What Is Active and Reactive Power? Definition & Guide

Key Takeaways

Active power (P, measured in kW) is the energy that does real work — running motors, lighting, and equipment
Reactive power (Q, measured in kVAR) maintains voltage and magnetic fields in inductive loads like transformers and motors
Apparent power (S, measured in kVA) is the vector sum of active and reactive power and determines inverter sizing
Power factor (PF = P/S) measures how efficiently a system converts apparent power into useful work — ideal is 1.0
Modern smart inverters can inject or absorb reactive power to support grid voltage stability
Utilities may penalize commercial customers with power factors below 0.90 through demand surcharges

What Is Active and Reactive Power?

In any AC electrical system, power has two components. Active power (P) is the portion of electricity that performs real, useful work — turning motors, producing light, running compressors. It’s measured in watts (W) or kilowatts (kW) and is what your electricity meter bills you for.

Reactive power (Q) doesn’t do useful work. Instead, it sustains the magnetic and electric fields needed by inductive loads like transformers, motors, and fluorescent lighting ballasts. Reactive power oscillates between the source and load without being consumed, but the conductors and equipment must still carry it. It’s measured in volt-amperes reactive (VAR) or kilovolt-amperes reactive (kVAR).

The combination of active and reactive power is apparent power (S), measured in kilovolt-amperes (kVA). This is the total power that the inverter and wiring must handle, and it’s what determines equipment sizing. Understanding these relationships is essential for proper solar design software modeling and utility interconnection compliance.

The relationship between active, reactive, and apparent power is often shown as a right triangle: active power is the horizontal leg, reactive power is the vertical leg, and apparent power is the hypotenuse. The angle between them determines the power factor.

How Active and Reactive Power Work

Solar Inverter Generates AC

The solar inverter converts DC from the panels to AC. By default, most grid-tied inverters output power at unity power factor (PF = 1.0), meaning all output is active power with zero reactive power.

Building Loads Consume Both Components

Resistive loads (heaters, incandescent lights) consume only active power. Inductive loads (motors, compressors, transformers) consume both active and reactive power. Most commercial buildings have a lagging power factor of 0.80–0.95.

Grid Supplies the Difference

The utility grid supplies whatever active and reactive power the solar system doesn’t cover. If the building’s reactive power demand is high, the grid must supply it — increasing apparent current flow and transmission losses.

Smart Inverters Provide Reactive Support

Under IEEE 1547-2018 and utility-specific rules, smart inverters can inject or absorb reactive power to regulate local voltage. This Volt-VAR function reduces grid stress and may be mandatory for interconnection approval.

Utility Meters and Bills Reflect Power Factor

Commercial utility meters track both kW (active) and kVAR (reactive). If the building’s power factor falls below the utility’s threshold (typically 0.90), a demand surcharge or reactive power penalty applies to the monthly bill.

Power Triangle Relationships

S² = P² + Q² | PF = P / S = cos(φ) | Q = P × tan(φ)

Types of Power in Solar Systems

Billable

Active Power (P)

Measured in kW. This is the useful power that runs equipment and is recorded by the electricity meter. Solar systems offset active power consumption, directly reducing the electricity bill.

Non-Billable (Usually)

Reactive Power (Q)

Measured in kVAR. Supports magnetic fields in motors and transformers. Not directly billed in residential tariffs, but commercial utilities often penalize low power factor caused by excess reactive demand.

Equipment Sizing

Apparent Power (S)

Measured in kVA. The total power the inverter and wiring must carry. A 10 kVA inverter at 0.90 PF delivers only 9 kW of active power. Inverter datasheets rate capacity in kVA or kW — check which one.

Quality Metric

Power Factor (PF)

Ratio of active to apparent power (0 to 1.0). A PF of 1.0 means all power is active (ideal). Commercial buildings typically run 0.80–0.95. Smart inverters can improve the building’s PF by providing reactive support.

