Does running the AC in your car really burn gas?

Jason Brown, B.S. Mechanical Engineering & Mathematics, Western Michigan University (2007)

Updated Apr 11 · Upvoted by Serhat Erpolat, PhD Mechanical Engineering, Loughborough University (2004)

I've spent years designing mobile AC systems and the answer to this question comes down to one simple thing.

A gas engine generates power by burning fuel and air. Part of the power that it generates is consumed by the accessories attached to the engine via the serpentine belt. An AC compressor consumes about 3–4 horsepower to run. So yes, a small part of the horsepower is consumed by the AC system. That means that some small portion of gas goes into producing that power that would otherwise not be used without an AC system.

Also on really hot summer days the AC system may cause your engine fan to turn on when idling or at low speed because of excessively high temperatures and pressures in the AC system. That will require more power from the alternator and may kick your vehicle into a high idle to handle it which would also consume more fuel.