Maximum open-circuit voltage U_{OC MAX}

The datasheet voltage U_{OC STC} is measured at 25 °C. In winter the panel is colder and its voltage is higher. DSTU HD 60364-7-712:2022 (Annex B.1) sets how to account for it:

Here α_Uoc is the panel’s voltage temperature coefficient from the datasheet (%/K, negative) and T_min is the lowest air temperature of the site. If either is unknown, the standard requires U_{OC MAX} = 1.2·U_{OC STC}. Clause 712.512.1.1 says that U_{OC MAX} is taken as the rated voltage when selecting devices for a PV array.

Example for Kyiv (T_min = −40 °C) and an illustrative panel with U_oc 49.5 V, α −0.25 %/K: K_U = 1.1625, U_{OC MAX} = 57.5 V per panel. A string of 13 such panels gives 747.5 V, below 800 V. Without α and temperature data the 1.2 rule gives 59.4 V per panel and 772.2 V for 13 panels. A fourteenth panel in the first case would give 805 V, above the 800 V limit.

U_{OC MAX} above the inverter’s maximum input voltage is one of the few design errors that can destroy an inverter on the first frosty morning. That is why string length is always counted from the cold end, not from STC.

Where α_Uoc comes from: the panel datasheet gives it as the temperature coefficient of open-circuit voltage, usually in %/°C. If the maker gives it in mV/K, convert it to a percentage of U_oc. If you enter your own U_oc instead of the catalogue value and do not know α, it is more honest to use the 1.2 rule, which gives a larger margin. Everything in the DC circuit is checked against U_{OC MAX}: the inverter input, isolators, fuses, connectors. Each device's rated voltage must be at least the string's U_{OC MAX}. A quick check of a panel and inverter pair can be done in the string calculator.

Solispec computes U_{OC MAX} for each MPPT input: K_U from the catalogue panel’s α and the site T_min, or 1.2·U_{OC STC} when α or T_min is missing. The report shows which rule was used. The engine caps panels per string at ⌊V_max / U_{OC MAX}⌋ of a panel. If a string’s U_{OC MAX} exceeds 1000 V, it asks you to check the maximum system voltage of the panels and devices against their datasheets (cl. 712.512.1.1).

String calculator