Site minimum temperature T_min

In DSTU HD 60364-7-712:2022 (Annex B.1), T_min is the lowest ambient air temperature at the PV installation site. Not a winter average and not the heating design temperature, but the absolute minimum. A panel’s open-circuit voltage rises in frost, so this temperature decides how many panels fit into a string without exceeding the inverter’s voltage limit.

Where the value comes from: in Ukraine, absolute minima are given in DSTU-N B V.1.1-27:2010 (Table 1) by climatic region. For Kyiv (region I) it is −40 °C. Example for an illustrative panel with U_oc = 49.5 V and α = −0.25 %/K: K_U = 1 + (−0.25/100)·(−40 − 25) = 1.1625, so in frost one panel gives up to 57.5 V. At −25 °C the same factor would be 1.125 and the voltage 55.7 V; 1.8 V per panel on an 18-panel string adds up to more than 30 V.

If T_min is unknown, the standard does not let you guess it: U_{OC MAX} is then taken as 1.2·U_{OC STC}. The −25 °C found in older tools and tables is too optimistic for most of Ukraine.

A few practical notes. The standard uses the site's air temperature, not the module's, and the absolute minimum at that. T_min is only needed for the upper string limit. For the lower end of the MPPT window heat matters instead, and Solispec uses the panel's hot operating voltage there. If you have weather observations near the site with a lower minimum, you can enter them in the T_min field. Do not use heating design temperatures from other tables: they are not the absolute minimum and are warmer than it. The lower T_min, the shorter the allowed string, so rounding 'towards warm' always works against safety.

In Solispec T_min comes from: your value in the field if you entered one; otherwise the minimum of the climatic region at the site coordinates; if no site is placed, the 1.2·U_{OC STC} rule. The MPPT check line states which rule was used: "K_U = 1 + α·(T_min − 25)/100 = …" with the temperature and its source, or "1.2·U_oc STC (site T_min not set)".

String calculator