How to Improve Power Factor: Capacitors, Motor Loading, and Utility Penalties

If your utility bill includes a power factor surcharge, or if you're seeing unexplained heating in transformers and conductors, the fix usually comes down to one of two things: capacitors or load management. This article covers both, and explains why the math works the way it does.

Why low power factor happens

Most industrial and commercial loads are inductive. Motors, transformers, and fluorescent ballasts with magnetic ballasts all draw reactive power from the supply to build their magnetic fields. That reactive current flows through your wiring without doing any work, but it heats conductors and loads transformers.

Induction motors are the main culprit in most facilities. A motor rated at 0.9 power factor at full load might drop to 0.6 or lower when running at 30% load. That's not a defect. It's how induction motors work: the magnetizing current is roughly constant regardless of mechanical load, so at light loads it represents a larger fraction of the total current.

Common causes of poor facility power factor:

How utilities measure and charge for it

Utilities calculate power factor from recorded real power (kW) and apparent power (kVA) over a billing period, usually a 15- or 30-minute demand interval. Some bill on average PF; others on minimum measured PF during the month.

The penalty structure varies by rate schedule. Common approaches:

The threshold is most commonly 0.85 or 0.90. Check your rate schedule under "reactive demand" or "power factor adjustment."

Run your actual numbers through the power factor calculator to see what your apparent power is and how much reactive compensation would bring you above the threshold.

Power factor correction with capacitors

Capacitors are the standard fix because they supply reactive power locally, reducing how much the source needs to provide. You're not eliminating the reactive current requirement; you're just meeting it from a capacitor on your panel instead of drawing it from the utility.

The reactive power a capacitor supplies:

Q_C = V² × 2πfC

At 60 Hz and 480 V, a 100 µF capacitor supplies roughly 8.7 kVAR. For practical purposes, capacitors are specified in kVAR, not farads.

Sizing capacitors

To raise power factor from PF1 to PF2, the required capacitor rating in kVAR is:

Q_C = P × (tan(φ1) − tan(φ2))

Where P is your real power in kW, φ1 is the current phase angle, and φ2 is the target phase angle.

Example: A facility draws 200 kW at 0.72 PF and wants to reach 0.92 PF.

Round up to the next standard size. If you over-correct past unity, you get a leading power factor, which can cause voltage rise and other issues, so don't go far past your target.

Where to install capacitors

Motor terminals: Putting capacitors directly at the motor terminals is the most effective location. The reactive current circulates within the motor circuit and doesn't travel through any upstream wiring. This also reduces the kVA on the motor's starter and feeder.

Main panel or switchboard: Easier to install and maintain but doesn't reduce current in the branch circuits. Good for overall billing correction; not as good for reducing branch conductor heating.

Automatic capacitor banks: For facilities with highly variable loads, automatic banks switch capacitor steps in and out based on measured reactive demand. This prevents over-correction during light-load periods.

Capacitors and harmonics

Facilities with significant harmonic currents (from VFDs, rectifiers, UPS systems) need to be careful. Capacitors have low impedance at harmonic frequencies and can amplify harmonic voltages through resonance. In these environments, consider harmonic filters or detuned capacitor banks (capacitors in series with reactors, tuned to resonate below the dominant harmonic frequency).

Improving PF through load management

Capacitors are sometimes the wrong tool when the root cause is heavily underloaded motors.

Running a 50 hp motor at 10% load is wasteful regardless of power factor correction: you're spinning large amounts of iron and copper to move very little load. The better fix is right-sizing the motor for the actual mechanical load.

Options:

What a realistic improvement looks like

Correcting from 0.75 to 0.92 PF on a 500 kW facility:

That 124 kVA reduction frees up transformer and feeder capacity. If your utility charges a demand penalty at, say, $10/kVAR-month for reactive demand above the 0.90 threshold, the savings can be several hundred dollars per month on a facility this size.

Capacitor banks for this correction typically cost $8,000–20,000 installed, depending on whether you need automatic switching and harmonic filtering. Payback periods of 1–3 years are common in facilities with aggressive power factor penalties.

Checking your numbers

The calculate power factor tool on this site will give you your current PF from measured kW and kVA readings. From there, you can work backward to the capacitor kVAR needed for any target PF. Most capacitor vendors will do the sizing calculation for free if you provide your utility billing data.

Power factor correction is one of those improvements that pays for itself and then keeps paying. The upfront investment is concrete; the ongoing savings are predictable. If your current PF is below 0.85, it's worth running the numbers.