Reactive Power

What is reactive power

Reactive power, measured in kVAR, is the component of AC electrical power that oscillates between source and load without doing useful work. It is required to maintain voltage on AC networks and to support magnetic fields in inductive devices like motors and transformers.

In phasor terms, voltage and current in an AC circuit are not always in phase. Active power is the in-phase product, useful for work. Reactive power is the quadrature product, useful for voltage and magnetisation but not for work. Apparent power is the vector magnitude that combines both.

Modern grid-tied solar inverters are reactive-power-capable devices. They can inject leading or lagging reactive on demand, supporting site power factor, contributing to grid voltage regulation, and meeting smart-inverter requirements under CEA Grid Code 2019.

Reactive power explained simply

Picture pouring water down a slide. The water doing work is like active power: it lands at the bottom and powers something. Some water sloshes back and forth on the slide, never reaching the bottom, but its motion keeps the slide wet and smooth. That sloshing is like reactive power: it does not turn anything on, but the slide cannot work without it.

Inductive loads like motors and fluorescent lights need that sloshing to magnetise. Capacitors do the opposite, pushing the slosh in the other direction. A grid in balance has equal slosh in both directions, and voltage stays stable.

Why reactive power matters

Voltage stability. Voltage on AC networks is governed by reactive balance. Reactive shortage causes voltage collapse.

Power factor. C&I customers pay reactive penalties when power factor falls below state thresholds; see reactive power in solar inverters, an India buyer’s guide for how factories eliminate this charge without a dedicated correction panel.

Grid integration of solar. Inverters injecting controlled reactive power help DISCOMs absorb more rooftop solar without voltage issues.

Smart-inverter functions. Volt-var, volt-watt and power-factor control all rely on reactive power capability.

Capacitor bank replacement. Smart inverters can replace traditional capacitor banks at industrial sites, with dynamic response instead of stepped switching.

How reactive power works

In an AC circuit:

• v(t) = Vm sin(ωt)
• i(t) = Im sin(ωt − φ)

Active power P = Vrms × Irms × cos(φ). Reactive power Q = Vrms × Irms × sin(φ). Apparent power S = Vrms × Irms = √(P² + Q²).

φ is the phase angle between voltage and current. Power factor PF = cos(φ).

A solar inverter modulates the current phase relative to the grid voltage to inject or absorb reactive power. The control loop sets the desired reactive level based on a fixed setpoint, a Q(V) curve or a Q(P) curve.

Technical deep dive

Inverter capability curve

The capability curve is bounded by the rated apparent power S.

Smart-inverter modes

Capability vs grid code

CEA Grid Code 2019 requires generators above 10 kW to provide power factor between 0.95 leading and 0.95 lagging at full output, with extended reactive capability at lower outputs.

Reactive billing

DISCOMs typically meter reactive energy (kVARh) separately. Penalty kicks in when:

PF < threshold (typically 0.85 to 0.95)

Penalty rate is usually a fraction of the active tariff applied to excess reactive.

Real-world applications

Industrial sites using solar inverters as dynamic reactive sources, eliminating older capacitor banks; see solar inverter for factory and industrial loads for sizing guidance.

Utility solar plants providing scheduled reactive support per dispatch instructions.

C&I rooftop systems configured to maintain power factor near unity at the site.

Hybrid wind-solar plants coordinating reactive at the point of common coupling.

Distribution feeder support, where rooftop solar volt-var control prevents overvoltage.

Advantages of inverter reactive support

Dynamic response in milliseconds, faster than capacitor banks.

No mechanical wear or switching transients.

Capacity available without additional hardware.

Avoids reactive penalties on the customer bill.

Helps DISCOMs integrate more distributed solar.

Limitations

Reactive capability reduces with rising active output.

Reactive injection during high voltage can worsen the problem if not coordinated.

Inverter losses include reactive support overhead.

Requires correct configuration. Default factory settings often disable reactive.

Voltage instability risk if multiple inverters inject reactive without coordination.

Common problems

Default inverter PF setting at unity, missing site reactive penalty.

Reactive injection at high voltage causing voltage rise nuisance.

Capability curve misunderstood, leading to overcommitment in design.

Multi-inverter sites with no plant controller, each inverter reacting independently.

DISCOM reactive penalty continues despite inverter setting, because reactive meter not in same compartment.

Volt-var curve too aggressive, causing oscillation.

Best practices

Read the inverter capability curve carefully. The 33 percent figure at full active is a default, not a constant.

Configure power factor or Q setpoint at commissioning based on actual site load.

Use volt-var mode for distributed plants where voltage regulation matters.

Coordinate multi-inverter sites with a plant controller.

Audit the first three reactive bills. Penalty should drop materially after inverter PF tuning.

Maintain firmware updates. Smart-inverter functions improve with each release.

Comparison: solar inverter vs traditional reactive sources

Indian market context

CEA Grid Code 2019 mandates reactive capability and smart-inverter functions for generators above 10 kW. State DISCOMs increasingly use inverter reactive capability for feeder voltage regulation.

Large C&I customers actively configure solar inverters to maintain power factor at unity, avoiding penalty.

Utility solar plants under SECI PPA include reactive dispatch terms, where the plant controller responds to TSO commands.

Major inverter brands support full reactive capability with documentation in the Indian market.

Standards and certifications

Common mistakes

Leaving default unity PF setting active despite site reactive penalty.

Sizing inverter without considering reactive headroom.

Activating volt-var without coordinating across the feeder.

Forgetting that reactive capability reduces with active output.

Ignoring power factor on the customer bill.

Using old capacitor banks when smart inverters could do the job, and skipping a resonance check when both are present on the same feeder; see THD in solar inverters for how fixed capacitor banks can amplify a specific harmonic order once a solar inverter joins the circuit.

Not specifying reactive capability in inverter purchase orders.

Key takeaways

Reactive power is what keeps AC voltage stable. Solar inverters can now serve as dynamic reactive sources, replacing capacitor banks and providing voltage support to distribution feeders. CEA Grid Code 2019 mandates the capability. Configuration at commissioning, plant-level coordination and firmware updates decide whether the capability is actually used.

Further reading

For how Reactive Power plays out in real projects, these guides go deeper:

• Reactive Power Support from Solar Inverters India
• How Does a Solar Power System Work in India?
• String vs Microinverter, Indian Roof Reality
• Enphase vs SolarEdge in India 2026: Are Premium Micros Worth It?

Sources

• CEA Technical Standards for Connectivity to the Grid (Amendment) Regulations 2019.
• IEEE 1547-2018, Standard for Interconnection and Interoperability of Distributed Energy Resources.
• UL 1741 SA and SB, Inverters, Converters, Controllers and Interconnection System Equipment.
• IEC 61727:2004, Photovoltaic systems: Characteristics of the utility interface.
• NREL Technical Report TP-5D00-72528, Grid-Forming Inverter Performance.
• Sandia National Laboratories Report SAND2018-2168, Volt-Var Control with Smart Inverters.
• IEA PVPS Task 14 Report on smart inverter functions.
• Mohan N., Power Electronics: Converters, Applications, and Design, Wiley.