Air-conditioning relies on a compressor driven by the engine, and the radiator fan and heater blower will run continuously while it is turned on, so it will burn some extra fuel.
Obviously it takes more energy to run the air-con in warmer weather when use of the system can make a dent in fuel consumption if you’re doing lots of short trips. On long journeys, though, the difference will be hardly noticeable (because once the car is cool, keeping it that way burns much less fuel).
If you’re worried about fuel economy around town at low speed, driving with the windows open is an option instead of using the air con – but at higher speeds this will cause so much additional drag that closing the windows and using the air con will be the cheaper option.
According to
airconco.com, motorists should be opening their windows at speeds up to 55mph to save fuel economy.
Having said that, even a 10% penalty is hardly wrecking the car’s fuel economy — an efficient driving style and keeping the tyres properly inflated will easily save more than this.
A good tip, come the hot weather, is to drive with the windows down for a few minutes to clear the hot air from the cabin, then close them and turn on the air-con.
Here in Spain I could not live without it as today it's already 35C and will get to 40C by 14:00hrs...............
