What is "volt-ampere"

n. a unit of electrical power in an AC circuit equal to the power dissipated when 1 volt produces a current of 1 ampere [syn: var]

A volt-ampere (VA) is the unit used for the apparent power in an electrical circuit, equal to the product of root-mean-square (RMS) voltage and RMS current. In direct current (DC) circuits, this product is equal to the real power (active power) in watts. Volt-amperes are useful only in the context of alternating current (AC) circuits ( sinusoidal voltages and currents of the same frequency).

Some devices, including uninterruptible power supplies (UPSs), have ratings both for maximum volt-amperes and maximum watts. The VA rating is limited by the maximum permissible current, and the watt rating by the power-handling capacity of the device. When a UPS powers equipment which presents a reactive load with a low power factor, neither limit may safely be exceeded. For example, a (large) UPS system rated to deliver 400,000 volt-amperes at 220 volts can deliver a current of 1818 amperes.

The volt-ampere is not an SI unit. This is because even if reactive or apparent, the quantity the unit represents is power, and SI is a coherent system that does not use different units for the same quantity in different contexts.