Solar PV
Panel voltage rises in cold weather — a detail that matters for isolator and cable voltage rating, not just string length.
It's a common intuition that heat is the main enemy of solar system performance — and for power output, that's broadly true (see the temperature coefficient discussion elsewhere in OhmWorks content). For string voltage specifically, though, cold weather is the more demanding condition.
A solar panel's open-circuit voltage (Voc) has a negative temperature coefficient — meaning voltage increases as cell temperature decreases below the 25°C standard test condition reference. On a cold, clear morning, a string's actual Voc can run meaningfully higher than its datasheet STC rating suggests.
Cable voltage rating and DC isolator voltage rating both need to be selected against the string's maximum realistic Voc — calculated using the module's actual voltage temperature coefficient and the site's genuinely coldest expected temperature, not the STC datasheet figure alone.
V(max) = V(STC) × N(panels in series) × [1 + β × (T(min) - 25)] β = voltage temperature coefficient (%/°C, negative) T(min) = lowest realistic site temperature
Getting this wrong in the direction of underestimating string voltage is a genuine safety risk — an isolator or cable rated below the string's real maximum voltage is a compliance and safety failure waiting for a cold morning to expose it.
Engr. Jason Morales — Founder, SolarEnergyPH & OhmWorks