What power factor measures
Power factor is the ratio of real power to apparent power: PF = P ÷ S. It runs from 0 to 1. A PF of 1 means all the current does useful work; lower values mean extra current circulates to supply reactive power, mostly for motor and transformer magnetic fields.
For sinusoidal supplies, PF = cos φ, where φ is the phase angle between voltage and current. A PF of 0.8 corresponds to an angle of 36.9°.
Sizing correction capacitors
Capacitors supply reactive power locally so the supply does not have to. The kvar needed to raise the power factor from PF₁ to PF₂ at a constant real power P is Qc = P × (tan φ₁ − tan φ₂).
Example: an 80 kW load at PF 0.80 has tan φ₁ = 0.75. To reach 0.95, tan φ₂ = 0.329, so Qc = 80 × (0.75 − 0.329) = 33.7 kvar.
Capacitance follows from Q = 2πf × C × V². For a single-phase capacitor, C = Qc ÷ (2πfV²). For a delta-connected three-phase bank, each of the three capacitors carries a third of the kvar at line-to-line voltage.
Limits of this estimate
The calculation assumes sinusoidal voltage and current. Variable-speed drives and other electronic loads add harmonics, which a displacement power factor does not capture and which can overload capacitors. Over-correction to a leading power factor can also raise voltage. Real installations should be designed with the supplier's data and the applicable electrical code.