Active and Reactive Power
Active and Reactive Power are two fundamental components of AC electrical power flow that determine how solar systems interact with the electrical grid. In solar PV systems—especially grid-tied systems using modern inverters—understanding the balance between active and reactive power is essential for system stability, voltage control, interconnection compliance, and energy delivery.
Active power (P) is the real, usable power that performs actual work: running appliances, charging batteries, powering lighting, and feeding the grid.
Reactive power (Q) does not perform “useful” work, but it is required to maintain voltage levels and electromagnetic fields for motors, transformers, and grid stability.
Solar inverters must manage both components when converting DC to AC. As solar adoption grows, utilities increasingly require inverters to provide reactive power support. Modern design tools like Solar Designing incorporate this behavior when modeling AC output and grid interaction.
Key Takeaways
- Active power (P) performs real work, measured in kW.
- Reactive power (Q) supports voltage and grid stability, measured in kVAR.
- Inverters must manage both to meet grid code requirements.
- Poor power factor reduces system efficiency and may incur penalties.
- Understanding active and reactive power is essential for inverter sizing, interconnection design, and grid-tied PV operation.
What Are Active and Reactive Power?
Active Power (P)
Measured in watts (W) or kilowatts (kW), active power is the real power that flows through the AC system to supply useful energy.
Solar PV systems aim to maximize active power production because it directly impacts:
- Site energy yield
- Net metering value
- System ROI
- Utility export
Whenever you look at a system’s “production in kWh,” you’re looking at active power over time.
Reactive Power (Q)
Measured in VAR (Volt-Ampere Reactive) or kVAR, reactive power maintains voltage and the magnetic fields required by:
- Motors
- Transformers
- Inductive loads (HVACs, compressors, pumps)
Reactive power does not produce usable work but is essential for power system stability. Without it, voltage would collapse, and equipment across the grid could malfunction.
Modern solar inverters can supply or absorb reactive power to support the grid—known as Volt/VAR control.
Foundational related concepts include Power Factor, Voltage, and Solar Inverter.
How Active and Reactive Power Work
1. Solar panels generate DC power
Panels produce DC power, which contains only active power because DC has no frequency component.
2. Inverters convert DC → AC
During conversion, the inverter must match the grid's:
- Frequency (50 or 60 Hz)
- Voltage
- Waveform
3. Inverter injects active power into the grid
This is the usable energy consumers rely on.
4. Inverter supplies or absorbs reactive power
Reactive power is used to maintain voltage stability.
Utilities often require solar inverters to:
- Correct power factor
- Support local voltage
- Prevent overvoltage on feeder lines
- Respond to grid disturbances
5. Power Factor ties active and reactive power together
Power Factor (PF) = Active Power (kW) ÷ Apparent Power (kVA)
A power factor close to 1.0 means the system is efficient.
Types / Variants
1. Active Power (P)
Delivered power (kW) that performs measurable work.
2. Reactive Power (Q)
Non-working power (kVAR) necessary for voltage support.
3. Apparent Power (S)
Combination of active + reactive power, measured in kVA.
4. Leading and Lagging Reactive Power
- Lagging: supplied by inductive loads like motors
- Leading: supplied by capacitive equipment or inverters
Solar inverters can generate both to support the grid.
How Active and Reactive Power Are Measured
Active Power (P)
- Units: W, kW, MW
- Calculated as:
P = V × I × PF
Reactive Power (Q)
- Units: VAR, kVAR, MVAR
- Calculated as:
Q = V × I × sin(θ)
Apparent Power (S)
- Units: VA, kVA, MVA
- Calculated as:
S = √(P² + Q²)
Power Factor (PF)
Shows how effectively the system uses active power.
See Power Factor.
Typical Values / Ranges
Residential PV Systems
- Active power: 3 kW – 15 kW
- Reactive power: ±10% to ±20% of inverter capacity
Commercial Rooftop PV
- Active power: 50 kW – 2 MW
- Reactive power: ±20% to ±33%
Utility-Scale Solar
- Inverters may supply ±50% reactive power
- Must follow strict grid codes (IEEE 1547, UL 1741 SA)
Typical system PF targets range from 0.95 lagging to 0.95 leading depending on utility requirements.
Practical Guidance for Solar Designers & Installers
1. Always check inverter reactive power capability
Different inverters provide different VAR support.
2. Confirm utility power factor requirements
Many utilities require exporting at 0.98 PF or better.
3. Model AC behavior in design software
Tools like Solar Designing show how reactive power affects system sizing and compliance.
4. Use reactive power to reduce overvoltage issues
Helpful for long feeder lines or high PV penetration zones.
5. Consider AC cable sizing and voltage drop
Reactive power increases current flow—use the Voltage Drop Calculator.
6. Validate inverter settings during commissioning
Volt/VAR curves must match utility interconnection agreement.
7. Understand that reactive power reduces active power at times
Inverters have a total kVA capacity; absorbing or supplying reactive power reduces available kW.
Real-World Examples
1. Residential PV with Overvoltage Issues
A 7 kW system in a dense neighborhood experiences inverter shutdown due to high feeder voltage.
Reactive power control is enabled, allowing the inverter to absorb VARs and stabilize voltage, eliminating shutdowns.
2. Commercial 500 kW Rooftop System
Utility requires operation at 0.98 lagging PF.
Designers size inverters accordingly and configure their Volt/VAR settings to meet interconnection rules.
3. Utility-Scale Solar Farm
A 50 MW project provides ±30% reactive power support to manage voltage fluctuations on a weak rural grid.
This capability becomes essential for meeting IEEE 1547 requirements.
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