Power supply efficiency is without a doubt the most talked about quality in recent years. It's the percentage of current that actually reaches the PC components. If components use a total of 400W, a system with an 80 percent efficient PSU uses 400 / 0.80 = 500W. The same PC with a 90 percent efficient PSU only draws 400 / 0.90 = 444W. It can potentially make a difference in your energy bill, but also translates to other areas. The wasted capacity is turned into heat. The more efficient the PSU, the cooler it stays, the less cooling you need, which improves the lifespan of the components.
The graphs below first show the average effeciency of the tested power supplies, and then the efficiency from 200-500 watts.
The Be Quiet Dark Power Pro 10 550W does really well here. An average efficiency of 90 percent for a 550-watt power supply is outstanding. Only the Enermax Platimax 500W is better, but also a lot more expensive.