The 26 kW photovoltaic (solar cells) plant installed in Rocca di Capri Leone, Sicily. This is a small-medium solar cell power plant, and at this plant the data coming from the Meteosat satellites allows for the monitoring of malfunctions and of irradiance sensor calibration failure. By comparison between satellite and sensor data it has been possible to detect a 10% difference of the measured irradiance, helping optimising the power plant.