The instantaneous global power which weighs on an oriented surface is given by the sum of the direct component that is obtained from the eqn (28), the diffuse component, which comes from the celestial vault portion seen from the surface, and the part reflected by the soil and nearby objects towards the same surface.

If the sky’s behaviour is assimilated to that of an isotropic spring of diffuse radiation, it is possible to determine the diffuse component which reaches the surface as:

Gd=/do Rd (31)

where

Rd = cos2 (b/2) = (1+cos b)/2 (32)

where Ido is the diffuse radiation on the horizontal plane and Rd is the inclination factor of diffuse radiation.

We can express the radiation (direct and diffuse) reflected by the soil on a certain surface as:

(Ibo +Ido)Rr (33)

Rr, the inclination factor of reflected radiation, is equal to:

Rr = psen2 (b/2) = p(1 — cos b)/2 (34)

p is the soil’s reflection coefficient and it can assume values between 0.2 (grass, concrete) and 0.7 (snow). Therefore, the instantaneous solar power, which is received on a arbitrary oriented surface, in the case of isotropic sky, is equal to [1]:

G = Ibo Rb+Ido Rd+(Ibo+Ido)Rr (35)