Tagan TG530-U15 and TG530-U22 power supplies
 

Power demands have shifted around a bit, but the ratings of the higher wattage PSUs for the last few years have been outrageously higher than practically any computer could use, then or now. Usually, people buy PSUs like that either because they think their pimped and pumped PC actually does need a 650W-plus PSU (it very probably doesn't), or because they like the idea of having a lot of headroom. That's fair enough; it's not as if higher rated PSUs consume any more power when running a given system. Or, of course, they buy the big-*** PSU because it looks cool and doesn't cost much more than a more modestly rated grey-box unit, when compared with the overall cost of a new PC.