The law of conservation of energy comes into effect here. When there is zero load on the alternator you only have to overcome the friction in its bearings and whatever it takes to spin its little cooling fan which is very little. When you put a load on it it takes energy to overcome the magnetic fields and keep it spinning. 1 horsepower is ~750 watts of energy. The alternator is far from 100% efficient though so your actually putting about 1hp of load on the engine for every 500 watts your drawing from the alternator. At full load that means a 120 amp alternator pulls 3-5 horsepower. That's also why your engine rpm will dip a bit when you turn on a heavy load like the cooling fans or rear defrost.
