Power of the Sun.

After these necessary preambles, I must write about the sun, because it represents the starting point for the new technologies development.

Leaving aside technical description of our star as the diameter and composition, to analyze the properties of solar energy we should consider the radiation emissions, and for this reason we define the spectral irradiance Is {[W/(m² · μm)] o [W/(m² · nm)]}. It's the power incident on the surface area per unit time per wavelength. Integrating we get the uniform irradiance G {[W/m²]}.

We know that the Earth's orbit is slightly elliptical with an average distance equal to 149.5 x 10⁹. The average irradiance outside the atmosphere on a plane perpendicular to rays is 1367 W/m², and it's called solar constant Gcs. This value was proposed for the first time by Samuel Pierpont Langley and then it was refined between 1322 W/m² and 1412 W/m² , depending on the terrestrial latitudes.

Through the atmosphere, the solar radiation decreases; it is partly absorbed, partly reflected and partly diffused. This energy loss is defined by the atmosphere thickness that sun's rays have to travers measured at sea level, and it's known as Air Mass, AM. At mediterranean latitudes it is 1.5, with a value of irradiance G about 963 W/m².

A model called ASTM G173-03 was created to represent the spectrum solar irradiance on a surface in the one and only one set of standard conditions and orientation. [1]
More detailed information can be found at:
ASTM G173-03 AM 1.5
 
Here reported it's a plot from the same site.

Solar radiation dies at 2500 nm, and consists in three parts: UV up to 380 nm (it carries only a few percent of the incoming energy), the visible between 380 nm and 780 nm (about 50% of the incident solar energy), infrared over 780 nm (close to 50%).

[1]: http://rredc.nrel.gov/solar/spectra/am1.5/