Power Factor

What is power factor?

Power factor (PF) = active power (kW) ÷ apparent power (kVA) at the same measurement point and time. If a circuit carries 5 kVA and delivers 4 kW of active power, the PF magnitude is 4 ÷ 5 = 0.8. The direction and sign convention for leading or lagging operation should be stated separately.

At the same active power, a lower PF generally means more current and greater loading of cables, transformers or an inverter. It does not mean that “20% of the electricity is wasted” in the 0.8 example. Reactive power and losses are distinct quantities.

Power factor explained simply

Imagine pulling a boat along a canal from the bank with a rope. Only the part of your pull that points along the canal moves the boat, and the sideways part strains the rope without moving anything. Active power is the along-the-canal pull. Reactive power is the sideways pull. Apparent power is the total pull on the rope. Power factor tells you how much of the total is useful.

An inductive load such as a motor needs current to build its magnetic field, and that current is out of step with the voltage. The supply has to carry it, even though it does no work. That is why a lightly loaded motor can have a poor power factor.

The power triangle

Active power (P, in kW), reactive power (Q, in kVAR) and apparent power (S, in kVA) form a right-angled triangle:

• S² = P² + Q²
• PF = P ÷ S = cos φ for sinusoidal waveforms, where φ is the phase angle between voltage and current
• Q = S × sin φ

Leading, lagging and waveform distortion

An inductive load commonly draws lagging reactive power; capacitive behaviour can be leading. Solar inverters that support reactive-power control can operate within a specified capability envelope, but this may constrain simultaneous active-power output. The exact limits and permitted settings come from the model and grid approval.

For ideal sinusoidal waveforms, displacement PF relates to the phase angle as cos φ. With harmonics, true PF also includes a distortion term. For a sinusoidal voltage and a current with total harmonic distortion THD_I (as a fraction), true PF = displacement PF ÷ √(1 + THD_I²). A current with 10% THD lowers true PF by only about 0.5%, while a heavily distorted current lowers it more. A meter, invoice and inverter datasheet may report different PF definitions or boundaries; compare them carefully. The THD guide explains harmonic distortion, while kVA explains apparent power.

Why power factor matters

• Equipment loading. More current for the same kW loads transformers, cables and switchgear and can limit how much more load they can carry.
• Network losses. Reactive current adds I²R losses in the network.
• Billing. Some commercial and industrial tariffs include a power-factor penalty or incentive, or bill on apparent energy (kVAh) rather than active energy (kWh).
• Grid connection. Utilities and grid codes may state a reactive-power capability or operating range for larger generators.

What does it mean for a solar project?

Inverter reactive capability: a worked example

An inverter’s output current limit is set by its apparent power S. Whatever S it cannot use for active power is available for reactive power:

Take the Qbits QB-6KTLC three-phase model, whose datasheet lists 6,000 W rated output and 6.6 kVA maximum output power, with a power factor above 0.99 under rated power and an adjustable range of 0.8 leading to 0.8 lagging (checked October 2026). If the 6.6 kVA limit is what governs:

So running at a 0.8 power factor can reduce the active power the inverter may deliver at that moment, compared with its rating at unity power factor. Confirm the exact derating behaviour with the manufacturer. The same arithmetic applies to any model, but the figures belong to that model’s datasheet.

Power factor correction methods

Capacitor banks can interact with harmonics from inverters and drives. A resonance between the bank and the network can amplify a harmonic and overheat the capacitors, which is one reason to measure before adding or changing compensation. The THD in solar inverters guide covers distortion at the output.

Solar and power factor in practice

• Daytime only. Inverter reactive support is available while the inverter is generating, so a site that is billed on whole-day or evening power factor may still need other compensation.
• Approval. Utilities set the allowed operating mode, so a power-factor or voltage-control setting should follow the connection approval and not a generic setpoint.
• Interaction with export limits. Reactive output, active output and any zero-export control all share the same current limit.
• Measurement point. The PF the utility bills is measured at its meter, which may be some distance from the inverter, so what the inverter reports and what the meter reports can differ.

The reactive power guide discusses the topic in more depth for commercial sites.

Common problems

• A power-factor penalty continues after installing solar because the inverter was never configured for reactive output.
• A capacitor bank over-compensates and produces leading PF at low load.
• Resonance between capacitors and harmonics.
• Large swings through the day that a fixed bank cannot follow.
• Comparing a displacement PF on a datasheet with a true PF on a bill.
• A PF measured at the inverter used to explain a PF measured at the meter.

Best practices

1. Read the tariff order for the PF threshold, penalty method and billing basis.
2. Measure the load’s PF over a representative period before choosing a remedy.
3. Check harmonics before adding capacitors.
4. Set inverter reactive controls only within the model’s capability and the approved mode.
5. Review the PF on the bills after any change.
6. Have a qualified engineer review a persistent penalty.

Indian context

State commissions set power-factor thresholds, incentives and penalties in tariff orders, so the same load can be treated differently in two states. Many tariffs apply to commercial and industrial categories, and some residential categories are not billed on PF at all. Grid-connection requirements for distributed generation are set in the CEA connectivity regulations and in state rules and approvals. Those regulations address matters such as harmonic injection and DC injection, and any reactive-power operating range for your plant should come from the connection approval or the applicable grid code, not a generic figure. Check the current text for your connection.

For Qbits models, obtain the current product datasheet and site requirements before setting reactive controls. The on-grid inverter range page lists the models, and the C&I solutions page covers commercial projects. See also the reactive power entry.

Standards and references

Common mistakes

• Using kW instead of kVA to size equipment at a low PF.
• Assuming an inverter can supply reactive power at full active output.
• Treating one tariff’s threshold as a national rule.
• Adding capacitors without measuring harmonics.
• Reading displacement PF and true PF as the same number.

Key takeaways

Power factor is kW divided by kVA. A low value means more current for the same work, which loads equipment and can raise charges. A solar inverter may help within its kVA limit and the approved settings, but it is not a universal substitute for compensation. Check the tariff, measure the site, and use the datasheet of the exact model.