Why power factor matters

Power factor (PF) relates to energy efficiency; it is the ratio of working power to apparent power. Measured between 0 and 1 as a unitless quantity, the closer the Power Factor is to 1 the more efficient the power delivery system is at utilising the supplied source. In terms of generators, it is a nominal power factor used to calculate the kW output of an engine to supply the power for a particular alternator kVA output. Generators are typically rated at a power factor of 0.8.

To understand the calculation first we must know that:

Working power (kW)

Also known as real, active, watt-full, useful or true power, is the actual power consumed or utilised in an alternating current (AC) circuit and is measured in Watts.

Reactive power (VAR)

Also known as phantom power, it is the energy which returns to source in each complete cycle of the AC waveform. This power does not perform work but is essential for providing the voltage levels necessary for working power to be useful. Reactive power is measured in units of Volt-Amps-Reactive (VAR).

Apparent power (kVA)

Is the combination of working power and reactive power. A generator must be rated accordingly to carry the sum of both the waste energy and that which does the work. It is measured in units of Volt-Amps (VA).  

Using the classic ‘pint of beer’ analogy: The working power is the liquid beer you want to consume, and the reactive power is the froth; not useful but an essential biproduct of creating and pouring the beer. The total of both liquid and froth in the glass is the apparent power.

Why is power factor important?

Poor power factor means that more current is required to perform the same amount of work, for example with a 70% (0.7) power factor, it takes 35% more current to do the same work. Ultimately, any loss of power lowers efficiency and is a waste of money for the end user. Large currents at a low power factor can also cause voltage losses in the alternator, which in turn can lead to reduced loading handling capacity.