A volt-ampere (VA) is the unit used for the apparent power in an electrical circuit. In direct current (DC) circuits, this product is equal to the real power in watts. Volt-amperes are also used in alternating current (AC) circuits, but it is less precise in this application.
In DC circuits, 1 VA is equal to one watt (1W). The power, P (in watts) in a DC circuit is equal to the product of the voltage V (in volts) and the current I (in amperes): P = VI.
In AC circuits, power and VA represent the same thing only when there is no reactance. Reactance exists when a circuit has an inductor or capacitor. Because most AC circuits contain reactance, the VA figure is greater than the actual delivered power in watts, and this causes confusion in specifications for power supplies. For example, a supply might be rated at 600 VA. This does not mean it can deliver 600 watts, unless the equipment is reactance-free. In real life, the true wattage rating of a power supply is 1/2 to 2/3 of the VA rating.