work it like this sparky.
if you have a 500 watt ups, that is 80% efficient, means, you in effect, have a max of 400 watts that is usable
calculate it like this, take your psu's RATED watts, ,say 500, and then multiply it by .8 (80% efficient)
you get 400,
%60 is .6, %75 is .75, you get...