I. Introduction
The Interest toward renewable energy, such as solar radiation, wind, and tides, has been hugely increasing in the last years because of a growing attention with respect to the problems related to the environmental pollution. The employment of such energy sources allows the use of fossil fuels to be drastically reduced, thus reducing the emission of greenhouse gases in the atmosphere: every gigawatthour of energy produced thanks to renewable sources prevents up to of emissions. One of the most interesting renewable sources is the solar radiation, which allows thermal and electrical energy to be obtained. The latter, which is obtained by means of photovoltaic (PV) cells, is more useful, since it can be used at the production site as well as at great distances thanks to the possibility to integrate a PV plant into the mains. For this reason, in the last decade, PV modules based on several technologies have been developed, which are characterized by different cost and performance. Nameplate specifications are usually available for these PV modules that refer to the standard test conditions (STC: solar irradiance 1 , temperature 25 °C, air mass 1.5), but very poor data are available that refer to the long-term drift of the module specifications: this is mainly important for the module efficiency, which is employed to estimate the producibility of a PV plant and, consequently, the payback time of the initial investment. A drift of the module efficiency could be taken into account in order to obtain a more realistic estimation.