Photovoltaic Power Production is increasing nowadays. Forecast information is essential for an efficient use, the management of the electricity grid and for solar energy trading. Solar Photovoltaic systems transform solar energy into electric power. The power output depends on the incoming radiation and on the solar panel characteristics. To achieve an accurate forecast we need to have a deep knowledge of the Sun´s path, the atmosphere´s condition and the scattering processes. We also have to take into account the characteristics of the solar plant.