Designer’s Note

When a utility requires a smart inverter to operate at less than unity power factor (e.g., PF = 0.95 absorbing), the inverter’s usable active power output decreases. A 10 kVA inverter at PF 0.95 delivers only 9.5 kW of active power. Factor this into your system sizing.

Key Metrics & Calculations

Metric	Unit	Typical Range	What It Means for Solar
Active Power (P)	kW	System rated capacity	Directly offsets electricity consumption
Reactive Power (Q)	kVAR	0–30% of apparent power	Affects inverter sizing and grid compliance
Apparent Power (S)	kVA	Inverter nameplate rating	Total power the inverter must handle
Power Factor (PF)	0–1.0	0.90–1.0 for solar inverters	Required setting for grid interconnection
THD (Total Harmonic Distortion)	%	Under 5% per IEEE 1547	Waveform quality affecting power quality
Reactive Power Capability	±kVAR	Per inverter spec sheet	Range of voltage support the inverter can provide

Inverter Active Power at Non-Unity PF

P_available (kW) = S_rated (kVA) × Power Factor Setting

Practical Guidance

Check the utility’s reactive power requirements. Some utilities require inverters to operate at a fixed power factor (e.g., 0.95 lagging). Others use dynamic Volt-VAR curves. These settings reduce available active power — size the inverter accordingly.
Size inverters by kVA, not just kW. If the interconnection agreement requires reactive power injection, you need an inverter with enough kVA headroom. A 10 kW system at PF 0.90 needs an 11.1 kVA inverter to deliver full active output.
Model power factor correction for commercial sites. If the building has a poor power factor, the solar inverter’s Volt-VAR function can improve it — potentially eliminating the utility’s PF penalty. Quantify this as an additional savings in the financial model.
Verify inverter reactive power range. Not all inverters have the same Q capability. Check the datasheet for the reactive power operating envelope — some can deliver ±60% of rated kVA as reactive power, others only ±30%.

Configure Volt-VAR settings per the interconnection agreement. Default inverter settings may not match utility requirements. Program the correct power factor or Volt-VAR curve before commissioning — incorrect settings can cause interconnection test failures.
Measure power factor at commissioning. Use a power quality meter at the point of interconnection to verify the inverter’s power factor output matches the required setting. Document the reading for the utility’s inspection records.
Check for harmonic issues. Non-linear loads (VFDs, LED drivers, computers) create harmonics that distort the AC waveform. If the building has high harmonic distortion, the solar inverter may interact poorly with existing loads. Measure THD before and after commissioning.
Size conductors for apparent power, not active. Wiring must carry the full apparent current (kVA), not just the active component. At PF 0.90, conductor current is 11% higher than the active power alone would suggest.

Identify power factor penalties on the utility bill. Many commercial customers don’t know they’re paying a PF surcharge. Review the bill for “reactive demand” or “power factor adjustment” line items — a solar software proposal that eliminates this penalty adds tangible savings.
Don’t oversimplify for commercial clients. Building managers and facility engineers understand power factor. Showing that your proposal accounts for reactive power demonstrates technical competence and differentiates you from competitors who only quote kW.
Quantify the full value stack. Active power saves on kWh charges. Peak shaving saves on demand charges. Reactive power correction saves on PF penalties. Present all three in the solar proposal for maximum financial impact.
Keep it simple for residential. Homeowners don’t need to understand reactive power. Their loads are mostly resistive, and residential meters don’t charge for PF. Focus on kWh savings and bill reduction — that’s what matters to them.

Model Power Factor and Reactive Power in Proposals

SurgePV’s financial modeling accounts for demand charges, power factor penalties, and inverter reactive power capabilities — giving commercial customers the complete savings picture.

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Real-World Examples

Residential: 6 kW System at Unity Power Factor

A homeowner installs a 6 kW single-phase system with a 6 kW string inverter operating at PF 1.0. The inverter outputs 6 kW of active power and 0 kVAR of reactive power. The home’s loads are mostly resistive (lighting, appliances, electric heating), so no reactive compensation is needed. Annual production: 8,400 kWh. The residential meter only tracks kWh — no power factor penalty applies.

