In Electrical engineering, Apparent power is used to describe the power load as seen by a generating source. It is the vector sum of the real power, which represents energy transferred from the source to the load, and reactive power, which represents energy that circulates between the source and the inductive and capacitive energy storage elements of the load. Apparent power is expressed in units of volt-amperes (VA), not watts. It is typically of most interest in power transmission and distribution.
The definition of the relationship between Apparent power (S), Real power (P), and Reactive power (Q) is : S = P + jQ and S2 = P2 + Q2
For a network containing only linear elements of resistance, capacitance and inductance, excited by a sine wave of a single frequency, the Apparent power is:
, where φ is the phase difference between the current and voltage.
The power factor is the ratio of real power to apparent power. For two systems transmitting the same amount of real power, the system with the lower power factor will have higher circulating currents due to energy that returns to the source from energy storage in the load. These higher currents in a practical system may produce higher losses and reduce over all transmission efficiency. A lower power factor circuit will have a higher apparent power and higher losses for the same amount of real power transfer.
The concept of the apparent power may be extended to multi-phase systems by summing up the apparent powers of the individual phases.