There are two kinds of power which are existing in the AC power grid. One is true power (P) measured in watts (W), and the other is reactive power (Q) which is expressed in volt-amperes-reactive (VAR).
True power is used for keeping electrical equipment working normally. In other words, transforming electricity into other forms of energy. For instance, kinetic energy, luminous energy, thermal energy.
Reactive power is existing in electrical circuits, used for the exchange between an electric field and a magnetic field. Besides, it builds and maintains the magnetic field of electric equipment.
Apparent power (S), measured in voltamperes (VA), consists of true power and reactive power. When calculating the efficiency of power supply, we need to calculate by true power (W) instead of apparent power (VA).