Hello André, According to the post you sent me, it sais that the Voc(Tmin) requirement corresponds to: - Choosing 1000 W/m2 means that in worst case, in winter the sun may suddenly appear on the array (cloud), and in this case the cell's temperature may be the ambient temperature. These choices are a universal common practice, adopted by everyone (not only PVsyst users). However I don't know whether they are mentioned in an IEC norm about system sizing. As I understand, PVsyst takes into account the coldest day of winter and considers that the minumum temperature will be during the day while the sky is cloudy. When there are clouds the irradance is not zero, so the cell temperature will not the same as the ambient temperature according the theorical formula (according to IEC 60891). The cell temperature will be the same just when there is no sunlight (or very low irradiance). On other hand, to suppose that the irrandiance will be 1000W/m2 when the minimum ambient temperature is done, is a consideration very conservative. It will depend on each placement and it would be interesting that the irradance could be changed for the maximum Voc scenario (like the reference low temperature can be modificaded). The IEC 60891 tells the procedure for temperature and irradiance corrections, so I consider that PVsyst should allow to change the irradance for the maximum Voc calcule. With a historical meteorological data is easy to know the irradance at the lowest temperatures. Thank you in advance.