What Is Power Factor? Formula, Meaning & Cost Impact
Power factor is the ratio of active power to apparent power: PF = kW ÷ kVA. It shows how much RMS voltage-and-current capacity is required to deliver a given amount of active power. A 0.80 power factor means that 1 kW requires 1.25 kVA—not that 20% of your electricity is automatically “wasted.”
Power factor is not efficiency. Before selecting correction equipment, verify the tariff, load profile, true versus displacement PF, harmonic distortion and system resonance risk.
What is power factor in simple terms?
Efficiency compares useful output with active-power input; power factor compares active with apparent power at the electrical input. A motor can therefore have high efficiency and a lower power factor, or vice versa.
For sinusoidal loads, lagging PF means current lags voltage and leading PF means it leads. With nonlinear loads, distortion can lower true PF even when the fundamental current is nearly in phase with voltage.
- Use measured kW and kVA to obtain true power factor.
- Use cosφ only when discussing the fundamental displacement angle under appropriate sinusoidal assumptions.
- Use the applicable tariff and a power-quality study—not a generic threshold—to choose a correction target.
Power factor in three practical parts
The number becomes useful only when the formula, the physical meaning and the commercial consequence are considered together.
Formula
Divide active by apparent power. Confirm whether the meter reports true or displacement PF and use the correct system inputs.
Meaning
Interpret P, Q and S without treating them as slices of one energy total. Understand phase shift, distortion and the power triangle’s limits.
Cost impact
Identify kVA demand, reactive-energy or PF adjustments, then compare avoidable charges with correction cost and risk.
How is power factor calculated?
The universal starting point is active power divided by apparent power. Other familiar equations require additional assumptions.
Use matching units: W ÷ VA or kW ÷ kVA. P is average active power and S is apparent power based on RMS voltage and current under the applicable system definition.
Uses total measured RMS quantities and active power, so harmonic current is reflected in the result.
Uses the phase angle between the fundamental-frequency voltage and current components.
Useful for sinusoidal steady-state analysis; distorted or unbalanced systems need the meter’s stated definitions and a fuller power-quality analysis.
Technical foundation: the US Department of Energy’s Electrical Science handbook, Volume 3, and Schneider Electric’s explanation of true versus displacement power factor.
Power factor needs simultaneous voltage, current and power data
A power analyzer measures active power while RMS voltage and current determine apparent power. For three-phase systems, use the correct wiring and total values; current alone cannot reveal PF.
For variable or nonlinear loads, log true PF, displacement PF, kW, kVA, kVAR and harmonic distortion at the relevant measurement point.
Diagram: Constant314, via Wikimedia Commons, CC BY-SA 4.0.
Single-phase and balanced three-phase examples
These examples use measured active power and calculated apparent power. The three-phase shortcut assumes a balanced sinusoidal system and appropriate RMS readings.
230 V, 20 A, 3.68 kW
- Apparent power: S = 230 V × 20 A = 4,600 VA = 4.60 kVA.
- Active power from the wattmeter: P = 3.68 kW.
- Power factor: PF = 3.68 ÷ 4.60 = 0.80.
400 V L-L, 50 A, 29.44 kW
- Apparent power: S = √3 × 400 V × 50 A = 34.64 kVA.
- Total active power from the analyzer: P = 29.44 kW.
- Power factor: PF = 29.44 ÷ 34.64 ≈ 0.85.
If the phases are unbalanced or waveforms are distorted, use a correctly connected three-phase power analyzer and the instrument’s documented method. Confirm CT/PT ratios, polarity, phase sequence and measurement location before relying on the number.
Real, reactive and apparent power are related—but not simple percentages
Under sinusoidal conditions, P and Q are perpendicular components whose vector magnitude is S. Adding or subtracting them like ordinary energy portions produces the wrong interpretation.
Converted into shaft output, heat, light and internal losses. Energy billing commonly records its time integral in kWh.
Supports magnetic and electric fields and affects current and voltage, but has zero net active-energy transfer over a complete ideal cycle.
Represents the voltage-current capacity seen by sources and conductors under the applicable definition.
What does 80% power factor actually mean?
For an 800 kW load, PF = 0.80 gives S = 800 ÷ 0.80 = 1,000 kVA. Under sinusoidal power-triangle assumptions, Q = √(1,000² − 800²) = 600 kVAR.
