What Is Power Factor and How Is It Calculated?
Power factor (PF) is the ratio of real power (kW), which does useful work, to apparent power (kVA), which is the total power drawn from the supply. It ranges from 0 to 1 and equals the cosine of the phase angle between voltage and current in an AC circuit.
The power factor formula comes from the power triangle: PF = kW ÷ kVA = cos φ, where kW is real power, kVA is apparent power, and φ is the phase angle. The sides are related by kVA² = kW² + kVAR², where kVAR is reactive power.
The capacitor needed to correct a load is Qc = kW × (tan φ1 − tan φ2), where φ1 is the existing angle and φ2 is the target angle. For example, raising a 100 kW load from 0.80 to 0.95 PF needs about 42 kVAR of capacitors.
According to the U.S. Department of Energy, induction motors are the largest source of lagging power factor in industry, and many utilities bill demand charges or penalties when facility power factor falls below roughly 0.90 to 0.95.
How to Use This Power Factor Calculator
Pick which second value you know alongside real power, and everything else updates instantly:
- Real Power (kW): The working power of the load, from the nameplate or a power meter.
- kVA, Power Factor, or kVAR: Whichever second quantity you know. Apparent power must be at least as large as real power, and PF must be between 0 and 1.
- Target Power Factor: The PF you want after correction. 0.95 is a common utility threshold; leave it at or below your current PF to skip the capacitor sizing.
The formulas are identical for single-phase and three-phase systems and for 50 Hz and 60 Hz grids, because kW, kVA and kVAR are already total-system values. Some utilities outside North America (for example in the UK, Australia and South Africa) bill on kVA or kVAR demand, which makes correction especially valuable there.
What Is a Good Power Factor?
A power factor of 0.95 or higher is generally considered good, while values below 0.85 usually trigger utility surcharges. The table shows typical values for common loads and how much extra current they draw compared with a unity-PF load.
| Load Type | Typical PF | Extra Current vs. PF 1.0 |
|---|---|---|
| Resistive heater, incandescent | 1.00 | 0% |
| Corrected industrial plant | 0.95 | 5% |
| Fully loaded induction motor | 0.85 | 18% |
| Typical uncorrected factory | 0.80 | 25% |
| Lightly loaded motor | 0.50 | 100% |
Frequently Asked Questions
What is a good power factor?
A good power factor is 0.95 or above. Most utilities require at least 0.90 to 0.95 and charge penalties or higher demand fees below that, while a value of 1.0 means all current is doing useful work.
How do I calculate power factor from kW and kVA?
Divide real power by apparent power: PF = kW ÷ kVA. For example, a load drawing 80 kW and 100 kVA has a power factor of 80 ÷ 100 = 0.80, with a phase angle of about 36.9 degrees.
What is the difference between kW, kVA and kVAR?
kW is real power that performs useful work, kVAR is reactive power that sustains magnetic fields in motors and transformers, and kVA is their vector sum, the total power the supply must deliver. They are related by kVA² = kW² + kVAR².
How do I improve my power factor?
Install shunt capacitors or an automatic capacitor bank sized with Qc = kW × (tan φ1 − tan φ2), avoid running motors lightly loaded, and use high-efficiency motors or variable frequency drives. Check for harmonics before adding capacitors, since they can cause resonance.