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Power factor correction calculator
Work out the capacitor bank size in kvar to raise the power factor of a three-phase load, and see how much the current and the line losses drop. Free, no signup.
- kvar, capacitance and current
- Results update as you type
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Capacitor bank size
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Line current: — → —
Check the inputs: power factors between 0 and 1, with the target higher than the present value.
- Reactive power before
- —
- Reactive power after
- —
- Apparent power before
- —
- Apparent power after
- —
- Current reduction
- —
- Line loss reduction
- —
- Capacitance per phase
- —
- Capacitor bank current
- —
Losses are the I²R losses in the supply cables and transformer. Capacitance is per phase for the chosen connection.
How the calculation works
Capacitors supply the reactive power the load needs locally, so less of it flows through the supply. The active power stays the same.
- Capacitor bank sizeQc = P · (tan φ1 − tan φ2), tan φ = √(1 − cos²φ) / cos φ
- Apparent power and currentS = P / cos φ, I = S / (√3 · U)
- Line lossesPloss,2 / Ploss,1 = (cos φ1 / cos φ2)²Losses scale with the square of the current.
- Capacitance per phasedelta: C = Qc / (3 · 2πf · U²), star: C = Qc / (2πf · U²)
Worked example
A 100 kW load at cos φ = 0.80, corrected to 0.95, on 400 V, 50 Hz, with a delta-connected bank:
| Capacitor bank size | 42.13 kvar |
|---|---|
| Line current | 180.4 A → 151.9 A |
| Line loss reduction | 29.1 % |
| Capacitance per phase (delta) | 279.4 µF |
Before you buy a bank
- Banks come in steps. Pick the next standard size and avoid overcompensation when the load is light.
- Capacitors and the network inductance can resonate near a harmonic. With drives or inverters, run a harmonic analysis and consider a detuned bank.
- Switching the bank on raises the voltage at the bus; see the FAQ below.
Frequently asked questions
What target power factor should I choose?
Many utilities charge for reactive energy below a power factor of about 0.9 to 0.95, so check your tariff. Correcting all the way to 1.0 is rarely worth it: the last few kvar save little and risk overcompensation at light load.
Why is the capacitance different for delta and star?
For the same kvar, star connection needs three times the capacitance of delta, because each star-connected capacitor sees only U/√3 instead of U.
Can capacitors cause harmonic problems?
Yes. A capacitor bank and the network inductance form a resonant circuit. If its resonant frequency is near a harmonic produced by drives or inverters, voltages and currents can be amplified. Detuned banks with series reactors avoid this, and a harmonic analysis shows the risk.
How much does the voltage rise when the bank is switched on?
Roughly ΔU ≈ Qc / S″k, where S″k is the short-circuit power at the bus. A 100 kvar bank on a bus with a 10 MVA fault level raises the voltage by about 1 %.
What about single-phase loads?
The kvar is the same, Qc = P (tan φ1 − tan φ2). The current is S / U and the capacitance is Qc / (2πf · U²).