As regards Italy, there are medium-height solar radiation regimes with a big varia­tion between northern and southern regions. Keeping in mind that the parameters required to determine univocally the position of an intercepting surface are the surface’s inclination and its azimuth orientation, we now list the principal sources for the retrieval of radiation data.

‘La radiazione globale al suolo in Italia’ [10], a paper edited by ENEA, is with­out any doubt the bibliographic source which supplies, on a national level, the most detailed information about the average global radiation (i. e. the one which includes direct, diffuse and reflected components) received on a square metre of a horizontal surface per month and year. The same kind of information, which refers to a smaller number of Italian places, is also provided by UNI ISO 10349 international norms.

‘L’atlante europeo della Radiazione Solare’ [11] is without any doubt one of the most authoritative sources for the valuation of solar radiation received in a certain period of time on a surface which is exposed in any manner. This atlas gathers all the data supplied by national metrological offices. These data, gathered in maps and tables, are the result of a 10-year study. The atlas is divided into two volumes: the first takes into consideration the horizontal surfaces and the second the inclined sur­faces. The first volume reports, for every Italian place we consider, the values of the daily average radiation expressed in W-h/m2 or in kW-h/m2. Every place is charac­terized by its latitude, longitude and height above sea level.

As regards the design of solar panels, the results of the second volume appear to be much more interesting than the first. As a matter of fact, solar panels are usually arranged with a certain inclination on a horizontal plane, and the second volume reports the values of the daily average radiation (global and diffuse) per month and year for different positions of the intercepting surface. However, in the European Atlas of Solar Radiation, only the principal cities of each country are mapped.

To obtain the values of radiation received on variously oriented and inclined sur­faces, there are a few algorithms (which can be easily found in the currently avail­able design software) among which the most well known and used is that of Liu and Jordan which is discussed in par. 11, 12 and 13. The steps to get a correct extrapo­lation of the data for a surface which is positioned in any manner, starting from the values for a horizontal surface, are outlined in the UNI 8477 norms (first part) [4, 5].