Partially correct.
Efficiency not really have to be at peak load. If you read some PSU reviews, you will notice the different efficiency at different load.
e.g
Take this:- FSP FSP400-60PFB as example
Light (20% Load) Effiency is 82.09%
Typical (50% Load) Efficiency is 83.96%
Full (100% Load) Efficiency is 81.41%
Average Efficiency is 82.49%

So, back to the point, efficiency means, how many percent of power comsume (input) in order to produce the output power.
e.g
if your system need 400W to run, a 80% efficiency PSU will consume 500W of power. 500W x 80% = 400W
if you have a 50% effiency PSU, then you will consume 800W in order to supply 400W for your system.
the higher efficiency PSU you have, the less electricity bill you will pay.

Got it?

This post has been edited by YiQi: May 6 2007, 10:01 PM