Power factor correction calculator
with a harmonic resonance check.

How the calculator sizes the capacitor bank

Power factor is the ratio of active power (kW, the power that does work) to apparent power (kVA, what the cables and transformer must carry). A low power factor means reactive power (kVAR) is flowing back and forth, loading the network without doing work. Capacitors supply that reactive power locally, so it no longer has to come through the supply.

The reactive power the bank must supply is:

Q_C = P × (tan φ₁ − tan φ₂)

where P is the active power in kW, φ₁ = arccos(present power factor) and φ₂ = arccos(target power factor). For example, 500 kW at 0.78 corrected to 0.95 needs about 237 kVAR.

The calculator also shows apparent power and line current before and after correction (I = S ÷ (√3 × V) for three-phase), and the capacitance per phase. For a three-phase bank, each phase supplies one third of Q_C; a delta-connected capacitor sees the line voltage, a star-connected one sees the phase voltage, so star needs three times the capacitance for the same kVAR.

Why the target is usually 0.95, not 1.0

• Utility tariffs commonly penalise power factor below a threshold such as 0.9 or 0.95. Check your tariff: the target that pays back is the one the bill rewards.
• Load changes through the day. A fixed bank sized for full load can push the site into a leading power factor at night, which raises voltage. Automatic banks switch steps in and out to follow the load.
• Diminishing returns. Going from 0.95 to 1.0 needs a large extra kVAR for a small current reduction.

The resonance check: why capacitors and harmonics do not mix

A capacitor bank and the supply transformer form a parallel resonant circuit. Its approximate resonant harmonic is:

h_r ≈ √( kVA_transformer ÷ ( Z_pu × kVAR_bank ) )

If h_r lands near the 5th, 7th, 11th or 13th harmonic, which variable-speed drives and rectifiers produce in quantity, the bank can amplify those harmonics instead of absorbing them. The symptoms are blown capacitor fuses, overheated capacitors, and higher voltage distortion after the bank is installed. The usual answer is a detuned bank: capacitors with series reactors that move the resonance below the 5th harmonic (a common choice is a 7 % reactor, which tunes the bank to about 189 Hz on a 50 Hz system, the 3.8th harmonic).

Before installing, measure the harmonic spectrum on the bus. Our THD calculator and the guide to harmonics and power factor explain what to look for.

Displacement vs true power factor

The calculator works with displacement power factor, the part caused by the phase shift between voltage and current, which capacitors correct. On sites with many drives and rectifiers, the true power factor is lower, because harmonic current adds apparent power without any phase shift. Capacitors do not fix that part; harmonic filters do. A meter that shows only one power factor figure may be showing either, so check which one before sizing a bank.

Frequently asked questions

How much kVAR do I need per 100 kW?
It depends on where you start. From 0.70 to 0.95 takes about 69 kVAR per 100 kW; from 0.80 to 0.95, about 42; from 0.90 to 0.95, about 16.

Will power factor correction lower my energy bill?
It lowers the reactive power and demand charges where the tariff has them, and reduces current and losses in cables and transformers. It does not reduce the kWh your machines consume.

Fixed or automatic bank?
A fixed bank suits a single large motor that runs at steady load. A site whose load changes through the day needs an automatic bank with several steps.

Want the bank sized on a week of real data instead of one operating point? Talk to an Inspector engineer.