Quote:
Originally Posted by Spinynorman
The energy to drive the compressor has to come from somewhere. The only energy sources are the engine and the battery. The battery only stores energy from the engine (via the alternator). So yes, it will affect the gas mileage.
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That's not the entire picture. When you generate more electricity from the engine and the brakes than what the battery can take, either because the battery is full or you produce more than can be used for charging, you have electric surplus energy for use that will just get lost if you don't use it.
That said, that surplus is not enough to run an AC continuously, even if it might when you brake.
Yes, running an AC makes a
significant difference to gas mileage. Recirculating the air instead of cooling fresh air can make a big difference. Leave the smelly kids at home.
For the heater, it depends on how much of the car engine heat can be used for heat exchange, but in general it's far less than the AC uses.
With gas prices going up, it's just a matter of time before we get better insulated cars, with thermopane windows, well insulated doors and roofs, top- and bottom-mounted exhaust fans, and other ways to keep the cabin temperature stable.