Most PCs have a eff rating, ie 80% is most common. If No rating then use 70%.
Power is based on Load, ie MB, GPU, RAM, GPU and Hard drives ect. The CPU and the GPU are the two single biggest power consumtion units. The PSU has to provide this power and is NOT 100% eff. So based on 80 % lets say the computer uses 200 Watts therefore at the wall outlet you would use 200 + 20% x 200 = 220 Watts this is what you pay for. PS the Wattage rating of the PSU has no bearing on this - a 500 Watt or 750 watt PSU makes no difference.
PS The PSU is a VERY important part of your Build and I recommend that the brand/model be at least a tier 3 Unit.http://www.eggxpert.com/forums/thread/323050.aspx