Rus, it will get you in the ballpark. To accurately figure the actual power, you need to know how much work the motor is really doing. The nameplate amperage is the maximum load that should be imposed on it. For example, a bench grinder could have a nameplate amperage of 6A, draw 3A just to spin the wheel, and 12A if you get too aggressive and try to burn it up by pushing the bush hog blade into it. 6A is the most you SHOULD make it draw. If the grinder is sitting there just spinning the wheel, it will be cheaper than when you are grinding away.
With inductive (motor and other coil loads) or capacitive circuits, there's another component - power factor. Not all the current is in phase with the voltage. The real formula for power in a single phase AC circuit with inductive or capacitive loads is Volts X Amps X Power Factor.
The power company bills you for KWH (kilowatt hours) not KVA (kilovolt amperes). Generator manufacturers monkey with the nameplate ratings of their units, partly to cover this difference and usually rate the gensets in KW.
Transformer manufacturers on the other hand usually rate their equipment in KVA. If you have low power factor loads, you will hit your current limit before you get to the maximum power capability of the circuit.
Awww, too much coffee today, sorry I got carried away and am only scratching the surface anyway. Short answer is it will get you in the ballpark......................chim
