The electrical power in a DC circuit is defined as the product of voltage times current. Mathematically:

V = IR

However, in AC circuits the case is not that simple. Here we have three types of powers:

  • Real Power: The actual power that is supplied from load to source is known as real power. It is also sometimes referred to as active power. Its unit is watts
  • Reactive Power: The power that continuously bounces back and forth between source and load is known as reactive power. Reactive power is measured in volt-amperes reactive, 1var = 1V * 1A
  • Apparent power: A DC load having voltage V across it and current I flowing through it has power VI. However, in case of AC circuit (circuit having resistance and reactance) the product represents neither real nor reactive power. This VI is known as apparent power. Symbol S represents apparent power. Mathematically S = P + Q = VI (VA) For power apparatus VA is very small unit and kVA is used instead. S = VI/1000 (kVA)

Real, reactive, and apparent powers can also be related in terms of power triangle: