When you turn the AC on in your car, it uses energy supplied to it by the alternator. This energy is coming from the engine, which is using the fuel in your gas tank. The AC won’t run properly without the car started because the belt that engages the compressor (which is used to compress the coolant and make it cold) will only run with the engine started. This is the same belt that is responsible for running the alternator and charging your battery So, using the AC will consume more fuel, because more energy will be required from the engine; which is in the detriment of your Fuel