What Is Power Factor? Definition & Guide

Quick answer

Ratio of active power in kW to apparent power in kVA at a defined AC measurement point; distinct from energy efficiency.

Key Takeaways

Power factor compares active power with apparent power at the same measurement point.
A PF of 0.8 is not a statement that 20% of delivered energy is wasted.
Distorted waveforms require a distinction between total PF and displacement PF.
PV can reduce grid imports while worsening grid-connection PF.
Reactive-power capability, billing and approved control settings require separate checks.

What Is Power Factor?

Answer: Power factor is active power in kilowatts divided by apparent power in kilovolt-amperes at a defined AC measurement point. It describes the relationship between power and electrical loading, rather than conversion efficiency. For sinusoidal waveforms, it equals the cosine of the voltage–current phase angle. In a solar installation, the inverter’s PF and the facility’s grid-connection PF can differ.

Power factor

PF = P / S; for an importing load, PF magnitude = active kW / apparent kVA

Schneider Electric distinguishes total PF from displacement PF. Total PF includes waveform distortion; displacement PF describes the fundamental-frequency phase relationship. Check which quantity a meter or report actually displays. A magnitude between zero and one does not describe direction: signed export readings and leading/lagging labels depend on the instrument’s convention.

The Power Triangle

Active power: kW

Average power transferred to operate equipment. Integrating it over time gives active energy in kWh. A demand reading and an energy reading have different time bases.

Reactive power: kvar

The reactive component associated with phase-shifted AC voltage and current. Its presence is not equivalent to a matching percentage of energy dissipated as heat.

Apparent power: kVA

A measure of AC electrical loading. Use a measurement method appropriate to the phase arrangement, balance and waveform rather than assuming every circuit is a balanced sinusoidal system.

For the sinusoidal examples below, the magnitude relationship is:

Sinusoidal power triangle

S = √(P² + Q²); PF = P / S = cos(phi)

With appreciable harmonics and power-quality distortion, the simple fundamental-frequency triangle does not describe all apparent-power contributions. Do not substitute a displacement-PF reading for total PF without checking the measurement method.

Why Power Factor Matters for Solar

The location of the reading matters. An inverter measures its AC output; the utility connection measures the net exchange after facility loads and other sources. A unity-PF inverter does not guarantee a unity-PF facility.

Schneider’s solar self-consumption guidance explains how PV supplying active power alone can reduce grid PF: grid active-power demand falls while the facility still needs reactive power. Compensation therefore needs measurement and coordinated control, not a generic inverter PF setting. Available reactive support also has equipment and operating limits; daytime PV production is not proof of overnight compensation.

Facility objective

Understand net active and reactive exchange at the billing or control point over relevant operating periods.

Grid objective

Follow the applicable interconnection agreement and approved operating mode. A voltage-support requirement need not be identical to a facility billing objective.

Power Factor Penalties on Commercial Bills

Do not quote a universal penalty threshold or savings percentage. Obtain the actual account tariff and effective date, then identify the billed quantity, measurement interval, import/export treatment and any adjustment formula. A momentary inverter PF reading alone cannot establish a monthly saving.

A review should distinguish active-energy charges in kWh, demand charges, reactive-energy treatment and any PF adjustment that the tariff expressly defines. Keep the tariff calculation separate from the active and reactive power definitions. Reducing purchased active energy does not by itself establish that every other bill component falls.

Practical Guidance

Specify the measurement boundary, waveform assumptions and required operating mode.
Read the exact inverter’s kW rating, kVA limit and active/reactive capability curve at the applicable conditions.
Coordinate facility controls with the interconnection requirements and existing compensation equipment.

Use the approved design, manufacturer instructions and authorized commissioning procedure.
Document actual settings, meter sign conventions and readings at the required connection point.
Refer conflicts between plant controls, compensation equipment and utility requirements to the responsible engineer.

Request tariff documents and representative facility measurements before promising savings.
Explain that PF is not an energy-efficiency percentage.
State assumptions and unmodeled bill components explicitly in a proposal.

Discuss your commercial design workflow

Bring the facility measurements, selected equipment and utility requirements to a project discussion.

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Real-World Examples

The following are hypothetical calculations, not customer installations, tariff estimates or approved equipment selections. They assume sinusoidal operation at one common measurement boundary and ignore network losses.

PV reduces imports but lowers grid PF

Before PV, a load takes 80 kW and 60 kvar. Apparent power is √(80² + 60²) = 100 kVA, giving PF 0.80. If PV supplies 50 kW at unity PF, grid exchange becomes 30 kW and 60 kvar. Apparent power is approximately 67.08 kVA, and grid PF falls to approximately 0.447. Purchased active power has fallen; reactive demand has not. This calculation makes no claim about the account’s bill.

A kVA limit is different from a kW rating

If an inverter has an applicable 10 kVA apparent-power limit and operates at PF 0.90 at that limit, active output is 9 kW. The sinusoidal reactive-power magnitude is √(10² − 9²), approximately 4.36 kvar. These are boundary calculations, not a claim that every 10 kW inverter loses 10% of output.

PVsyst’s overload documentation distinguishes nominal ratings in active kW from apparent kVA in its simulation. Use the actual manufacturer capability information and model settings when evaluating inverter sizing.

Frequently Asked Questions

Is power factor the same as inverter efficiency?

No. PF compares active and apparent AC power. Conversion efficiency compares useful output with energy or power input at a stated boundary. A PF of 0.8 does not mean an inverter converts only 80% of its DC input.

Can solar improve a facility’s power factor?

It may, with suitable capability, measurements and coordinated control. Unity-PF PV alone can worsen grid PF by reducing active imports while reactive demand remains. Verify the complete facility response rather than only the inverter reading.

What power factor does a solar utility require?

There is no universal setting. Check the current jurisdiction, utility agreement, approved inverter configuration and operating conditions. Do not treat a generic fixed-PF example as a commissioning instruction.

Does reactive support always reduce active output?

No fixed percentage applies to every inverter. An apparent-power limit can constrain active output when reactive power is supplied. The relevant limit depends on the exact equipment, operating conditions and approved capability envelope.

Sources

Primary documentation checked September 30, 2026: Schneider Electric’s Electrical Installation Guide on total/displacement PF and solar self-consumption; PVsyst’s current power-factor and overload documentation. Worked numbers above are illustrative calculations. Project settings and billing conclusions require the applicable equipment and account documents.