Power Factor Correction Calculator

What Is Power Factor? Power factor (PF) is the ratio of real power (kW) to apparent power (kVA). It tells you how efficiently your electrical system is using the current being supplied. A power factor of 1.0 means all the current you draw does useful work.

At 0.75, it is tempting to say a quarter of the current is wasted. That is not how the triangle works. The reactive component is sin(θ), and at PF 0.75 that is √(1 − 0.75²) = 0.66, so two thirds of the current magnitude is reactive. The real and reactive parts combine as the legs of a right triangle, not by subtraction, which is exactly why correcting a poor power factor pays back so quickly.

Why Low Power Factor Is Costly Utilities supply apparent power (kVA) but bill most customers for real power (kW). With a low power factor you pull more current for the same useful output, which means larger cables, transformers and switchgear, all of which you buy. Many utilities add a penalty surcharge for large customers below 0.85 or 0.90. Correcting it reduces demand charges and frees up capacity you have already paid to install.

Causes of Low Power Factor Inductive loads are the primary cause: AC induction motors, transformers, welders, and fluorescent lighting. These loads draw current that lags behind voltage by a phase angle θ. The worse the lag, the lower the power factor.

The Correction Principle Capacitors produce reactive power that is 180° opposite in phase to inductive reactive power. Adding capacitors in parallel with the load cancels out a portion of the inductive reactive current. The goal is not to over-correct to a leading power factor — target just below or at unity (1.0).

The Formulas Current reactive power: Q1 = P × tan(arccos(PF1)) Target reactive power: Q2 = P × tan(arccos(PF2)) Required capacitor kVAR: Qc = Q1 − Q2

For the capacitance value: C = (Qc × 1000) / (2π × f × V²), giving Farads; multiply by 1,000,000 for microfarads.

Read that capacitance carefully. With V as the line-to-line voltage, the figure it produces is the capacitance per phase in a star (wye) connected bank. A delta-connected bank at the same voltage needs one third of it per phase, because each delta capacitor sees the full line voltage rather than V/√3 and so produces three times the reactive power for the same capacitance. Getting this backwards is a classic way to end up with a bank a factor of three off. The calculator shows both.

Current and Apparent Power Savings Before correction: S1 = P / PF1 (kVA), I1 = S1 × 1000 / (√3 × V) for 3-phase After correction: S2 = P / PF2 (kVA), I2 = S2 × 1000 / (√3 × V) for 3-phase Reduction in apparent power = S1 − S2 Reduction in current = (1 − PF1/PF2) × 100% This directly reduces losses in cables and transformers proportional to I².

Practical Considerations Capacitor banks are available in fixed steps (e.g., 5 kVAR, 10 kVAR, 25 kVAR). Always round UP to the nearest standard size — slight over-correction to unity is acceptable. Automatic power factor correction (APFC) panels switch capacitor steps in and out based on real-time PF measurement. Do not over-correct past unity — a leading power factor can also cause problems (voltage rise, resonance). Capacitors must be rated for the system voltage with a 10–15% safety margin.