Voc (Open-Circuit Voltage)

Quick Facts

What is Voc?

Voc is the voltage across a PV module’s terminals when the circuit is open (no current flows). It is one of the four characteristic points on the module’s I-V curve, along with Vmp, Imp, and Isc.

Relationship to I-V Curve

• Isc at V = 0 (terminals shorted).
• Voc at I = 0 (open circuit).
• Pmax at Vmp × Imp.
• Voc > Vmp; Isc > Imp.

All four values are read at STC off the module datasheet, and all four shift together once the array is running under real irradiance and cell temperature.

Temperature Dependence

Voc(T) = Voc(STC) × [1 + β_Voc × (T − 25°C)]

• β_Voc, the temperature coefficient of Voc: typically −0.0025 to −0.0030 /°C (datasheet value).
• T: cell temperature (use site ambient for cold-day calculation; cell temperature is approx. ambient for no-load conditions).

Cold temperature effect

At −10°C: Voc increases ~9% over STC. At −20°C: Voc increases ~13.5% over STC.

Hot temperature effect

At 65°C: Voc decreases ~12% from STC. At 80°C: Voc decreases ~16.5% from STC.

Worked Example

Module Voc(STC) = 41.6 V, β_Voc = −0.27 %/°C.

Site: Boston, MA. ASHRAE 99.6% min = −18°C.

Voc(−18°C) = 41.6 × [1 + (−0.0027)(−18 − 25)]
           = 41.6 × [1 + 0.1161]
           = 41.6 × 1.1161
           = 46.4 V per module

For NEC 690.7 limit 1,000 V: floor(1000 / 46.4) = 21 modules max in series. This cold-day Voc figure is the input every string sizing calculation starts from, get it wrong and the rest of the string count follows it off a cliff.

Engineering Considerations

• Always use site-specific ASHRAE extreme low: not annual average.
• β_Voc must come from manufacturer datasheet; varies by cell technology.
• TOPCon and HJT modules have tighter (less negative) β_Voc, allowing slightly longer strings in cold climates.
• Cable voltage drop doesn’t apply to Voc check (no current at open circuit).
• Bifacial modules can have slightly higher Voc due to rear-side irradiance effects.

Common Mistakes

1. Using STC Voc without temperature correction.
2. Using annual average minimum instead of extreme low.
3. Forgetting to apply β_Voc as negative when cold.
4. Mixing modules with different Voc on the same string.

Best Practices

• Document the cold-day Voc calculation directly on the SLD, most solar SLD software with NEC auto-generation will flag a string that violates 690.7 before the drawing goes out for permit.
• Cite the ASHRAE table or local code for the cold temperature used.
• Cross-check string sizing software output against the manual formula.
• Design string Voc to 90% of NEC limit for thermal and measurement margin.

Related Reading

The Voc number on a datasheet only becomes real once it’s checked against hardware limits and confirmed in the field. On the inverter side, the maximum DC input voltage rating that a string design must respect traces back to IEC 62109 inverter safety standard, which governs how that ceiling is set and tested.

During commissioning, technicians re-measure open-circuit voltage on every string as part of the solar inverter commissioning protocol to confirm the design-stage Voc calculation held up against as-built conditions. Readers who want the string sizing math laid out end-to-end, from Voc through NEC 690.7 to final module count, can also see it walked through in Qbits Energy’s string sizing glossary entry.

Standards & Certifications

• IEC 60904, STC measurement procedure.
• IEC 61215, Crystalline module qualification.
• NEC 690.7, Maximum system voltage compliance.

Key Takeaways

• Voc is the open-circuit voltage of a PV module at STC; the controlling parameter for NEC 690.7 string sizing.
• Voc rises with falling temperature (typical β = −0.25 to −0.30 %/°C).
• Always temperature-correct using site-specific extreme low (ASHRAE or local code).
• Design to 90% of the NEC limit for safety margin.
• The cold-day Voc calculation must appear on the SLD.

Disclosure: Qbits Energy is a Heaven Designs group company.