Reactive power

Direct-current electric power can be computed simply via the product of voltage and current.
In the case of alternating current, it is more complicated. The product of both quantities at every moment is referred to as instantaneous power. This power varies in sinusoidal fashion, and its frequency is twice that of current or voltage. However, as shown in Fig.1, this function differs according to three cases.
(a) When voltage and current are in phase, the instantaneous power is never negative; always positive or 0, and this means that the electric power is being transmitted effectively.
(b) When there is a phase difference between voltage and current, there are time zones where the power is negative.
(c) When the phase difference is 90 degrees, the mean value of the electric power is zero. The dissipation i₂R in the wires is always positive, and so the system functions pointlessly.
The product of the RMS voltage and current is superficial power and called apparent power. In general, we believe that the apparent power includes a real power and a reactive power. Figure 2 shows this.
The phase difference between voltage and current is generated by the reactance component (the addition of an inductive component due to the windings and a capacitive component due to the capacitors included in the system) in the circuit, and the term reactive power is used. In the case of polyphase voltage and current, the instantaneous power is constant or independent of time.