Power Factor
Definition
Power factor (PF) is the ratio of real power (kW, the power that does useful work) to apparent power (kVA, the total power supplied to the circuit), expressed as a number between 0 and 1 or as a percentage. A power factor of 1.0 (unity) means all supplied power is used for productive work. Lower power factors indicate wasted energy in the form of reactive power. Purely resistive loads have PF = 1.0; electric motors typically have PF = 0.80โ0.90; fluorescent lighting PF = 0.50โ0.90. Utility companies may charge penalties for commercial facilities with power factor below 0.85โ0.90. Power factor correction capacitors can improve PF.
Why is Power Factor Important?
In electrical engineering and everyday applications, Power Factor is a fundamental concept for understanding how electrical systems work. Whether you are an engineer designing circuits, an electrician sizing wires, or a homeowner estimating energy costs, this metric is essential for safety, efficiency, and accurate calculations.
Our electrical conversion calculators help you quickly convert between units and verify calculations, reducing errors and saving time in both professional and DIY electrical work.
What is Power Factor?
Power factor (PF) is a measure of how efficiently electrical power is being used, expressed as a ratio between 0 and 1 (or 0% to 100%). It represents the relationship between real power (watts โ the power doing actual work) and apparent power (volt-amps โ total power drawn from the source).
Power Triangle
| Power Type | Unit | Symbol | What It Represents |
|---|---|---|---|
| Real Power | Watts (W) | P | Actual work performed (heat, motion, light) |
| Reactive Power | Volt-Amps Reactive (VAR) | Q | Power stored/returned by inductors and capacitors |
| Apparent Power | Volt-Amps (VA) | S | Total power delivered by the source |
Power Factor = Real Power (W) รท Apparent Power (VA) = cos ฮธ
Power Factor of Common Loads
| Load Type | Typical PF | Why |
|---|---|---|
| Incandescent bulbs, heaters | 1.0 | Purely resistive โ perfect PF |
| LED bulbs (with driver) | 0.85โ0.95 | Switching power supply |
| Desktop computer | 0.95โ0.99 | Active PFC in power supply |
| Fluorescent lights | 0.50โ0.85 | Ballast inductance |
| Induction motor (full load) | 0.80โ0.90 | Inductive |
| Induction motor (no load/light) | 0.20โ0.40 | Nearly all reactive power |
| Welding machine | 0.40โ0.60 | Highly inductive |
Why Power Factor Matters
- Residential: PF has minimal direct impact on your electric bill (residential meters measure real power in kWh)
- Commercial/Industrial: Utilities charge power factor penalties when PF drops below 0.85โ0.90. A facility with PF of 0.70 may pay 15โ30% more than one at PF 0.95
- Generators/UPS: A 10 kVA generator with a 0.80 PF load can only deliver 8 kW of real power โ not the full 10 kW. Sizing equipment requires knowing the PF