What is Reactive Power?


What is Reactive Power ?

Reactive power(Q) is the amount of power consumed or supplied by the reactive components, i.e. capacitors and inductors.

  • While active power is the energy supplied to run a motor, heat a home, or illuminate an electric light bulb, reactive power provides the important function of regulating voltage. If voltage on the system is not high enough, active power cannot be supplied.
  • Reactive power is used to provide the voltage levels necessary for active power to do useful work. Q is essential to move active power through the transmission and distribution system to the customer.

It can be expressed as

power triangle, reactive power

  • (Reactive power)Q = S sin ϕ

Q = VI sin ϕ also calculated as  =P tan ϕ

Where,  S = apparent power and P = active power.

  • Reactive power is temporarily stored in the form of electric or magnetic fields that flows back and forth due to capacitive and inductive components.
  • Q can be generated as well as absorbed by power transmission system elements by virtue of shunt susceptance and series reactance respectively.
  • It has its origin in phase shift, if current through a device lags the voltage, then the device consumes reactive power.
  • Depending on the phase shift between the voltage and current, the amount of Q consumption of the device is decided.

Reactive Power Limitations

  1. It does not travel very far.
  2. Usually necessary to produce it close to the location where it is needed
  3. A source close to the location of the need is in a much better position to provide reactive power versus one that is located far from the location of the need
  4. It supplies are closely tied to the ability to deliver real or active power.

Need of reactive power 

  1. Reactive power (VARs) is required to maintain the voltage to deliver active power (watts) through transmission lines.
  2. Motor loads and other loads require Q to convert the flow of electrons into useful work.
  3. When there is not enough Q power, the voltage sags down and it is not possible to push the power demanded by loads through the lines.


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