Same with computer PSUs. What they're rated at is their max, not what it will suck down. Their efficiency is what they'll actually send to your components compared to the amount of power they draw, and that varies by the operating temperature. My OC'ed quad will draw near 300w at full load (which it stays at a majority of the time, 2.4ghz running @ 3.6ghz), and I have another quad that's not oc'ed and probably sucks down around 150w at load. One of the best quality PSUs you can buy is Corsair brand. I forget the name of the company that actually builds the internals, and they are probably rebranded by other high-end companies as well, but for example my Corsair 750w PSU was reviewed, and retained it's 80+ percent efficiency even when they were drawing 900w from the 750w PSU...at operating temperature. Most efficiency ratings you see will be based on a temp that you won't have ambient unless you have one hell of an AC bill.