Hi Hexit, I dissagree, if the 1000W supply @ 300W is only 70% efficient (which is an unreasonabley low estimate @300W) and the 500W supply is 85% efficient at 300W.
The 1000W would then draw 300/0.7 = 428W and the 500W supply would draw 300/0.85= 352W 428/352 is 1.21 i.e. 21% more power is used. = 73W
Given a more normal estimate of 80% and 85%
300/0.8 = 375, 375/352 = 6% more power = 23W
The size of PSU is only relevant in that the efficiency at a given wattage will be different, yes 20% of 1000 is higher than 20% of 500, but that only matters @1000W at which point the 500W has failed.
The other case is at very low, idle, power levels say 100W, here you might have 60% efficiency for the 1000W and 80% for the 500W.
100/0.6 = 166W and 100/0.8 = 125W 166/125 = 32% more power consumed, or to be useful 40W more power consumed.
40W extra for 24hrs a day is about $35/£35 of electricity.
Under load (300W) the extra electricity is about half that.