Power Factor Explained
Real, Reactive, and Apparent Power
Understand Power FactorYour electricity meter measures watts, but electrical infrastructure must handle volt-amps (VA). The relationship between them—power factor—affects efficiency, equipment sizing, and for commercial customers, electricity bills. Understanding power factor helps explain why motors and electronics behave differently from simple heaters.
Understanding Power Factor
What It Represents
Power factor is the ratio of real power (useful work) to apparent power (total power supplied). It ranges from 0 to 1:
- PF = 1.0: All power does useful work (resistive load)
- PF = 0.8: 80% does useful work, 20% is reactive
- PF = 0.5: Only 50% does useful work
The Water Analogy
Imagine a beer glass: real power is the beer, reactive power is the foam. You pay for a full glass (apparent power) but only the beer (real power) satisfies thirst. Low power factor means more foam.
Power Factor by Load Type
| Load Type | Typical PF | Examples |
|---|---|---|
| Resistive | 1.0 | Heaters, incandescent bulbs |
| Induction motor (loaded) | 0.80-0.90 | Fans, pumps, compressors |
| Induction motor (light load) | 0.40-0.70 | Idling motors |
| Fluorescent lights | 0.50-0.95 | Depends on ballast |
| LED drivers | 0.70-0.95 | Varies by quality |
| Computer power supplies | 0.60-0.95 | PFC-equipped = higher |
| Welding machines | 0.50-0.70 | Highly inductive |
The Power Triangle
The relationship between real, reactive, and apparent power forms a right triangle:
- Real power (P): Horizontal leg (watts)
- Reactive power (Q): Vertical leg (VAR)
- Apparent power (S): Hypotenuse (VA)
The Formula
S² = P² + Q²
Power factor = P/S = cos(θ)
Where θ is the phase angle between voltage and current.
Example Calculation
A motor draws 10 amps at 240V with power factor 0.80:
Finding Powers
- Apparent power: S = V × I = 240 × 10 = 2,400 VA
- Real power: P = S × PF = 2,400 × 0.80 = 1,920 W
- Reactive power: Q = √(S² - P²) = √(2,400² - 1,920²) = 1,440 VAR
If PF Were 1.0
Same motor (1,920 W) would only draw:
I = P / V = 1,920 / 240 = 8 amps (instead of 10)
Power Factor Correction
Capacitors can counteract the reactive power of inductive loads:
Methods
- Individual correction: Capacitor at each motor
- Group correction: Capacitors at distribution panel
- Central correction: Automatic capacitor banks at main
Benefits
- Reduced current draw
- Freed-up transformer/wire capacity
- Lower losses and heat
- Avoided utility penalties
For Residential Users
Residential customers typically don't pay for power factor because:
- Meters measure real power (watts)
- Loads are relatively small
- Many loads are resistive (heating, lighting)
However, low power factor still means:
- Higher currents in home wiring
- More heat in conductors
- Need for larger breakers/wiring for same useful power
Conclusion
Power factor is the ratio of real power (watts) to apparent power (VA) in AC circuits. Inductive loads like motors have power factors below 1.0, meaning more current flows than necessary for the useful work done. Commercial facilities often correct power factor with capacitors to reduce costs and improve efficiency. For most residential applications, power factor is less critical but still affects wiring and breaker sizing.