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.

Temperature Dependence

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

• β_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.

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.
• 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.

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.