Power Factor Calculator
Calculate real, reactive, and apparent power — and the capacitor needed for correction.
Reviewed by the ToolNestr Editorial Team — July 2026
Enter any 2 values to solve for the rest
Power factor formulas
Power factor (PF) is the ratio of real power to apparent power in an AC electrical system. It indicates how effectively electrical power is converted into useful work. The fundamental relationships are:
Power factor correction formula
To correct the power factor from PF₁ to PF₂, the required capacitor reactive power Qc is:
where φ₁ = acos(PF₁) and φ₂ = acos(PF₂)
Worked example
A factory has a 100 kW load operating at 0.8 power factor lagging. What is the apparent power, reactive power, and what capacitor size is needed to correct the power factor to 0.95?
After correction, the apparent power reduces from 125 kVA to 100 / 0.95 ≈ 105.3 kVA, freeing 19.7 kVA of system capacity.
Industrial Electrical
Factories with large motors, pumps, compressors, and welders typically have low power factor. Correcting it reduces electricity bills by eliminating utility penalties and lowering I²R losses in distribution equipment.
Reducing Utility Bills
Many utility companies charge a penalty when the power factor drops below a threshold (typically 0.85 or 0.90). Power factor correction capacitors can pay for themselves in months through reduced demand charges.
Equipment Sizing
Transformers, generators, switchgear, and cables must be sized for apparent power (kVA), not real power (kW). A load with low PF requires larger, more expensive equipment for the same useful power output.
Generator & UPS Loading
Backup generators and UPS systems are rated in kVA. A poor power factor reduces the real power they can deliver. Correction improves the usable capacity of standby power systems.
Tips for power factor correction
Size capacitors correctly
Use the formula Qc = P × (tan φ₁ − tan φ₂) to calculate the exact capacitor kVAR needed. Undersizing won't achieve the target PF, while oversizing wastes money and can cause overcorrection (leading PF), which is also penalised by some utilities. Always verify with a power analyser before installing.
Don't overcorrect
Correcting beyond 0.98-0.99 can push the power factor into leading territory. A leading PF means the capacitive reactance exceeds the inductive reactance, causing voltage rise and potential equipment damage. Most utilities target 0.95 to 0.98 lagging as the ideal range.
Use automatic capacitor banks for variable loads
For loads that vary throughout the day (like manufacturing plants), use an automatic power factor correction (APFC) panel. These systems switch capacitor steps in and out using a controller that monitors the real-time PF, maintaining the target even as load changes.
Harmonics can damage capacitors
Non-linear loads like VFDs, rectifiers, and UPS systems generate harmonic currents. Capacitors have low impedance at harmonic frequencies, which can lead to overheating and failure. In high-harmonic environments, use detuned reactor-capacitor banks tuned to 189 Hz or 210 Hz.
Capacitor placement matters
For maximum benefit, place capacitors as close to the inductive load as possible. This reduces I²R losses in the wiring between the capacitor and the load. Centralised correction at the main switchboard is simpler but doesn't reduce distribution losses.
Single-phase vs three-phase correction
This calculator assumes single-phase. In three-phase systems, each phase requires its own capacitor, and the total kVAR is distributed across phases. For balanced three-phase loads, divide the total kVAR by three (or use star/delta capacitor bank configurations).
Understanding power factor
Power factor is a measure of how effectively electrical power is being used. In an AC circuit, voltage and current waveforms alternate sinusoidally. When the load is purely resistive (like a heater), voltage and current rise and fall together — they are in phase, and the power factor is 1.0 (unity). When the load contains inductance (motors, transformers, ballasts), the current waveform lags behind the voltage waveform. This phase shift means that for part of each cycle, energy is stored in the magnetic field and returned to the source rather than doing useful work. The cosine of the phase angle between voltage and current gives the power factor.
Why utilities penalise low power factor
Low power factor increases the current drawn from the utility grid for the same amount of real power. Higher current means larger I²R losses in transmission lines and transformers, requiring thicker cables and larger capacity equipment. Utilities must invest in infrastructure to handle this extra current, so they pass the cost on to customers with low PF through demand charge adjustments or explicit power factor penalties.
Leading vs lagging power factor
Most industrial loads are inductive (lagging PF), meaning current lags voltage. Capacitive loads cause leading PF (current leads voltage). While some leading PF can offset nearby inductive loads, excessive leading PF causes voltage rise and can damage equipment. The goal of correction is to bring the PF close to unity but still slightly lagging — typically 0.95 to 0.98 lagging.
Power triangle explained
The power triangle visualises the relationship between the three types of power. Real power P (kW) is on the horizontal axis — it does useful work. Reactive power Q (kVAR) is on the vertical axis — it represents energy stored in magnetic fields. Apparent power S (kVA) is the hypotenuse, the vector sum of P and Q. The angle φ between P and S is the phase angle, and the power factor is cos φ. As the angle increases, the PF decreases, and more apparent power is needed to deliver the same real power. Correcting the PF means reducing the angle φ by cancelling some of the reactive power with capacitors.
How to use the Power Factor Calculator
Enter any two values
Fill in any two of the six fields: voltage, current, real power, reactive power, apparent power, or power factor. Leave the rest blank.
Results update live
All missing values are computed and filled automatically as you type. The power factor correction box shows the capacitor size needed if PF is below target.
Adjust the correction target
The default correction target is 0.95. Change the "Correct to PF" value to see the capacitor size needed for any target power factor between 0 and 1.
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Frequently asked questions
What is power factor?
The ratio of real power (kW) to apparent power (kVA), ranging from 0 to 1. A low power factor means poor electrical efficiency.
Why correct power factor?
Utilities charge penalties for low power factor, and it reduces system capacity. Correction saves money and improves efficiency.
What is a good power factor?
Above 0.95 is excellent, 0.9-0.95 is good, below 0.8 may incur penalties.
How is power factor corrected?
By adding capacitors in parallel to offset reactive power from inductive loads like motors.
Sources & references
This tool uses standard formulas and reference values from:
- • NFPA 70 — National Electrical Code (NEC), conduit fill, box fill and conductor ampacity. nfpa.org
- • IPC-2221, Generic Standard on Printed Board Design (trace width / current). ipc.org
- • NIST reference constants and unit definitions; IEEE standards where applicable. nist.gov
For educational and preliminary use. Verify against full engineering calculations and the governing standard before any real design.