"700-850W is towards the upper end of the performance mainstream sector, where people need the power to run the latest dual core CPUs as well as a couple of hard drives and high end graphics cards, possibly in a multi-GPU arrangement. Each of the PSU's power rails was loaded at 50, 75 and 100 percent to test if it could sustain what it said it was capable of. While the PSUs were under load, temperatures were also taken to determine how well they were keeping cool as they reacted to the sustained load."
Check it out at Bit Tech.