system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why does running the AC in your car use gas/lower your MPG?

I get that it needs energy to run the system, but can't that come from the battery or another source? Does it cause the engine to work extra hard? Is it better to keep it off most of the time to reduce wear?
gpt
There are two parts to the AC system in your car: the compressor and the fan.  The fan uses electricity from the battery which is constantly recharged (if the car is running) by the alternator.  This does not significantly affect your MPG, although it will kill your battery if you run the fan while the engine is off.
The compressor is what actually makes the air cold by driving a refrigeration cycle.  It is powered by a belt that connects it directly to the engine and it turns on and off as it is needed.  If you are ever standing near a car that has its engine running and AC on, you will can hear the compressor cycling.  When it turns on the engine will sound lower due to the added load which is fairly significant.  This load causes the engine to do extra work which uses more gasoline and reduces MPG.
Also on many cars if you pay attention, you may notice that under heavy acceleration the AC will not be as cold.  This is because many cars can shut off the compressor to free up more power for the engine when you are flooring it, sacrificing AC performance for a brief period.