I did some research and to use the AC in summer is about the same as having another average size passenger in your car.  So what does that mean ? 

Your AC cost you about 2-4 miles per gallon.  By the way keeping all the windows down changes the aerodynamics of your car and cost you more than the 2-4 miles per gallon with it on, so it about even on the AC call.  Did you also know having a luggage rack you do not use causes 5% less gas mileage.