How does the aircon affects the fuel consumption?

[dr. d]

you can actually feel the power drain when you turn on the aircon. you can feel the engine/car lurch just a little (suddenly slow down a little) if you aren't running fast... in some (older?) cars, you can read a drop of engine rpm..
the energy drain isn't too small.. up to 10% or so, some say... depending on the engine's size..
to compensate for this loss in engine power, there is the automatic idle-up circuit, and there's also your right foot..