system
Given the following post, respond as if you're commenting on Reddit:
human
Does running air conditioning in your car impact gas milage?

I've heard some people swear it's true, and others say it's a myth. Would it not just run off the battery and not affect the gas usage?
gpt
Yes, it does.  Most AC units draw mechanical power directly from the motor via a belt attached to the main pulley at the end of the crankshaft.  The unit has an electric clutch that disengages and allows the pulley to turn freely, or engages and runs the freon pump (thereby extracting mechanical energy from the engine).  It's a small power draw compared to (say) driving on the freeway, but significant - especially for city driving - because it is continuous.   

Even if the AC only pulled power from the battery, that power is not produced magically.  Energy stored in the battery comes from the car's alternator, which works by extracting mechanical energy from the motor the same way the AC unit does: it imposes a drag force on the motor via a belt attached to the pulley at the end of the crankshaft.  The more electricity you use, the more drag force is imposed.  Nothing is for free.

On modern hybrid cars the effect is even more pronounced because the AC forces the engine to run (to supply the needed mechanical energy) even if it would otherwise shut off.