In alternating current power transmission and distribution, volt-ampere reactive (var) is a unit used to measure reactive power in an AC electric power system. Reactive power exists in an ac circuit when the current and voltage are not changing at the same time. The correct symbol is var and not VAr or VAR, but the latter two terms are widely used. The term var was proposed by the Romanian electrical engineer Constantin Budeanu and introduced in 1930 by the IEC in Stockholm, which has adopted it as the unit for reactive power.
Vars may be considered as either the imaginary part of apparent power, or the power flowing into a reactive load, where voltage and current are specified in volts and amperes. The two definitions are equivalent.
A sinusoidally alternating voltage applied to a purely resistive load results in an alternating current that is fully in phase with the voltage. In many applications it is however common for there to be a reactive component to the system, that is, the system possesses capacitance, inductance, or both. These electrical properties cause the current to change phase with respect to the voltage: capacitance tending the current to lead the voltage in phase, and inductance to lag it.
For sinusoid currents and voltages at the same frequency, reactive power in vars is the product of the RMS voltage and current, or the apparent power, multiplied by the sine of the phase angle between the voltage and the current. The reactive power Q, (measured in units of volt-amperes reactive or var), is given by:
where φ is the phase angle between the voltage and current.
Only effective power, the actual power delivered to or consumed by the load, is expressed in watts. Imaginary power is properly expressed in volt-amperes reactive.
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