Power Factor

$PF = \frac{P}{S} = \cos\phi \quad \text{(for sinusoidal waveforms)}$

Induction motors and lightly loaded transformers make PF lagging (inductive). A low PF means more current for the same power, so more $I^2R$ losses and bigger cables. Commercial and industrial consumers are typically required to maintain a high power factor (often around 0.9 or above) and may face penalties otherwise.

For non-sinusoidal currents (e.g. rectifier loads), PF also includes distortion:

$PF = \frac{\cos\phi}{\sqrt{1 + THD^2}}$