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#41
The wattage rating of the PSU has nothing to do with watts drawn from the wall plug.
A PSU that is 90% efficient will deliver 90 watts to the PC when it draws 100 watts from the wall.
A PSU that is 83% efficient will deliver 83 watts to the PC when it draws 100 watts from the wall. If your components require 90 watts, that PSU will have to draw 109 watts from the wall to deliver the 90 watts.
109 x .83 = 90.47.
That's the advantage of efficiency. In the above case, the 90% efficient PSU pulls 9 fewer watts from the wall than the 83% efficient PSU, and yet both deliver 90 usable watts to the components.
The more efficient PSU saves 1 KWH per 111 hours of operation. 9 watts x 111 hours is about 1000 (1 KWH).
So--if you were paying 38 cents per KWH, you'd save about 57 cents a week if the thing were running 24/7, with the more efficient PSU.
That's for a PC requiring 90 watts. Double that for a PC using 180 watts. It would be over a dollar a week.
I wouldn't save anywhere near that much as I don't pay 38 cents and don't run 24/7.