US power is, for the most part, 120 volts. The average circuit breaker is, I think, 15 amps. 120 x 15 = 1800 watts. If you had two VERY beefy PCs (dual quad core + SLI 8800 running at full tilt) maybe it would pull 1800 watts.
Note: I am not an electrician, nor did I stay at a Howard Johnson last night, but I think this is how it works.