OT question for electronics wizards

"Daniel T. Burrows" <dburrows(a)netpath.net> wrote: The 7A figure (for the AC input) printed on the supply is the peak current; the power supply will not ever draw 7A continuous. Keep in mind that for AC, it is NOT necessarily the case that watts = volts * amps. That relation is only true for a perfectly resistive load. For such a load, the voltage vs. time and current vs. time waveforms and sinusoidal and exactly in phase. Most computers and other devices with switching power supplies present a highly non-linear load and draw a disproportionate fraction of their power consumption at the peaks of the cycle. If the computer consumed 300W on 115V mains and presented a perfectly resistive load, you would expect the average current to be about 2.6A, and the maximum to be about 3.7A (at the peak of the cycle when the line voltage is around 170V). But because it is not a resistive load, it actually will draw more than 3.7A peak. This is why the maximum load of a UPS is rated in VA or KVA, rather than in watts. Inductive loads are even less well-behaved than switching power supplies. Their current waveform is sinusoidal, but out of phase with the voltage. The electric utilities don't like this very much, which is why Power Factor Correction has become such a big thing recently. Eric