Does the AC use more petrol?

The short answer is - yes it does.
But in most cases not much – it all depends on how the car is used. For example if you get in your car and drive up the motorway for a hundred miles at seventy miles an hour the amount of extra petrol or diesel used to run the AC is negligible. Indeed if you were to turn the AC off and drive instead with a window open to keep cool, you may well find that the increased drag on the car would increase the fuel consumption more than by using the AC button. On the other hand if your car is mostly used for short journeys there may be a penalty to pay in petrol for the comfort of continuous AC. For example, you get into a stinking hot car and drive 15 minutes to the supermarket. By the time you get there the car is comfortably cool and you leave it in the full sun for an hour while you do the shop. After the hour the car has returned to its stinking hot status and you have to have the AC on full again for the short journey home. If this is the sort of travelling this car has to do all its life with only short journeys then the fuel consumption is definitely going to deteriorate but after all you get comfort in exchange. For a few minutes each day the compressor is going flat out, taking power from the engine and in addition both the internal blowers and the electric condenser fans are working hard to cool the car down, causing the alternator to work hard to power them and loading the engine further. But if this same car is then used for a long journey, after perhaps ten minutes, once the internal temperature is reduced comfortably, the AC throttles itself back – its done the hard work, now it only has to keep the car cool and so now the fuel consumption returns to a much more acceptable level. Similarly if on a hot day you drive along and perhaps note that the fuel consumption on your onboard computer shows 34mpg and you turn on the AC and it immediately drops to 28 mpg it would be easy to assume that you were loosing 6 mpg for the AC. Having read the previous few sentences you can now work out that this simplistic assumption is not in fact correct and that within a few minutes the computer will show a gradual rise to near the point at which it started.
I have found little official research on this but in June 2003 the UK Department of Transport sponsored some research into fuel economy on trucks which included the use of AC. This technical evaluation was done by BTAC/IRTE (British Transport Advisory Committee/Institute of Road Transport Engineers) at the MIRA test track at Nuneaton at the highest speed lorries are able to do with their speed limiters (56mph). At this relatively low speed the effects of an open window are nothing like so serious as they would be at 70 mph, but even so the effect on fuel consumption of the AC switched off and the window open was to increase the consumption by 7% - quite a staggering increase for such a moderate speed. With the windows closed and with the AC switched on, to quote the starchy language of the official report, “the consolidated data suggest that air conditioning has a minimal affect on fuel consumption”.

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