Most cars on the road today have air conditioning. The average car’s air conditioner uses 5-10 horsepower. On most cars, when the heater is set to the defrost setting the air conditioner turns on automatically. Even if you didn’t turn on your AC manually. They do this to dehumidify the car and defog as well as defrost. So, when you are driving your car, remember to use the floor setting on the heater when the windshield is clear. Over the course of the year you can save a couple hundred bucks on fuel consumption.