What shortstuff said but only if you are comparing PSU's of the same efficiency.
EX: a 60% efficient PSU will draw 167w to make 100w whereas an 80% PSU will only draw 125w and a 90% would only draw 111w
Depends on ur Hardware, if ur hardware needs minimum 500 watt, it means it needs around 500 watt to run the system or maybe around 480... The Component that steals most of the Energy, is the GPU(Videocard). IF u have a high end GPu it will steal around 300-400 watt alone!!!! And the Cpu steals of course some too, 70-100 watt! and dont forget the other components, they dont take much but still!