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A lagging (inductive) load draws more reactive power than necessary, forcing the utility to supply a higher apparent power (kVA) than the real power (kW) actually used. Adding a capacitor bank in parallel with the load supplies local reactive power, reducing the reactive current drawn from the source and raising the measured power factor toward 1.0 — without changing the real power consumed by the load at all.
| Quantity | Formula | Meaning |
|---|---|---|
| Existing reactive power | Q1 = P×tan(acos(PF1)) | Reactive power before correction |
| Target reactive power | Q2 = P×tan(acos(PF2)) | Reactive power allowed after correction |
| Correction capacitor size | Qc = Q1−Q2 = P(tanφ1−tanφ2) | Reactive power the capacitor must supply |
| Single-phase capacitance | C = Qc / (2πfV²) | Capacitor value across the supply |
| Three-phase capacitance (per phase) | C = Qc / (3×2πfVph²) | Vph=VLL for delta, VLL/√3 for star |
Correcting all the way to PF=1.0 is rarely done in practice — most utilities and standards target 0.9–0.98 to avoid the cost and risk (over-correction can create a leading PF and resonance/overvoltage issues) of a perfectly unity system.
Find the reactive power reduction needed, Qc=P×(tanφ1−tanφ2), where φ1 and φ2 are acos of the existing and target power factors, then convert Qc to capacitance using C=Qc/(2πfV²).
A parallel capacitor supplies reactive current locally, cancelling part of the lagging reactive current drawn from the source, while keeping the load voltage unchanged — a series capacitor would instead drop voltage and is used for line compensation, not local PF correction.
Most utilities require 0.9–0.95 minimum; many industrial standards target 0.95–0.98. Correcting to exactly 1.0 is usually unnecessary and risks over-correction (leading PF).
The load becomes net capacitive (leading PF), which can cause overvoltage, resonance with other capacitive/inductive equipment, and is often penalized by utilities just like a low lagging PF.
Yes — PF correction is calculated for a specific real power P; if the load varies significantly, automatic switched capacitor banks are used to add/remove capacitance stages as load changes.
A delta-connected bank sees the full line-to-line voltage across each capacitor, while a star-connected bank sees only the phase voltage (VLL/√3); since Q=ωCV², the lower star voltage needs a larger capacitance for the same kVAR.
It does not reduce kWh (real energy) charges, but it can eliminate low-PF demand penalties and, by reducing current draw, may allow smaller conductors/transformers and reduce I²R losses in the facility wiring.
Incorrectly sized capacitor banks can cause resonance with system inductance (harmonics amplification) or overvoltage from over-correction; always size correction capacitors conservatively and consider harmonic-filter-rated capacitors in facilities with variable-frequency drives.
C = Qc(in VAR) / (2πfV²), where V is the RMS voltage across the capacitor and f is the supply frequency; remember to convert kVAR to VAR (×1000) before using this formula.
Because reactive power is proportional to P×tanφ, not directly to PF; subtracting tanφ values (not PF values) gives the exact reactive power difference needed.
This calculator addresses displacement power factor (fundamental-frequency phase shift) only; loads with significant harmonics (VFDs, switch-mode supplies) need harmonic-rated capacitors and possibly detuning reactors — consult a power quality specialist for those cases.
Current is roughly proportional to kVA for a fixed voltage, so raising PF from 0.7 to 0.95 reduces apparent power (and thus current) by about 26% for the same real power delivered.
Yes, for a balanced three-phase load, the correction capacitance should be split equally across all three phases (each sized per the per-phase Qc/3 as computed by the three-phase tab).
Yes — capacitor banks are sold in standard kVAR steps (e.g. 5, 10, 25, 50 kVAR); always select the nearest standard size at or slightly above the calculated Qc, never below it, to reliably reach your target PF.
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