kvar

kVAR, short for kilovolt-ampere reactive, is the unit of reactive power in alternating-current electrical systems. Reactive power, denoted Q, represents the energy alternately stored and released by inductive and capacitive elements in circuits. It does not perform real work by itself but is necessary to establish magnetic and electric fields within devices such as motors, transformers, and capacitors.

Reactive power relates to voltage, current, and the phase angle between them. For a single-phase system, S = VI is the apparent power in volt-amperes, P = VI cos(phi) is the real power in watts, and Q = VI sin(phi) is the reactive power in volt-amperes reactive. In a balanced three-phase system, Q = sqrt(3) V_L I_L sin(phi) and S = sqrt(3) V_L I_L. The power factor is cos(phi) = P/S, describing how effectively the current converts to useful work.

Q has a sign: it is positive for inductive loads (current lags voltage) and negative for capacitive loads (current leads voltage). In practical terms, high reactive power increases current and losses, can cause voltage drop, and may incur charges based on kvar demand.

Applications and measurement: kvar is used to size and specify power factor correction equipment, such as capacitor banks and reactors, to improve system efficiency and voltage regulation. It is monitored alongside real power (kW) and apparent power (kVA) using power meters.

Note: kvar is a rate quantity (power). Reactive energy over time is expressed in kilovar-hours (kVArh), analogous to kilowatt-hours for real energy, but kvarh is less commonly billed.