Volt-ampere is a measurement of power in a direct current (DC) electrical circuit.The VA specification is also used in alternating current (AC) circuits, but it is less precisein this application, because it represents apparent power, which often differs from truepower.

In a DC circuit, 1 VA is the equivalent of one watt (1 W). The power, P (in watts) in a DC circuit is equal to the product of the voltage V (in volts) and the current I (inamperes):

P = VI

In an AC circuit, power and VA mean the same thing only when there is no reactance. Reactance is introduced when a circuit contains an inductor or capacitor. Because most AC circuits contain reactance, the VA figure is greater than the actual dissipated ordelivered power in watts. This can cause confusion in specifications for power supplies.For example, a supply might be rated at 600 VA. This does not mean it can deliver 600watts, unless the equipment is reactance-free. In real life, the true wattage rating of apower supply is 1/2 to 2/3 of the VA rating.

When purchasing a power source such as an uninterruptible power supply (UPS) for usewith electronic equipment (including computers, monitors, and other peripherals), be surethe VA specifications for the equipment are used when determining the minimum ratingsfor the power supply. The VA figure is nominally 1.67 times (167 percent of) the powerconsumption in watts. Alternatively, you can multiply the VA rating of the power supplyby 0.6 (60 percent) to get a good idea of its power-delivering capability in watts.