Key Takeaways
- Autonomously adjusts reactive power (VARs) to keep grid voltage within acceptable limits
- Required by IEEE 1547-2018 for all new grid-connected inverters in the U.S.
- Absorbs VARs when voltage is high and injects VARs when voltage is low
- Operates continuously without curtailing real power output in most conditions
- Often paired with volt-watt mode for comprehensive voltage management
- Settings are configured through inverter parameters based on utility requirements
What Is Volt-VAR Mode?
Volt-VAR mode is a smart inverter function that automatically adjusts the amount of reactive power (measured in volt-amperes reactive, or VARs) the inverter produces or absorbs based on the local grid voltage at the point of connection. When grid voltage rises above the target range, the inverter absorbs reactive power to help pull voltage down. When voltage drops below the target, it injects reactive power to push voltage up.
This autonomous voltage regulation happens continuously and transparently, without any action from the system owner or the utility. The inverter follows a predefined curve — called a volt-VAR curve — that maps voltage levels to reactive power setpoints.
Volt-VAR mode is the first line of defense against voltage issues on circuits with high solar penetration. It addresses voltage fluctuations without reducing real power output, preserving energy production and customer savings.
How Volt-VAR Mode Works
The volt-VAR function follows a configurable response curve that defines inverter behavior at different voltage levels.
Voltage Measurement
The inverter continuously measures AC voltage at the point of common coupling (PCC) — typically the service entrance or meter location.
Curve Lookup
The measured voltage is compared against the configured volt-VAR curve. The curve defines how much reactive power to inject or absorb at each voltage level.
Reactive Power Adjustment
The inverter adjusts its reactive power output accordingly. At high voltage, it absorbs VARs (lagging). At low voltage, it injects VARs (leading). Within the deadband, it produces zero reactive power.
Continuous Operation
The process repeats every few cycles (milliseconds), providing real-time voltage support without waiting for utility commands or manual intervention.
Q_range = S_rated × sin(arccos(Power Factor)) — typically ±44% of rated apparent power at 0.9 PFThe Volt-VAR Curve
The volt-VAR curve is the core configuration element. It defines the relationship between measured voltage and reactive power response.
| Voltage Region | Inverter Action | Purpose |
|---|---|---|
| Below V1 (e.g., 0.92 pu) | Maximum VAR injection (leading) | Boost low voltage |
| V1 to V2 (e.g., 0.92–0.98 pu) | Proportional VAR injection | Gradual voltage support |
| V2 to V3 — Deadband (e.g., 0.98–1.02 pu) | Zero reactive power | No action needed, voltage is normal |
| V3 to V4 (e.g., 1.02–1.08 pu) | Proportional VAR absorption | Gradual voltage reduction |
| Above V4 (e.g., 1.08 pu) | Maximum VAR absorption (lagging) | Suppress high voltage |
Volt-VAR curve parameters vary by utility. When configuring inverters in solar design software, use the specific curve settings required by the interconnecting utility. Default IEEE 1547 Category B settings are a safe starting point, but many utilities specify custom curves.
Volt-VAR vs. Volt-Watt
These two smart inverter functions work together but address voltage differently.
Volt-VAR Mode
Adjusts reactive power to manage voltage. Does not curtail real power output under normal conditions. The preferred first response because it preserves energy production and customer savings.
Volt-Watt Mode
Reduces real power output when voltage exceeds critical thresholds. Only activates when volt-VAR alone cannot control voltage. Results in lost energy production and reduced customer savings.
Practical Guidance
Volt-VAR configuration affects system design, installation, and customer expectations.
- Check utility-specific volt-VAR requirements. Each utility may specify different curve parameters, deadband widths, and response times. Get the exact settings from the interconnection agreement.
- Size inverters with reactive power capacity in mind. Volt-VAR operation uses a portion of the inverter’s apparent power capacity. Ensure the inverter is sized to deliver rated real power while also providing required VARs.
- Document settings in the design package. Include volt-VAR curve parameters in the design documentation so installers configure the inverter correctly during commissioning.
- Consider grid voltage at the service point. Sites at the end of long distribution feeders are more prone to voltage issues. These systems benefit most from properly configured volt-VAR operation.
- Configure volt-VAR during commissioning. Don’t leave inverters on default settings. Program the utility-required volt-VAR curve before completing the interconnection process.
- Verify settings with monitoring data. After commissioning, check inverter monitoring to confirm volt-VAR is operating as expected. Look for reactive power output that responds to voltage changes.
- Keep firmware updated. Smart inverter functions including volt-VAR may receive firmware updates that improve performance or add utility-required features. Schedule updates during routine maintenance.
- Document configuration for the utility. Some utilities require proof of volt-VAR configuration as part of the interconnection approval. Screenshot the inverter settings during commissioning.
- Reassure customers about production impact. Volt-VAR mode does not reduce energy production under normal conditions. Customers can be told their system will produce at full capacity while also supporting grid stability.
- Explain it as a compliance requirement. IEEE 1547-2018 mandates volt-VAR capability. Frame it as a standard feature of modern solar software and equipment, not an optional add-on.
- Differentiate from curtailment. Some customers confuse volt-VAR with power curtailment. Clarify that reactive power adjustment is different from reducing real power output.
- Use it as a selling point for grid-friendly solar. Utilities increasingly prefer solar installations with smart inverter functions. Volt-VAR compliance can speed up interconnection approvals.
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Frequently Asked Questions
Does volt-VAR mode reduce solar energy production?
No, not under normal conditions. Volt-VAR mode adjusts reactive power, which is separate from the real power (watts) that generates energy and bill savings. The inverter can provide or absorb VARs while still producing its full rated real power output. Only in rare cases where the inverter reaches its apparent power limit might a small real power reduction occur.
Is volt-VAR mode required for all solar inverters?
IEEE 1547-2018 requires all new grid-connected distributed energy resource inverters to be capable of volt-VAR operation. However, whether the function is activated and what specific curve settings are used depends on the local utility’s requirements. Some utilities require it to be enabled at commissioning; others allow it to be activated later via remote command.
What is the difference between volt-VAR and volt-watt?
Volt-VAR adjusts reactive power to manage voltage without affecting energy production. Volt-watt mode reduces real power output when voltage exceeds critical limits. Volt-VAR is the primary voltage management tool; volt-watt acts as a secondary backup that only activates when reactive power alone cannot keep voltage within limits. Both functions often operate simultaneously.
About the Contributors
Content Head · SurgePV
Rainer Neumann is Content Head at SurgePV and a solar PV engineer with 10+ years of experience designing commercial and utility-scale systems across Europe and MENA. He has delivered 500+ installations, tested 15+ solar design software platforms firsthand, and specialises in shading analysis, string sizing, and international electrical code compliance.
CEO & Co-Founder · SurgePV
Keyur Rakholiya is CEO & Co-Founder of SurgePV and Founder of Heaven Green Energy Limited, where he has delivered over 1 GW of solar projects across commercial, utility, and rooftop sectors in India. With 10+ years in the solar industry, he has managed 800+ project deliveries, evaluated 20+ solar design platforms firsthand, and led engineering teams of 50+ people.