How much power does a 500w power supply actually use?

I’ve been looking at “green” drives and other equipment with low power requirements, and it makes me wonder: when you have a power supply with a certain rating, say 500 watts, does it consume 500 watts all the time, regardless of processing load or how much equipment is actually being supported? If I have only 200w of demand on a 500w unit, is it still burning all 500w?