To eliminate the effects of varying local features, such as obstructions that cast shadows and the specific ground covering, solar radiation is routinely measured on horizontal surfaces free of obstacles. Consequently, solar radiation data are most often given in the form of global radiation on a horizontal surface. Since PV modules are usually positioned at an angle to the horizontal plane, the radiation input to the system must be calculated from the data.
The assessment of radiation arriving on an inclined surface, using as input global horizontal data, raises two main problems: to separate the global horizontal radiation into their direct and diffuse components; and, from them, to estimate the radiation components falling on an inclined surface. In general, these problems may be posed for different time scales, for example, daily irradiation, hourly irradiation and so on. Individual or time-averaged values may be sought. Here, we will first focus on the monthly average daily irradiation values. This is not only convenient for presentation purposes, but also coherent with solar radiation data availability, and particularly suited to most PV engineering practical problems. Additional comments for other cases will also be given afterwards.