceleration, but I don’t know about other accessories. I’ve seen some “Fast & Furious” type street racers that have the world’s loudest sound systems installed, and I’ve always wondered what effect, if any, those sound systems have on the car’s power. I’m also curious as to whether a car will run faster with the headlights turned off.

Finally, assuming that these accessories do have an effect on a car’s HP, why would this be so? Does the drain on the electrical system cause the spark plugs to fire less often, or the belts to turn more slowly, or what?



They put a load on the alternator making it harder for the engine to turn.

Right, the load on the alternator causes the belt connected to the alternator to turn more slowly. The A/C causes drag on your engine not because of the alternator (or at least not ONLY because of the alternator), but because the A/C has its own pump that must be powered by the belt(s) also. Your typical auto accessories don’t put that much of a load on your alternator to make a noticeable difference, I don’t think. A 10,000-watt stereo system in a Honda Civic may be a noticeable load, either on the power or in the extra weight all the amps and speakers and crap add.

I just noticed, I said it this way to sort of tie into the OP’s theory. . .the belt doesn’t necessarily turn more slowly, it just takes a little more mechanical energy from the engine to turn it at the same speed.