Find the capacitor bank size in kVAR needed to raise your power factor from its present value to a target, with the nearest standard bank size, kVA and current before and after correction, and the reduction in cable losses.
Capacitors supply reactive power locally, so the supply no longer has to carry it. The real power (kW) stays the same; only the kVAR drawn from the supply, and therefore the kVA and current, fall.
The default values in the calculator above reproduce this example.
Multiply the load in kW by the factor below to get the capacitor kVAR. Each factor is tan φ1 − tan φ2.
| Present PF | to 0.90 | to 0.95 | to 0.98 | to 1.00 |
|---|---|---|---|---|
| 0.60 | 0.849 | 1.005 | 1.130 | 1.333 |
| 0.65 | 0.685 | 0.840 | 0.966 | 1.169 |
| 0.70 | 0.536 | 0.692 | 0.817 | 1.020 |
| 0.75 | 0.398 | 0.553 | 0.679 | 0.882 |
| 0.80 | 0.266 | 0.421 | 0.547 | 0.750 |
| 0.85 | 0.135 | 0.291 | 0.417 | 0.620 |
| 0.90 | — | 0.156 | 0.281 | 0.484 |
Example: 200 kW from 0.80 to 0.95 → 200 × 0.421 = 84.2 kVAR, so a 100 kVAR bank.
To check the present power factor from kW, voltage and current first, use the Power Factor Calculator. To size the cable feeding the capacitor bank, use the Cable Size Calculator.
Qc = kW × (tan φ1 − tan φ2), where φ1 = arccos of the present PF and φ2 = arccos of the target PF. Example: 500 kW from 0.75 to 0.95 needs 500 × (0.882 − 0.329) = 276.6 kVAR, so a 300 kVAR bank.
It lists the factor (tan φ1 − tan φ2) for each pair of present and target power factor. Multiply the load kW by the factor to get the capacitor kVAR. From 0.75 to 0.95 the factor is 0.553.
0.95 is a common target. It avoids most utility penalties without the risk of over-correction. Correcting all the way to 1.0 needs much more kVAR for little extra benefit and can push the system leading at light load.
It removes low power factor penalties and kVA-based demand charges where the tariff has them, and it reduces current and cable losses. It does not reduce the kWh consumed by the load itself.
An APFC panel splits the capacitor bank into several switched steps and uses a power factor controller to switch them in and out as the load changes, so the power factor stays near the target without over-correcting at light load.
When the installation has significant non-linear load such as VFDs, UPS systems or LED drivers. Harmonics can cause resonance with plain capacitors, so a detuned bank with series reactors is used. A harmonic survey decides this.
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