Recently a post was made on a group I subscribe to touting the amazing usefulness of a webpage that calculates amps * volts = watts for you. All well and good (if a bit simple), but I commented that the basic premise, although good for dc, was flawed with ac. This caused great consternation in the group, as if I said the Pope wasn’t Catholic. I then explained that if you have a 1200w resistive load at 120vac, then yes, 1200 / 120 = 10a. But, and it’s a BIG but, if the load is a reactive load (motors, computers, fluorescent lights, etc.), a fourth entity comes into play, called Power Factor.
In this case, lets say a motor has a PF of .5
Now things look very different. We have our 1200 watt load / 120 vac = 10 amps. Now divide the 10a by the power factor .5, and we get 20 amps. That’s a much heavier load, requiring heavier wiring, and a bigger generator or inverter. This may help explain why some folks have problems with some loads and not others, that go beyond simple start up surges.
For more info, see http://arduinotronics.blogspot.com/p/tutorials.html