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haxxiy said:

Does a PSU rating actually make any difference in the electricity bill? As far as I know, most countries charge residences based on real power, not apparent power. So in theory your PC can be as wildly power-draw inefficient as you like without paying more for it.

(Although it should work as a more general way of knowing the quality of the PSU's components, so not skimping on that).

I think you have a misunderstanding of what PSU power efficiency mean.  If your computer require 500 watts.  Your PSU is 80% efficient at 500 watts then that mean the PSU going to require you to draw 625 watts of power from the wall to provide your computer 500 watts of power.  If it 90% efficient it would only need to draw 555 watts from wall.  Therefore your real power draw from wall would be 70 watts less therefore reducing your power bill slightly.