What this calculator does
Low power factor means you pay for current you do not use. This calculator converts your active power and existing power factor into the reactive power in kVAr, then subtracts the reactive power at your target to give the capacitor bank you need.
How to use it
- Enter the active (real) power in kW.
- Enter the measured or nameplate power factor, which is normally between 0.6 and 0.85.
- Set the target power factor — 0.95 or 0.98 is typical for distribution.
- The result is the total kVAr of correction; divide by three for a balanced three-phase bank.
Governing formula
Q_c = P · (tan φ₁ − tan φ₂) Where:
-
P= Active power [kW] -
φ₁= acos of the existing power factor [rad] -
φ₂= acos of the target power factor [rad]
Worked example — 250 kW at 0.75 lifted to 0.95
The figures below were produced by running this page's own engine with the example inputs (Active power (kW), Existing power factor, Target power factor). If the calculator returns something different for the same numbers, that is a bug, not a rounding difference.
| Capacitor bank required | 138.31 kVAr |
|---|---|
| Reactive power now | 220.48 kVAr |
| Reactive power at target | 82.17 kVAr |
| What to install | Install ≈ 138 kVAr of shunt capacitance (a three-phase bank of 46 kVAr per phase) to lift power factor from 0.75 to 0.95. |
- acos(0.75) = 41.4°, giving tan φ₁ = 0.882; acos(0.95) = 18.2°, giving tan φ₂ = 0.329.
- Q_c = 250 × (0.882 − 0.329) ≈ 138 kVAr — roughly 46 kVAr per phase on a 400 V three-phase supply.
Assumptions & limitations
Engineering calculation. Computes the reactive power that must be removed with shunt capacitance to reach the target power factor, assuming a fixed active load.
- Assumes a constant active load; motors with varying load need switched or automatic correction.
- No harmonic study — capacitor banks must be detuned with reactors where significant harmonics are present.
- Results are the ideal bank size; allow for transformer and capacitor tolerance when ordering.
This tool supports professional engineering work; it does not replace a sealed design. Confirm every result against the applicable standard and a licensed engineer before it is built.
References & standards
- IEEE 141 (Green Book) — voltage control and reactive power compensation.
- IEC 60831 — shunt capacitors for power factor correction.