Commercial: 200 kW System with PF Correction

A manufacturing plant in Ohio installs a 200 kW three-phase system. The building has large motors and compressors that pull the facility’s power factor down to 0.82. The utility charges a $0.50/kVAR penalty above a 0.90 PF threshold, costing $2,400/month. The solar inverters are configured to inject reactive power at PF 0.95, raising the facility’s net PF to 0.93. This eliminates the penalty — adding $28,800/year in savings beyond the kWh offset.

Utility-Scale: 50 MW Plant with Grid Support

A 50 MW solar plant in Australia operates under a grid code requiring dynamic reactive power support. The plant’s central inverters provide ±15 MVAR of reactive power to stabilize the local distribution network voltage. During periods of high solar generation, the inverters absorb reactive power (leading PF) to prevent voltage rise. During cloud events when generation drops, they inject reactive power (lagging PF) to prevent voltage sag.

Impact on System Design

Design Consideration	Residential	Commercial	Utility-Scale
Power Factor Billing	No penalty	PF penalty common (PF threshold 0.85–0.95)	Contractual PF requirements
Inverter PF Setting	Unity (1.0) typical	May need 0.90–0.95 per utility	Dynamic Volt-VAR curve required
Reactive Power Value	None	$500–$5,000/month penalty avoidance	Grid ancillary service revenue
Inverter Sizing Impact	Minimal	5–10% oversizing for Q headroom	10–15% oversizing for full Q range
Wiring Impact	Negligible	Size for kVA, not kW	Significant cable sizing implications

Pro Tip

For commercial proposals, always pull 12 months of utility bills and check for power factor penalties before sizing the system. If the customer is paying $1,000+/month in PF surcharges, a solar system with reactive power support can eliminate that cost entirely — a selling point that many competitors miss.

Sources & References

IEEE 1547-2018 — Interconnection standard covering reactive power requirements, Volt-VAR, and power factor settings for distributed energy resources.
NREL Solar Glossary — Definitions of active, reactive, and apparent power in solar energy context.
U.S. DOE SETO — Research on smart inverter grid support functions including reactive power management.

Frequently Asked Questions

What is the difference between active and reactive power?

Active power (measured in kW) performs useful work — it runs equipment, produces light, and heats spaces. Reactive power (measured in kVAR) maintains the magnetic fields needed by motors, transformers, and other inductive devices. Reactive power doesn’t do useful work, but the electrical system must still carry it. Together, they make up apparent power (kVA), which determines how large your inverter and wiring need to be.

Does reactive power affect my electricity bill?

For residential customers, typically no — residential meters only measure active power (kWh). For commercial and industrial customers, yes. Many utilities charge a power factor penalty when the building’s PF drops below 0.85–0.95. This can add hundreds or thousands of dollars per month to the utility bill. A solar system with smart inverter reactive power support can correct the building’s power factor and eliminate these penalties.

Can solar inverters provide reactive power?

Yes. Modern smart inverters can both inject and absorb reactive power. This capability is used to support grid voltage stability (Volt-VAR function) and to correct a building’s power factor. The amount of reactive power available depends on the inverter’s kVA rating and current active power output. When the inverter is producing near its maximum kW output, less reactive power capacity remains available.

What power factor should a solar inverter be set to?

This depends on your utility’s interconnection requirements. Most residential systems operate at unity power factor (PF = 1.0), meaning no reactive power output. Some utilities — particularly in California (Rule 21), Hawaii, and Europe — require smart inverter settings of PF 0.90–0.98 or dynamic Volt-VAR curves. Always check the specific interconnection agreement before configuring the inverter.

How does power factor affect inverter sizing?

When an inverter must provide reactive power, its available active power output decreases. A 10 kVA inverter at PF 1.0 delivers 10 kW. At PF 0.90, it delivers only 9 kW of active power (the remaining 4.36 kVAR goes to reactive support). If you need the full 10 kW of active output plus reactive power support, you need a larger inverter — about 11.1 kVA in this case. Always size based on the required power factor setting, not just the DC array capacity.