Open Circuit Voltage

Voc is measured with the module terminals open (no current flowing). It represents the maximum voltage the module can produce and is the starting point for PV string design. Since Voc increases as cell temperature decreases, string voltage reaches its maximum on a cold, clear morning.

Temperature Dependence

Voc has a negative temperature coefficient, typically −0.25 to −0.35%/°C for crystalline silicon. On a cold winter morning in Shimla (−5°C), Voc can be 10–15% higher than at STC (25°C). String sizing must ensure that even in the coldest expected conditions, the combined string Voc does not exceed the inverter's maximum input voltage (typically 1000–1100 V DC for commercial inverters).

String Length Formula

Max strings in series = floor(Inverter max DC input V / (Voc_STC × (1 + TC_Voc × (T_min − 25)))

This calculation is mandatory per IEC 62548 and IEC 62446-1 commissioning documentation requirements.