Power factor
The ratio of active power to apparent power in an AC circuit, between 0 and 1, indicating how effectively current delivers useful work.
Power factor (PF) describes how much of the current flowing in an AC circuit contributes to real work. A PF of 1 means voltage and current are perfectly in phase; lower values mean extra current carries reactive power or harmonic distortion.
Inductive loads such as induction motors, transformers and magnetic ballasts give a lagging power factor, often 0.7 to 0.85 for lightly loaded motors. Capacitive loads give a leading power factor. Non-linear loads (rectifiers, cheap LED drivers, variable-speed drives) distort current, lowering the true power factor even when the displacement angle is small.
Low power factor means more current for the same useful power, so conductors, transformers and generators must be oversized and resistive losses rise. At PF 0.8, current is 25% higher than at unity, and I²R losses are about 56% higher.
Utilities therefore charge commercial and industrial customers for poor power factor, commonly penalising values below 0.90 to 0.95, or billing reactive energy in kvarh. Correction methods include capacitor banks, active power factor correction circuits in power supplies (driven by IEC 61000-3-2 harmonic limits), synchronous motors and inverters that inject reactive power. Solar inverters can be set to fixed PF or volt-var curves under standards such as IEEE 1547-2018 and EN 50549.
Related terms
All terms →- Reactive power
The component of AC power that oscillates between source and load without doing net work, needed to sustain magnetic and electric fields and control voltage.
- Transmission and distribution losses
Electricity lost as heat and through non-technical causes between power plants and end users, usually expressed as a share of energy supplied.
- Ancillary services
Services beyond bulk energy that system operators procure to keep the grid stable and secure, such as frequency reserves, voltage control and black start.
- Smart meter
An electronic meter that records consumption in short intervals and communicates the data remotely to suppliers and network operators.
- String inverter
An inverter that converts the DC output of one or more series strings of PV modules into grid-synchronised AC, typically with several independent MPPT inputs.
- Watt (W)
The SI unit of power, equal to one joule per second; it measures the rate at which energy is produced, transferred or used.