What is reactive power ?

The portion of the power due to stored energy ( in form of magnetic field in inductors and electric field in capacitors.) which returns to the source in each cycle is known as reactive power.
       If the load is purely reactive then the voltage and current are 90° out of phase . For half of each cycle product of voltage and current is positive but on the other half cycle it will be negative ,hence there is no net energy flow in one complete cycle . There is no net transfer of energy to load .
      The reactive power doesn't work at the load it heats the wires,wasting energy .Conductors , transformers,generators must be sized to carry the total current including that does useful work and heats up the conductor.
         Capacitors are considered to generate reactive power and inductors consume it. When capacitor is inserted in an electrical circuit it will cancel the reactive power consumed by load .