In electric power transmission and distribution, volt-ampere reactive (var) is a unit in which reactive power is expressed in an AC electric power system. Reactive power exists in an AC circuit when the current and voltage are not in phase. The correct symbol is var and not Var, VAr, or VAR, but all three terms are widely used, and VAR is widely used throughout the power industry infrastructure. 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.
The unit "var" does not follow the recommended practice of the International System of Units, because the quantity the unit var represents is power, and SI practice is not to include information about the type of power being measured in the unit name. 
A sinusoidally alternating voltage applied to a purely resistive load results in an alternating current that is fully in phase with the voltage. However, in many applications it is 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, between the voltage and the current. The reactive power , (measured in units of volt-amperes reactive or var), is given by:
where is the phase angle between the current and voltage. Q refers to the maximum value of the instantaneous power absorbed by the reactive component of the load.
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.
Physical significance of reactive power
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Reactive power (measured in vars) is present in a system containing reactive (inductive or capacitive) components and can be either produced or consumed by different load/generation elements. Though "imaginary", the reactive power has great physical significance and is essential to the operation of the electrical system as a whole. While the real power P is used to supply the energy required to perform actual work (such as running a motor), the reactive power regulates the voltage in the system. If the reactive power is too low, inductive loads such as transformers will be unable to maintain voltages necessary for the generation of electromagnetic fields, leading to a "voltage collapse" that create blackouts . Transmission line impedances also make it necessary to provide reactive power to maintain voltage levels necessary for active power to flow through. Therefore reactive power is essential to move active power through transmission and distribution systems to the customer. However if reactive power in a system is too high, there is increased heat loss in transmission lines and loads as the current flowing through the system is much higher, creating a potentially hazardous breakdown situation. The power factor of a load tells us what fraction of the apparent power is in the form of real power and performs actual work. A high power factor is desirable since it minimizes the amount of reactive power needed by the load, reducing heat losses and maximizing efficiency.
Notes and references
- Council Directive on units of measurements 80/181/EEC: "Special names for the unit of power: the name volt–ampere (symbol ‘VA’) when it is used to express the apparent power of alternating electric current, and var (symbol ‘var’) when it is used to express reactive electric power." Chapter 1.2.3., p. 6
- SI Brochure, 8th ed.: "Just as the quantity symbol should not imply any particular choice of unit, the unit symbol should not be used to provide specific information about the quantity [...]" Section 5.3.2.