Power supply technology is constantly evolving and I don't think it's too much to expect the current crop of high-end power supplies to maintain 80% efficiency across the board. The Mushkin Enhanced Joule 1000W produced satisfactory results until fully loaded, at which point, the efficiency dipped to 77%. It's not really a problem and in no way affects the ability to reliably power a PC, but it does mean a few extra dollars on the electricity bill at the end of the year.