Power factor correction (PFC) is one of the most cost-effective upgrades in commercial and industrial electrical systems. It does not make your equipment use less real power — but it can dramatically cut the apparent power your supply, cables and transformers have to carry.
What low power factor costs you
Inductive loads such as motors and transformers draw reactive power (kVAR) to build their magnetic fields. That reactive current flows back and forth without doing useful work, but it still:
- increases the kVA demand on the supply,
- increases current, and therefore I²R losses in cables,
- uses up capacity in transformers and generators,
- triggers utility penalties — many tariffs charge extra below 0.85 or 0.90 PF.
How correction works
Capacitors supply reactive power locally, cancelling part of the inductive kVAR drawn by the load. In the power triangle, the vertical kVAR side shrinks, the angle θ narrows, and the hypotenuse (kVA) moves closer to the kW base.
The capacitor rating needed to move from one power factor to another is:
where θ₁ = arccos(existing PF) and θ₂ = arccos(target PF).
Worked example: 400 kW from 0.75 to 0.95 PF
| Quantity | Before (0.75 PF) | After (0.95 PF) |
|---|---|---|
| Real power | 400 kW | 400 kW |
| tan θ | 0.882 | 0.329 |
| Reactive power | 352.8 kVAR | 131.5 kVAR |
| Apparent power | 533.3 kVA | 421.1 kVA |
Capacitor bank required: 400 × (0.882 − 0.329) ≈ 221 kVAR.
The result: the same 400 kW of useful work now needs 112 kVA less supply capacity — a reduction of about 21%, freeing headroom on the transformer and cutting line current by the same proportion.
Fixed vs automatic correction
- Fixed capacitors at individual motors suit loads that run steadily for long periods.
- Automatic (stepped) banks at the main switchboard switch capacitor stages in and out to follow a varying load and avoid over-correction.
- Detuned or active filters are needed where harmonics from drives and rectifiers are significant, to avoid resonance.
Before you install
Measure the actual power factor and harmonic content over a representative period, check your tariff’s penalty threshold, and have the installation designed by a qualified engineer — over-correction can create leading power factor and overvoltage problems.
Want to see how power factor affects real power? Try the power factor effect calculator on the homepage.