Voltage

Multiply the module's datasheet Voc (at STC, 25°C) by the number of modules in the string, then apply the temperature coefficient. For c-Si modules, Voc decreases by approximately 0.3% per °C above 25°C. At 50°C module temperature, a 30-module string rated at 49V Voc per module would be expected at approximately (49 × 30) × (1 − 0.003 × 25) = 1360V rather than 1470V STC.

A Voc reduction equal to precisely one module's contribution — at the temperature-corrected per-module Voc — almost always indicates an open-circuit module in the string. Causes include a failed or blown junction box fuse, an open-circuit MC4 connector, a cracked cell across the full module width, or a stuck-conducting bypass diode that has short-circuited the module's voltage contribution. Walk the string with a multimeter to locate where the voltage drops.

Voc is the open-circuit voltage — measured with no load connected. Vmpp is the voltage at maximum power point during normal operation. Vmpp is typically 75–80% of Voc for crystalline silicon modules. Field IV curve testing captures Voc at the right end of the curve and Vmpp at the knee. Voc is used to verify string configuration; Vmpp verifies the inverter is operating at the correct point on the curve.

At open circuit, Voc is primarily temperature-dependent. Strings in different rows, at different heights, or with different soiling levels will have different module temperatures and therefore different Voc readings — even under identical irradiance. A variation of 1–3% between strings is normal if module temperatures differ. Variations greater than 5%, or discrepancies not explained by temperature, warrant investigation for missing modules or wiring errors.