Don't forget the PSU isn't perfectly efficient; it's usually between 80 and 90 percent. So for safety, about 1100W (for the total of the DC power draws of the components fed by the PSU) is really the maximum for continuous draw, and as you pointed out, there's also monitors, sound, etc. A typical monitor now is probably 50W or so (which is actually really good compared to the CRT behemoths of yesteryear).