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Concept
Volt-ampere
Summary
The volt-ampere (SI symbol: VA, sometimes V⋅A or V A) is the unit of measurement for apparent power in an electrical circuit. It is the product of the root mean square voltage (in volts) and the root mean square current (in amperes). Volt-amperes are usually used for analyzing alternating current (AC) circuits. In direct current (DC) circuits, this product is equal to the real power, measured in watts. The volt-ampere is dimensionally equivalent to the watt: in SI units, 1 V⋅A = 1 W. VA rating is most used for generators and transformers, and other power handling equipment, where loads may be reactive (inductive or capacitive).
For a simple electrical circuit running on direct current, the electrical current and voltage are constant. In that case, the real power (P, measured in watts) is the product of the current (I, measured in amperes) and the voltage from one side of the circuit to the other (V, measured in volts):
For alternating current, both the voltage and current are oscillating. Instantaneous power is still the product of instantaneous current and instantaneous voltage, but if both of those are ideal sine waves driving a purely resistive load (like an incandescent light bulb), average power becomes (with subscripts designating average (av), peak amplitude (pk) and root mean square (rms)):
More generally, when voltage and current are not in phase, these products no longer represent average power but a new apparent power (S), measured in volt-amperes:
The relationship between real power and apparent power is described by the power factor. With a purely resistive load, they are the same: the apparent power is equal to the real power. Where a reactive (capacitive or inductive) component is present in the load, the apparent power is greater than the real power as voltage and current are no longer in phase. In the limiting case of a purely reactive load, current is drawn but no power is dissipated in the load.
Some devices, including uninterruptible power supplies (UPSs), have ratings both for maximum volt-amperes and maximum watts.
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