It does not mean that reactive power is the remaining 20% or that 20% of purchased kWh disappears. At the same voltage and phase arrangement, the 0.80 PF load draws 25% more current than a unity-PF load delivering the same kW.
| Power factor | kVA for an 800 kW load | Current relative to unity PF | Interpretation |
|---|---|---|---|
| 1.00 | 800 kVA | 1.000 × | Reference case; real installations may not remain exactly at unity. |
| 0.95 | 842 kVA | 1.053 × | A common engineering target, but not a universal tariff rule. |
| 0.80 | 1,000 kVA | 1.250 × | 25% more current than the unity reference at equal kW and voltage. |
| 0.70 | 1,143 kVA | 1.429 × | More feeder, transformer and source capacity is occupied by the same kW. |
In the simplified 0.80-versus-unity comparison, the current-dependent I²R component in the same resistance is 1.25² = 1.5625, or about 56% higher. That is not the same as saying the facility’s total kWh use or bill is 56% higher.
Leading, lagging and true power factor answer different questions
Many motors and transformers require magnetizing current, producing lagging displacement PF. Capacitors can offset part of that fundamental-frequency demand locally.
Nonlinear equipment—including many VFD, UPS and rectifier inputs—can draw harmonic current. Such a load may show a displacement PF near 1 while its true PF is lower. A meter that reports only cosφ can therefore hide the current distortion relevant to conductor loading and correction design.
Capacitors address displacement reactive power. Harmonics may require detuned reactors, passive filters, active filters, improved converter front ends or another engineered solution.
How can low power factor affect capacity and the utility bill?
The electrical effect is predictable; the bill effect is tariff-specific. Separate energy, demand, reactive and adjustment mechanisms before promising savings.
PF correction does not remove the active energy required by the process. It may reduce some upstream losses, but not the load’s useful work.
A tariff may bill peak kW, peak kVA or another demand determinant over a defined interval, sometimes with ratchets.
Some contracts meter reactive demand or reactive energy and apply a charge outside specified conditions.
Some utilities apply a debit, credit or adjusted billing demand based on a defined PF calculation and threshold.
A transparent before-and-after example
At 300 kW and 0.75 PF, apparent power is 300 ÷ 0.75 = 400 kVA. At 0.95 PF, it is 300 ÷ 0.95 = 315.8 kVA. The difference is 84.2 kVA, and line current falls by about 21% at the same voltage and phase arrangement.
If—and only if—the tariff bills this kVA demand directly, the gross monthly change can be estimated as 84.2 multiplied by the applicable currency-per-kVA rate. Actual savings still depend on the demand interval, ratchet, qualifying threshold, operating hours, metering point and whether corrected PF coincides with the billed peak.
Official examples show why one universal rule is unsafe: PG&E documents its own eligibility and averaging method, while Ofgem lists reactive-power charges as one possible business-energy cost.
Use at least 12 months of interval data and bills where seasonality, production cycles or demand ratchets matter. Include equipment, installation, study, protection, maintenance and loss effects in the payback model.
How do capacitor banks improve power factor?
A shunt capacitor bank supplies leading reactive power near an inductive load. This reduces the fundamental reactive current that must travel through upstream conductors and transformers. Fixed banks suit stable demand; automatic stepped banks switch stages as the load changes.
The target should leave operating margin and reflect the tariff, voltage profile and load variation. Correcting blindly to 1.00 can create leading operation, voltage rise or switching problems when loads fall.
For 500 kW from PF 0.75 to 0.95, the ideal sinusoidal estimate is about 277 kVAR. This is a starting calculation—not a final equipment specification.
| Method | Best fit | What it addresses | Key design check |
|---|---|---|---|
| Fixed capacitor | One stable inductive load with predictable duty | Fundamental displacement reactive power | Interlock with the load and prevent leading operation when it is off. |
| Automatic stepped bank | Facility demand that changes through the day | Variable displacement reactive power | Step size, controller settings, switching duty and load-cycle response. |
| Detuned capacitor bank | Systems where harmonic assessment supports reactor use | Displacement correction with reduced resonance risk | System impedance, tuning, harmonic spectrum and capacitor current. |
| Active harmonic filter | Dynamic nonlinear loads or targeted harmonic mitigation | Selected harmonic current and, for suitable designs, reactive compensation | Required spectrum, current rating, response and point of connection. |
Study resonance before adding capacitors
Network inductance and capacitors can create a resonant condition that amplifies harmonic voltage or current. Review transformer impedance, existing capacitors, nonlinear load spectrum and switching conditions. Follow equipment-manufacturer instructions, and never place ordinary correction capacitors on a VFD output.
