Power Factor Calculator
Compute power factor from kW and kVA, find reactive power (kVAR), and see the power triangle visualized.
Common conversions
| Input | Result |
|---|---|
| Resistive heater or incandescent bulb | PF 1.00 |
| LED lighting with driver | PF 0.90-0.95 |
| Induction motor, full load | PF 0.85 |
| Induction motor, no load | PF 0.35 |
| Arc welder | PF 0.50-0.60 |
| Fluorescent, magnetic ballast | PF 0.55-0.90 |
| VFD-driven motor | PF 0.95-0.98 |
| Uncorrected industrial plant average | PF 0.65-0.75 |
The math behind it
- PF = 42 / 50
- PF = 0.84
Everything you need to know
Power factor is the ratio of real power (kW), the power that actually does work, to apparent power (kVA), the power a generator, transformer, or utility feeder must be sized to deliver. A PF of 1.0 means all delivered power performs useful work. Anything lower means extra current flows through the system without producing any extra output, and that extra current still costs money in wire losses, transformer capacity, and, for many commercial accounts, penalty charges.
How power factor differs by load type
Purely resistive loads, such as electric heaters and incandescent bulbs, keep voltage and current perfectly in step, giving PF = 1.0. Inductive loads, the most common cause of low PF in industry, include motors, transformers, and magnetic ballasts; their current lags behind voltage, producing a lagging power factor typically between 0.5 and 0.9 depending on load level. Capacitive loads, such as correction capacitor banks, underground cable runs, and some electronic power supplies, do the opposite: current leads voltage, giving a leading power factor. Most real facilities carry a mix of resistive and inductive equipment, so the combined PF sits somewhere below 1.0 and lagging unless capacitors are added to compensate.
Correcting power factor with capacitor banks
Capacitors supply leading reactive power that cancels the lagging reactive power drawn by motors and transformers, raising the overall PF without changing the real power delivered. The kVAR needed to correct a load from one PF to a target PF is kW × (tan φ1 - tan φ2). Correcting a 100 kW load from PF 0.75 to PF 0.95, for example, requires roughly 55 kVAR of capacitance. Facilities typically install either fixed banks sized for the average load or automatically switched banks controlled by a PF relay that adds or removes capacitor steps as load changes through the day, which avoids the over-correction that a fixed bank can cause during light-load periods.
Utility power factor penalty billing
Low PF forces a utility to generate, transform, and deliver more current than the customer's real power consumption alone would require, so many commercial and industrial tariffs bill for it directly. Common structures include a flat surcharge once monthly average PF drops below a threshold, usually 0.90 to 0.95, or a switch to billing demand in kVA instead of kW once PF falls low enough that the difference becomes significant. For a motor-heavy facility running at PF 0.75, correcting to 0.95 with capacitor banks often pays for itself within 12 to 24 months purely through avoided penalty charges, before counting the added benefit of reduced conductor and transformer loading.
Common applications
Commercial tariffs commonly add a surcharge once monthly PF falls below 0.90 to 0.95. Correction capacitors typically pay back the investment in 12 to 24 months from avoided penalties alone.
A 100 kVA generator delivers only 80 kW of usable output at PF 0.8. Sizing backup equipment in kVA rather than kW avoids a surprise capacity shortfall when the real load has a lower PF than assumed.
Conductors carry current, not real power. A low-PF load draws more amps for the same kW, which forces heavier wire gauges and larger transformers than a PF-1.0 load of the same wattage would need.
Non-linear loads such as VFDs, LED drivers, and switch-mode power supplies add displacement and distortion components. True power factor combines both; a meter reporting only cos φ can miss a significant distortion component on heavily non-linear panels.
Common mistakes
PF varies with load level. A motor running at 25% load may show PF 0.5, while the same motor at full load reaches PF 0.88. Use the operating-point value, not a single nameplate number, for any correction calculation.
