Size PV strings correctly: check open-circuit voltage at low temperature against inverter maximum DC voltage and the MPP window - with min/max module count.
Fill in the fields below and click "Calculate".
Sizing PV strings: the cold limit decides
A PV module's open-circuit voltage rises as temperature falls – the Voc temperature coefficient is negative. The most critical operating point is the sunny winter morning: cold modules, inverter not yet on grid, full open-circuit voltage at the DC input. If the string voltage then exceeds the inverter's maximum DC input voltage, damage and loss of warranty loom.
The design formula
Uoc(Tmin) = Voc,STC × (1 + TC/100 × (Tmin − 25 K))
Example: a module with Voc = 45 V and TC = −0.29 %/K reaches 45 V × (1 + 0.0029 × 35) = 49.57 V at −10 °C. Twelve modules per string give 594.8 V – uncritical for a 1000 V inverter. The upper limit here would be 20 modules (991 V).
The second limit: the MPP window in summer
In midsummer cells reach 65–75 °C and the MPP voltage drops accordingly: 38 V × (1 − 0.0029 × 45) = 33.0 V per module. The string must still stay above the inverter's lower MPP limit, otherwise yield is lost. With MPP-min = 200 V at least 7 modules are needed.
FAQ
Does this apply to all module types? Yes – Voc, Vmpp and the temperature coefficient come straight from the datasheet; the physics is identical for mono, poly and glass-glass modules.
What about current? String current equals module current and must stay below the inverter input's max MPP current – with common designs, voltage is the limiting factor.
