String sizing: Voc in the cold, Vmp in the heat.

The open-circuit voltage of a solar cell rises as the cell gets colder. The highest voltage in a string therefore occurs on a cold, clear morning just after sunrise, before the inverter has started operating: the panels are cold and the string is unloaded. That voltage must not exceed the inverter’s maximum permitted DC voltage, nor the panel’s maximum system voltage.

In the other direction, the voltage at maximum power (Vmp) falls as the panels heat up. On a hot summer day a short string can drop below the lower limit of the MPPT window. The inverter can then no longer track the panels’ maximum power point and output falls – just when the sun is strongest. This is a question of yield, not of safety.

The short-circuit current is the hard limit. The sum of Isc for the strings connected in parallel on one MPPT must not exceed the input’s maximum short-circuit current in the datasheet. The datasheet also says how many strings each input takes, and the inputs can differ.

The maximum usable input current is a separate limit. If the operating current is above it, the inverter limits the current and shifts the operating point. The result is lower output when the sun is high, not a fault. High-current panels, like the one in the example, can exceed the limit with a single string.

Only connect strings of the same length and the same roof orientation to one MPPT. Parallel strings share the same voltage, so a shorter string, or one facing another way, pulls the other away from its maximum power point.

The basis is the same across Europe: CENELEC’s HD 60364-7-712:2016 on PV installations, with a normative Annex B on how the maximum open-circuit voltage (UOC MAX) and the maximum short-circuit current (ISC MAX) are determined. CENELEC’s member countries adopt it in their own installation rules. The international original, IEC 60364-7-712, came out in a new edition in 2025. The lowest temperature is chosen for the site; the national practice we have been able to verify is set out below.

Sweden: section 712 of SS 436 40 00:2023 (edition 4), the electrical installation rules from SEK Svensk Elstandard. We have not found a national standard temperature for the Voc calculation. Winters differ a great deal between the south and the north of the country, so start from the lowest local temperature and check that −25 °C is enough where you are building.

Germany: DIN VDE 0100-712:2016-10, the German edition of HD 60364-7-712:2016. SMA’s technical information on system design uses an outdoor temperature of −10 °C for systems in Germany as its example and marks −10 °C and +70 °C cell temperature as the usual design range, noting that the mounting and the local temperature have to be taken into account.

Spain: the low-voltage regulations REBT (Real Decreto 842/2002), in which ITC-BT-40 covers low-voltage generating installations. Under CENELEC rules, HD 60364-7-712 is to be adopted in Spain as well. We have not found an official standard temperature. The difference between the coast and the inland plateau is large, so start from the lowest local temperature.

Italy: CEI 64-8 part 7-712 for the installation, and CEI guide 82-25 for the design, construction and operation of PV systems. In Italian design documents, −10 °C and 70 °C are common check points: Voc at −10 °C against the maximum voltage of the inverter and the panel, Vmp at 70 °C against the lower limit of the MPPT window. That is practice; in mountain areas the lowest local temperature can be lower.