Power Factor Calculator (PF = W/VA)
Power factor tells you how effectively an AC circuit converts supplied power into useful work. A power factor of 1.0 means all the power does useful work. A factor of 0.7 means 30% of the current is wasted sloshing back and forth. Electric utilities charge commercial customers for poor power factor, so it directly affects your bill.
Understanding the Power Triangle
Every AC circuit has three types of power:
- Real Power (P) in Watts: the actual work done (heating, spinning motors, lighting)
- Reactive Power (Q) in VAR: energy stored and released by inductors and capacitors each cycle
- Apparent Power (S) in VA: the total power the source must deliver
They form a right triangle: S squared = P squared + Q squared
Power Factor Formula
PF = cos(phi) = P / S = Real Power / Apparent Power
| Power Factor | Phase Angle | Efficiency |
|---|---|---|
| 1.00 | 0 degrees | Perfect (resistive) |
| 0.95 | 18 degrees | Excellent |
| 0.85 | 32 degrees | Good (typical motor) |
| 0.70 | 46 degrees | Poor |
| 0.50 | 60 degrees | Very poor |
Common Power Factors
- Incandescent bulbs: 1.0 (purely resistive)
- LED drivers: 0.90 - 0.99
- Electric motors (loaded): 0.80 - 0.90
- Electric motors (unloaded): 0.10 - 0.30
- Welding machines: 0.50 - 0.70
Why It Matters
A motor with PF = 0.7 drawing 1,000W of real power needs 1,429 VA of apparent power. That means the wiring, transformer, and breaker must handle 43% more current than necessary. Utilities add power factor surcharges for commercial accounts below 0.90, and improving power factor with capacitor banks can save significant money.