Solar radiation is electromagnetic energy emitted by the fusion of hydrogen in the sun; such energy does not reach the Earth’s surface in a constant manner and its amount varies during the day, between seasons and depends on the cloudiness, from the angle of incidence and the reflectivity of the surfaces. The radiation that a square meter on a horizontal surface receives is called global radiation and is the result of the sum of the direct and diffuse radiation. The direct radiation is that which reaches the Earth directly from the sun while the diffuse radiation is that which is reflected by the sky, clouds and other surfaces. Direct radiation is present only when the sun is well visible. During the winter diffuse radiation is higher in percentage and on an annual basis and is equal to 55% of the global total. In the calculation of dimensions of the photovoltaic solar systems it is often opportune to take into consideration the amount of solar radiation that is reflected by the surfaces near the photovoltaic modules (Albedo’s coefficient).
The intensity of the solar radiation on the ground depends on the inclination angle of the radiation itself: the smaller the angle that the sun rays form with the horizontal surface and greater the amount of atmosphere they have to travel means consequent lower radiation that reaches the surface. As we have seen, a surface received the maximum amount when the solar rays hit perpendicularly on it. The sun’s position varies during the day and during the seasons and thus also the angle with which the sun’s rays hit a surface varies. The amounts thus depend on the position and inclination of the PV modules. A horizontal surface receives the maximum amount of energy during the summer when the sun’s position is high and the days are long while a vertical surface set toward the south receives higher amount during the winter than during the summer – about 1.5 times more than the horizontal surface. The best orientation of a vertical or inclined surface that must capture the most sun rays is thus that one set towards a southern direction.