The fine detail on what you ask is that the % Efficiency rating for most PSU's is not the same for all power levels - it may be 80% at 75% of maximum output, but it will vary from that at other power outputs. HOWEVER, the variation is so small over any reasonable range (say, from 25% to 95% or max output) that you really do not need to worry about it. In your case, check the % Efficiency rating of the PSU you already have, and of the new larger one you are considering. If they are the same then the new one will use exactly the same power as your old one until you change your other system components. And if you can get a new larger unit with even better % Efficiency, you'll actually come out ahead of the game.