What Are Power Factor and PFC?

Power factor (PF) is a ratio between 0 and 1 showing how much of the apparent power an AC load draws from the mains turns into real (useful) work. It's defined as PF = Real Power (W) / Apparent Power (VA). PFC (power factor correction) is the technique used to bring this ratio close to 1.

Real, Apparent and Reactive Power

In AC circuits there are three power concepts: real power (W, doing work), reactive power (VAR, sloshing back and forth in coils/capacitors) and apparent power (VA, their resultant). In an ideal resistive load PF = 1. In inductive (motor) or capacitive loads the current shifts in phase from the voltage; the cosine of this phase difference (cos φ) gives the classic power factor. Phase-shifted current means more current is drawn to do the same work.

Why Is PFC Needed?

Switching power supplies (computers, LED drivers, adapters) use a bridge rectifier + capacitor at the input and draw current only in pulses at the voltage peaks. This creates harmonic distortion and a low power factor independent of phase shift. PFC corrects this:

• Passive PFC: Smooths the current pulse with an input inductor (simple, limited improvement).
• Active PFC: Shapes the input current to match the voltage with a boost stage and controller (PF ~0.95–0.99).

Where Does It Come Up?

Standards for devices above a certain power (e.g. mains harmonic limits) require active PFC. When choosing a power supply, a "with PFC / high PF" label means the device places a cleaner load on the mains.

Things to Watch Out For

• PF ≠ efficiency: Power factor is the "quality" of the drawn current; efficiency is how much of the input power reaches the output. They are different things.
• No PF in DC: Power factor is an AC-specific concept (the DC power formula P = V×I is fixed).
• Harmonics: Low PF often comes not from phase shift but from harmonic currents; just adding a capacitor won't fix this — active PFC is needed.

Frequently Asked Questions

What is power factor in short?
It's a 0–1 ratio showing how much of an AC load's apparent power (VA) turns into real work (W): PF = W / VA.

What does PFC do?
It brings the power factor close to 1 by making the input current match the voltage; it places a cleaner load on the mains and reduces harmonics.

When choosing a device, check the PFC/PF figure in the power supply specs.