Open Circuit Voltage

What is open circuit voltage (Voc)

Open Circuit Voltage (Voc) is the voltage across the terminals of a solar PV module when no current is flowing because no electrical load is connected. It is the maximum voltage the module produces under the given conditions. At STC (1000 W/m² irradiance, 25°C cell temperature, AM1.5G spectrum), modern modules typically have Voc of 41-50 V.

Voc is determined by the cell technology (Voc per cell at STC is approximately 0.6-0.7 V for modern silicon cells) and the number of cells in series. A 60-cell PERC module has Voc ≈ 60 × 0.61 = 36.6 V. A 144 half-cell module has Voc ≈ 144 × 0.34 = 49 V (each half-cell ~0.34 V).

For solar system design, Voc is critical for string sizing. Cold-temperature Voc determines the maximum number of modules per string, bounded by the inverter’s absolute maximum DC input voltage. Exceeding this limit on the first cold morning destroys the inverter. See Solar String Sizing and Overcurrent Protection India, EPC Guide for the full 5-step sizing method EPC teams use to avoid this.

Why Voc matters

String sizing. Cold Voc bounds maximum string length.

Inverter safety. Exceeding inverter max DC input is catastrophic.

System voltage class. Sets DC bus voltage operating range, which the inverter’s MPPT window must also accommodate.

Temperature derating. Cold morning Voc rise must be calculated.

CEIG approval. SLD must demonstrate string Voc within limits.

Cold Voc calculation

Where:

• β = Voc temperature coefficient (typically -0.0027 for PERC)
• T_min = minimum expected cell temperature in operation

For typical Indian conditions with T_min = 5°C: Voc(cold) = Voc × (1 + (-0.0027) × (-20)) = Voc × 1.054

So cold Voc is ~5.4 percent higher than STC Voc. Colder zones see a much larger spike, up to about 12.2 percent above STC at -20°C (0.0027 × 45) in Himalayan winter sites, as detailed in Inverter Voltage: String Voltage vs MPPT Voltage in Solar Inverters India 2026.

Technical deep dive

Voc by cell technology

Module configurations

String Voc calculation

String Voc(cold) = N × Voc_module(cold)

Must stay below inverter max DC input (e.g., 1000 V residential, 1500 V utility).

Indian regional Tmin assumptions

Real-world applications

String sizing calculations.

Inverter selection.

CEIG drawing approval, coordinated with agencies such as Heaven Designs’ electrical and CEIG drawing service.

System voltage class selection (1000 V vs 1500 V).

Best practices

Apply cold Voc derate using site-specific T_min.

Include 5 percent safety margin on max DC input.

Verify with QBits Energy’s string sizing calculator or the inverter manufacturer’s own tool.

Document Voc values used in calculations.

Use cell-technology-specific β.

Indian market context

ALMM-listed modules report Voc and β in datasheets.

Indian solar inverters max DC input: 600 V residential, 1100 V C&I, 1500 V utility.

CEIG drawing approval verifies string Voc compliance.

Standards and certifications

Key takeaways

Open Circuit Voltage (Voc) is the maximum voltage a solar module produces with no current flowing. Modern modules: 41-50 V at STC. Voc rises in cold weather: ~5 percent higher at 5°C than at STC. String Voc is bounded by inverter maximum DC input. Indian cold-temperature derating uses site-specific T_min. CEIG drawing approval verifies string sizing compliance.

Further reading

For how Open Circuit Voltage plays out in real projects, these guides go deeper:

• String Voltage vs MPPT Voltage: Inverter Guide
• Solar String Sizing Software: Complete 2026 Guide
• Solar Inverter Sizing: 12 Key Questions (2026)
• Solar String Sizing & Overcurrent Protection

Sources

• IEC 60904, PV device measurement.
• IEC 61215, Module qualification.
• IS 14286, Indian module standard.
• ITRPV Roadmap, latest edition.
• Manufacturer datasheets from leading module brands.
• NREL PV Performance Studies.
• IEA PVPS Task 13 Reports.
• IEC 62548, PV array design.
• Inverter manufacturer string sizing tools.