Depends on the ambient temperature, the relative size of the car's engine to what it has to pull along, whether it is manual or automatic, and what sort of use you put it to. On a motorway at 50mph - 70mph the effect of a/c on economy is negligible for most cars. Opening the windows will affect economy far more significantly. But at low speed and especially stuck in traffic jams with the engine idling the a/c could increase fuel consumed by as much as 20%. So in that circumstance it's more efficient to open the windows, if you can stand it. The actual power consumed will vary according to load, ambient temperature, the amount of refrigeration selected by the driver and the size of cabin area to be refrigerated. So it will range from zero bhp for a small car in low ambient temperature to about 10bhp for an 8-seater in high ambient temperature.