Take a look at
The 80 PLUS Program | Home for efficiency info on different maker's power supplies. I've been looking into this a lot recently. My power supply is probably 8 years old or so, it claims 70% efficiency at full load. I look forward to finding a new 85+% for whenever I do a new build.
Its pretty horrible when you look at the numbers. 70% efficiency and my computer at idle consumes 150W. That means I'm wasting 45 watts as a best case scenario!