The most common planning question in photovoltaics: how many modules may be connected in series in one string? The PV String Calculator (Inverter Sizing) calculates with the coldest day instead of datasheet fair weather – because that is exactly where the risk lies.
The decisive formula
Uoc(Tmin) = Uoc,STC × (1 + TC ÷ 100 × (Tmin − 25)) – a module's open-circuit voltage rises in the cold because the temperature coefficient TC is negative. At −10 °C it is around 10% above the STC value. The sum of all module voltages in the string must never exceed the inverter's maximum input voltage.
Step by step
- Enter module data: Uoc and Umpp from the datasheet, temperature coefficient of Uoc (typically −0.25 to −0.30 %/K)
- Select design temperature: −10 °C is common in Central Europe
- Enter inverter data: maximum input voltage (e.g. 1000 V) and MPP voltage range
- Read the result: maximum module count (cold case) and minimum count so the MPP voltage is not undershot in summer
Worked example
Module with Uoc = 45.1 V, TC = −0.28 %/K: at −10 °C the open-circuit voltage rises to 45.1 × (1 + 0.0028 × 35) = 49.52 V. On a 1000 V inverter, a maximum of 20 modules is therefore permissible (990 V) – with 21 modules (1040 V) the limit would be exceeded on a frosty morning.
Why this matters so much
Overvoltage at the DC input is one of the few mistakes that destroys an inverter instantly and irreparably – and the warranty is void, because the string design is verifiably documented. Conversely, a string that is too short costs yield when the MPP voltage slips below the tracker's window on hot days. The calculation does not replace sign-off by a qualified planner, but it provides the solid basis for it.
