Power Factor Correction Calculator
Size the capacitor bank in kVAR needed to raise an existing power factor to a target, and see the effect on apparent power.
PF Correction Size
Power factor is the ratio of real power to apparent power. A facility drawing 500 kW at 0.75 power factor is pulling 667 kVA through its transformer, cables and switchgear — a third more current than the real work requires, and every component in the path has to be sized for it. Correction adds capacitive reactive power locally so the reactive component stops being drawn through the whole system. Where the utility bills a power-factor penalty, the correction usually pays for itself on that alone.
The formula
| kW | Real power drawn by the load |
| PF1 | Existing, uncorrected power factor |
| PF2 | Target power factor after correction |
| kVAR | Capacitor bank size required |
Worked example
500 kW at 0.75 power factor, corrected to 0.95:
tan(arccos 0.75) = 0.882, tan(arccos 0.95) = 0.329
kVAR = 500 × (0.882 − 0.329) = 277 kVAR
Apparent power falls from 667 kVA to 526 kVA — about 140 kVA of transformer and cable capacity released without adding a single kW of load.
Which standard governs this
IEEE Std 18 covers shunt power capacitors. NEC Article 460 governs capacitor installation, including the disconnect and overcurrent requirements.
What this calculation does not account for
This sizes a fixed bank for one operating point. Loads vary, and a fixed bank on a lightly loaded system can over-correct into a leading power factor, which causes its own voltage problems. It also assumes linear load: on a harmonic-rich system, capacitors can resonate with source inductance and amplify the harmonic instead of correcting anything.
Common mistakes
Correcting to unity rather than to a target just below it, which risks over-correction as load varies. Installing capacitors on a harmonic-heavy bus without a detuning reactor. Sizing from a peak-demand reading and leaving the bank connected through the night.
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