Watts vs VA (volt-amps) in Power Specifications

Techs are often confused when given the VA (volt-amps) power rating for equipment instead of Watts. The difference is relatively simple, VA takes into account the power factor for reactive loads and is determined by multiplying the Volts and Amps required to power the circuit. For a purely resistive load, Watts and VA are the same. In equipment with some capacitive or inductive reactance in the load, there is a phase shift between the Voltage and Current waveforms (remember this from school?) which is dependant on the amount of reactance. Because of this phase shift, the peak voltage and peak current do not occur at the same time. If the current waveform is at 70% of peak when the voltage waveform peaks, the power consumed will be only 70% of the power supplied. A piece of equipment with a VA rating of 100 and a the power factor of 70% will actually consume only 70 watts of power. However, the power source must be capable of providing a full 100 watts. Reactance in a load can be caused by a number of things, including a lightly loaded motor or power transformer and power supply rectifier/filter circuits. Enough theory, here is what you need to know:

1. If equipment input power is rated in VA, then the power source must provide the same number of Watts as the VA specified.

2. If equipment input power is rated in Watts, then find out what the power factor is and use this formula: VA = Watts ÷ Power Factor (remember that power factor is in percent, so for 70% power factor, use .70 (or 70/100) in the formula)

3. If you cannot find out the power factor, then try using 70%, this is a guesstimate of the minimum power factor for electronic equipment that is used by most UPS manufacturers and should work in nearly all cases.

4. If the power source's output is rated in VA (as are some UPS's) the Wattage is equal to the VA.