Application guidance: Eaton’s Power Factor Correction Guide for the Plant Engineer and ABB’s PFC capacitor installation guidance.
Use this five-step power-factor review
Move from billing evidence to measurement, system study, solution selection and verified commissioning.
Read the tariff
Identify the billed quantity, interval, threshold, ratchet, leading/lagging treatment and measurement point.
Log the real load
Measure kW, kVA, kVAR, true PF, displacement PF and THD across representative production cycles.
Model the system
Review transformer data, fault level, cable impedance, existing capacitors, harmonics and switching transients.
Select the method
Choose fixed, automatic, detuned or active equipment based on the measured cause—not PF magnitude alone.
Commission and verify
Confirm PF, voltage, THD, step behavior, protection, temperature and actual bill performance after installation.
Need a meter and panel-component review for your power-factor project?
Send the supply details, CT/PT ratios, parameters you need to monitor, communication protocol, panel layout and target market. SENTOP can help match digital panel-meter and low-voltage distribution product options; final correction design and installation should be completed by qualified power-quality professionals.
Related SENTOP resources
Power factor FAQ
Short, qualified answers to the questions facility teams and equipment buyers ask most often.
What is power factor in simple terms?
Power factor is the ratio of active power in kW to apparent power in kVA. It indicates how much voltage-and-current capacity is required to deliver a given active load; it is not the same as equipment efficiency or a direct percentage of energy wasted.
What is the power factor formula?
The general formula is PF = |P|/S, using matching units such as kW divided by kVA. For a single-phase load, apparent power is commonly Vrms × Irms. For a balanced sinusoidal three-phase load, it is √3 × line-to-line voltage × line current.
What does 80% power factor mean?
A 0.80 power factor means apparent power is 1.25 times active power. An 800 kW load therefore requires 1,000 kVA. It does not mean that the remaining 20% is reactive power or that 20% of billed energy is automatically lost.
Is power factor always equal to cosφ?
No. cosφ describes displacement power factor for the fundamental voltage-current phase angle under suitable sinusoidal conditions. True power factor is active power divided by apparent power and also reflects waveform distortion.
What is a good power factor?
There is no universal target. Many industrial projects consider approximately 0.95 lagging a useful planning goal, but the correct target depends on the tariff, utility requirements, voltage behavior, load variation, harmonics and equipment design.
Can power factor exceed 1?
The magnitude of true power factor cannot exceed 1 because active power cannot exceed apparent power under consistent definitions. A displayed value above 1 suggests rounding, configuration, polarity, CT/PT or measurement issues that should be investigated.
Does improving power factor reduce kWh consumption?
Not necessarily. Correction does not remove the active energy required by the process, although lower upstream current can reduce some I²R losses. Bill savings depend on the tariff’s kVA, reactive-energy or power-factor provisions and the timing of the corrected load.
Can I add capacitors to a system with VFDs?
Only after a qualified harmonic and resonance review. Ordinary correction capacitors must not be connected to a VFD output, and capacitors on the supply system may require detuned reactors or another engineered solution based on the network and manufacturer instructions.
References and further reading
Official and first-party technical material used to verify the formulas, measurement distinctions, billing cautions and correction guidance.
- US Department of Energy — Electrical Science, Volume 3. Basic AC power, power triangle and leading/lagging relationships.
- Eaton — Power Factor Correction: A Guide for the Plant Engineer. Capacitor sizing, location, harmonics and survey inputs.
- Schneider Electric — Understanding True Power Factor. Displacement, distortion and true PF.
- Fluke — Maximizing VFD and UPS Performance. Measurement of displacement PF and harmonic distortion.
- ABB — Film Capacitors for Power Factor Correction. Harmonic distortion, detuning and resonance cautions.
- PG&E — Power Factor. Utility-specific billing example and differences in eligibility and averaging.
- Ofgem — Get Energy for Your Business. Business contract costs, including possible reactive-power charges.
- SENTOP — Digital Panel Meter. Internal next step for reviewing power-monitoring product options.
Engineering note: This guide explains calculation and procurement logic; it is not a capacitor-bank design, tariff interpretation or installation instruction. Final equipment selection requires current bills, measured load and harmonic data, an applicable system study, local standards and qualified electrical professionals.
