Energy Academy
Level 1/Electrical Science Refresher
AC Fundamentals3 / 10

Power Factor & Reactive Power

Real vs reactive power, why power factor matters, and how to improve it.

10 min read

In AC systems, not all current is doing useful work. Some is reactive — oscillating back and forth without transferring energy. Power factor is the ratio of real (useful) to apparent (total) power, and improving it saves money.

Real vs reactive power

Real power (P) measured in watts: the power that does actual work (heating, light, motion).

Reactive power (Q) measured in VArs (volt-amps reactive): power that oscillates back and forth, driven by inductive and capacitive loads.

Apparent power (S) measured in VA: the total current × voltage = √(P² + Q²)

Power factor (PF) = P / S = cosine of the phase angle between voltage and current.

For a resistive load (heater), current and voltage are in phase: PF = 1.0 (100%). For an inductive load (motor), current lags voltage: PF < 1.0.

Why poor power factor costs money

When PF is low (say, 0.7), you draw more current to deliver the same real power. Extra current means:

  • Larger cables and switchgear needed
  • I² R losses in wiring (copper loss is proportional to current squared)
  • Utility penalties: most commercial meters charge for kVA, not just kW. Low PF means you're charged for reactive power you're not using.
Power factor < 1 is expensive

A 100 kW motor at PF 0.85 draws apparent power of 100 / 0.85 = 118 kVA. At PF 0.95, it's 105 kVA. The utility charges for the higher value.

Causes of low power factor

  • Induction motors and transformers when lightly loaded or oversized
  • Uncompensated reactive loads
  • Harmonic distortion from VFDs, LED drivers, and switching power supplies

Improving power factor

  1. Right-size motors — oversized motors run at low load with poor PF
  2. Capacitor banks — add capacitors near inductive loads to cancel reactive power
  3. Synchronous condensers — rotary devices that inject reactive power
  4. Active harmonic filters — in facilities with heavy distortion

Cost to install capacitors: £2,000–5,000 for a typical building. Savings: ~£500–1,000/year in reduced demand charges. Payback: 3–5 years.

Practical management

  • Check your utility bill — it will show demand (kVA or kW) and PF
  • If PF < 0.95, you're being charged for reactive power
  • Measure PF in key areas (motor rooms, HVAC plants) using a power-factor meter
  • A multi-meter or power analyser (£200–500) gives real-time PF