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In power transmissionand distribution, alternating current power is measured expressed as three different types. Real power (P) (kW), Apparent power (S) (kVA) and reactive power (Q) (kVAr), these form a Vector triangle such that:

${\displaystyle S^{2}=P^{2}+Q^{2}}$

The power factor is the ratio of real power (in watts) to apparent power (in volt-amperes). Giving

${\displaystyle s=p*cos(phi)}$

By definition, the power factor is a dimensionless number between 0 and 1. Power engineers are often interested in the the power factor as this determines how efficient a power system is.

The power factor is determined by the type of loads connected to the power system, these can be

a) Resistive

b) Reactive

c) Capacitive

If a purely resistive load is connected to a power supply, the power factor will be unity (1) and only real power will flow. Inductive loads such as transformers and motors (any type of wound coil) absorb reactive power. Capacitive loads such as capacitor banks or buried cable generate reactive power.

A power transmission system is working at its greatest efficiency when the power factor is at unity (i.e. when no reactive power is present, so that the real power is the same as the apparent power). When the power factor is less than unity, the transmission losses increase and the system capacity is reduced. Power companies therefore require customers, especially those with large loads, to maintain, within specified limits, the power factors of their respective loads or be subject to additional charges.

When the load is purely resistive, the power delivered to it is equal to the product of volts and amperes, so the power factor is unity. When the current lags the applied voltage (due to an inductive load) the power factor is said to be lagging. When the current leads the applied voltage (due to a capacitive load) the power factor is said to be leading.

Note that although the value of the power factor reveals the magnitude of the phase angle, it does not reveal whether it is positive or negative. Thus, the power factor is specified as leading or lagging.

For example, to get a 1kVA of apparent power if the power factor is unity, 1kW of power needs to be generated (1kVA = 1kW * 1). At low values of power factor, more power needs to be generated to get the same apparent power. To get 1kVA of apparent power at 0.2 power factor 5kW of pwoer needs to be generated (1kVA = 5kW * 0.2).

It is often possible to adjust the power factor of a system to very near unity. This practice is known as power factor correction and is achieved by switching in/out banks of inductors or capacitors.

• Electronics
• Electric power