Does an alternator produce more electricity than a car needs or only what is demanded?

Dear Car Talk

Dear Car Talk | May 01, 2001

Dear Tom and Ray:

Does a car's alternator always produce more electricity than the car and its accessories can ever use? Or does the alternator produce only as much electricity as the car demands? In other words, if I open and close the electric windows, turn up the radio, charge a cell phone and run the headlights, will the alternator have to work harder, and will that decrease my gas mileage? -- Andrew

RAY: Well, the answer to your key question, Andrew, is yes. The more electrical stuff you use, the more gas you use.

TOM: Electricity isn't a "free byproduct" of running the engine. If it were, there wouldn't be any rolling blackouts in California. Everyone would just have their hot-tub heaters plugged in to their cars!

RAY: Here's how the alternator works: Your car's gasoline engine turns a belt that, in turn, spins the shaft of the alternator. And that's what ultimately produces electricity (if you want to get into more detail than that, ask your seventh-grader to explain Faraday's Law to you).

TOM: Of course, there are limits to what an alternator can produce. But within its normal operating range, the more electricity you "ask for," the more it produces. And when it produces more electricity, the shaft gets harder to turn, the engine works harder to turn it, and it uses more gas.

RAY: If you want to see this in action, the next time your car is idling (unless you have a car with a huge engine), try operating several power windows at once. You should see the tachometer drop and/or hear the engine slow down. You'll actually be witnessing electrical demand robbing power from the engine.

TOM: Can I see this phenomena on my 1952 MG?

RAY: Sure. Turn on the lights, look under the hood, and you'll see the hamster breathing harder.

Get the Car Talk Newsletter