Nowadays, it’s easy to measure watts to home appliances with Kill-A-Watt meters. However, living in Mexico, ordering one across the border would be cost prohibitive as shipping charges would be higher than the cost of the meter itself, and that's without taking into account import taxes!
Knowing that inexpensive Kill-A-Watt meters measure power on 120V circuits, a way will be shown to calculate power on any single phase motor load at any voltage. This same technique can be used on three phase motors as well except that you’ll have to adapt the formulas for three phase power calculation. For sake of simplicity, we’ll measure power on a 120V, 5000 BTU/h, thru-the-wall air conditioner.
Power on Alternating Current motors
Most people versed in alternating current electricity know that in order to measure power on electric motors, Amps and Volts readings would not be enough. There is another parameter – known as power factor- which affects the calculation of power in Alternating Current loads.
Power = (Volts)(Amps)(Power Factor) [Watts] (1)
Power Factor is the ratio of real power (measured in Watts) to apparent power (measured in Volts-Amperes or VA). Power Factor is a quantity that cannot be higher than unity.
Power Factor = ----------------- (2)
In motors, real power is the one that does the actual work, the apparent power (VA) is made up of real power and reactive power. Reactive power produces the magnetic field that makes the shaft rotate. Unfortunately, an Amp meter measures total current in Amps which –in case of alternating current motors- is comprised of real current and reactive current, therefore, they are not easy to separate in order to calculate running Watts.