Because it (the AC) was an energy guzzler. Because it (the car) was an energy guzzler.
Also "energy hog" to both, provided that your car is eletric one", other than that you could say that it´s a gas guzzler. Of course if you have problems with your car, even if it is gas-powered, then something a radio for instance, could be an energy hog in your car. It would be the cause of the quick depleting of your battery.