Abstract In designing an efficient photovoltaic system, the meteorological data of the system location must be considered. Field data registered over years prove the importance of the implementation site characteristics in the design and optimal exploitation of a PV system, because wetting and atmospheric gaseous and particulate pollution can strongly distort the incident solar radiation. Thus, in addition to the efficiency of the solar module, the tracking efficiency, the clouds crossing factor and the solar radiation loss coefficient into the cloudless atmosphere play an important role in the final energy production. This paper studies the experimental evaluation of the clouds crossing factor for a mountain area. The data are gathered for the Brasov area but, based on the solar charts; the results can be extended on most of the Meridional Carpathian inhabited areas.