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String length is limited by open circuit voltage on the coldest morning, because Voc rises as temperature falls — typically 0.25-0.35% per degree below the 25 C rating. The cold Voc multiplied by module count must stay below the inverter's maximum DC voltage, or the inverter is destroyed.
Cold Voc
Voc_cold = Voc_STC x (1 + temperature coefficient x (T_min - 25))
String length
Maximum modules = floor(inverter maximum DC voltage / Voc_cold)
Photovoltaic string design must comply with the applicable electrical installation standard and the inverter manufacturer's limits. Exceeding maximum DC voltage is a fire and shock hazard.
The record minimum ambient at the site, not the average winter minimum. Standards typically require the lowest expected cell temperature, which for a clear cold morning approximately equals ambient.
Yes. At maximum cell temperature the string's maximum power point voltage must stay above the inverter's MPPT window minimum, or the inverter cannot track and output collapses.