Voltampere

Volt-ampere (symbol VA) is a unit of apparent power used in alternating current (AC) electrical systems. It is defined as the product of the RMS voltage and RMS current, and it represents the magnitude of the complex power S, which can be expressed as S = P + jQ, where P is real power in watts and Q is reactive power in volt-ampere reactive (VAR). For sinusoidal voltages and currents, apparent power is approximately S ≈ V_rms × I_rms.

Real power P (in watts) is the portion of apparent power that does useful work, while reactive power Q (in VAR) represents energy that alternately is stored and returned by inductive and capacitive elements. The power factor is the cosine of the phase angle between voltage and current (φ); P = S cos φ and Q = S sin φ. When φ = 0, as in purely resistive circuits, S equals P and Q is zero, so apparent power reduces to real power.

Applications and usage: The VA rating is used to specify the capacity of electrical equipment such as transformers, generators, and power supplies, indicating how much apparent power they can deliver. In many systems, devices are rated in kilovolt-amperes (kVA) or volt-ampere-hours, with the actual usable power depending on efficiency and power factor. Apparent power is typically measured using instruments that assess voltage, current, and phase relationships, such as wattmeters, voltmeters, ammeters, and power factor meters.