Kilovoltampere

Kilovolt-ampere (kVA) is the unit of apparent power in alternating current (AC) circuits. It equals 1000 volt-amperes and is used to rate electrical equipment such as transformers, generators, and uninterruptible power supplies. It describes the total magnitude of power in an AC system, regardless of how much of it is converted to useful work.

In AC circuits, real power P (measured in kilowatts, kW) and reactive power Q (measured in kilovolt-ampere reactive, kvar) together form apparent power S (kVA) such that S^2 = P^2 + Q^2. The power factor cos φ = P/S relates real power to apparent power, indicating how efficiently the device uses power.

For three-phase systems, apparent power is S = √3 V_L I_L, where V_L is the line-to-line voltage and I_L is the line current, or S = 3 V_Ph I_Ph in per-phase terms. The same S value in kilovolt-amperes applies whether using line or phase quantities.

Practical use: The kVA rating expresses capacity to supply both active and reactive power. The actual usable (real) power depends on the system’s power factor. Example: a 200 kVA transformer at a power factor of 0.8 can deliver about 160 kW of real power at full load.

Notes: kVA is not a unit of energy; it is a rate. It should not be confused with kW (real power) or kVAR (reactive power). 1 kVA equals 1000 VA, and 1 MVA equals 1000 kVA.