Going past unity flips PF leading. A leading PF causes voltage rise on long feeders and can damage generators and sensitive electronics. Aim for 0.95 to 0.98 lagging instead of chasing exactly 1.0.
Cos φ, the displacement power factor, only accounts for the fundamental frequency. True PF also includes harmonic distortion, and the two figures can differ by 10 to 20% on heavily non-linear loads like VFDs and LED drivers.
Some tariffs apply a flat surcharge below a PF threshold, others switch to billing demand in kVA once PF drops low enough, and some use a rolling average that ratchets the penalty over several months. Check the specific rate schedule before estimating a correction project's payback.
Background and theory
The power triangle is the geometric picture of AC power: real power (kW) along the horizontal axis, reactive power (kVAR) along the vertical axis, and apparent power (kVA) along the hypotenuse. The angle φ between kW and kVA is the same angle by which current lags or leads voltage, and cos φ is the displacement power factor.
Reactive power doesn't perform work, but it cycles between source and load every half-cycle. Inductors, including motors, transformers, and ballasts, absorb reactive power and produce a lagging PF. Capacitors source reactive power and produce a leading PF. Pairing the two cancels reactive flow between them, which is the basis of power factor correction.
Modern switched-mode power supplies introduce harmonics that distort the current waveform. True PF is the product of displacement PF and a distortion factor; a device with displacement PF of 0.99 but heavy harmonics can still have a true PF of 0.65. Active power factor correction circuits, now standard above 75 W in the EU, reshape input current into something closer to a sine wave and lift true PF above 0.95.
Frequently asked questions
What is a good power factor?+
A power factor of 0.95 or higher is considered good for most industrial and commercial tariffs. Utilities in the US typically start applying penalties below 0.90, and some rate schedules set the threshold as high as 0.95.
How do I improve power factor?+
Add capacitor banks to offset inductive loads like motors and transformers, or use variable frequency drives that change how a motor draws current. Correcting a load from PF 0.75 to 0.95 typically needs roughly 0.55 kVAR of capacitance for every kW of load.
Can power factor be greater than 1?+
No. Power factor is a ratio between 0 and 1 by definition, since real power can never exceed apparent power. A PF of 1.0, unity power factor, is the theoretical maximum, reached only by purely resistive loads.
Is a leading power factor bad?+
Yes, in most industrial settings. Leading PF, usually caused by over-correcting with capacitors, raises voltage on the feeder and can stress generators and some electronic equipment. Utilities generally want a lagging PF close to but not above 0.95 to 0.98.
Why do industrial customers get charged for low power factor?+
Low PF forces the utility to deliver more current to supply the same real power, which means building and maintaining larger generators, transformers, and conductors. Utilities recover that added infrastructure cost through a PF penalty or a kVA-based demand charge.
How much does a capacitor bank cost to install?+
Typically $15 to $40 per kVAR installed for a fixed bank, with automatically switched banks costing more per kVAR. A facility paying a few hundred dollars a month in PF penalties often recovers that cost within 12 to 24 months.
Does power factor affect my home electricity bill?+
No, not directly. Most residential meters bill only kWh regardless of power factor, since household PF variation is small and utilities don't meter it at the residential level. PF penalties apply almost exclusively to commercial and industrial accounts on demand-based tariffs.
What causes a low power factor?+
Inductive loads, especially motors, transformers, and magnetic ballasts, running below their rated capacity. A motor loaded to only 25% of its rating might show a PF around 0.5, while the same motor at full load can reach 0.85 to 0.88.
How is power factor different from efficiency?+
They measure different things entirely. Efficiency compares useful output to total input for a device doing work. Power factor compares real power to apparent power in the electrical supply, independent of how efficiently that power gets used once delivered.
Do LED lights have a good power factor?+
It depends entirely on the driver. Basic LED drivers can have PF as low as 0.5, while drivers with active power factor correction reach 0.9 to 0.95 or higher, a specification worth checking before a large-scale lighting retrofit.