The angle of incidence (i) is the angle between the sun's rays irradiated on a surface and the line normal to this surface. To calculate the angle of incidence i on any plane, including the horizontal, the following equation may be used:

                  cos i_T = sin b cos S + cos g cos b sin S

where T = slope angle of surface from horizontal. For a vertical surface, when S = 90^o,

                  cos i_v = cos b cos g
                  
For a horizontal surface, when  S = 0^o ,

                   cos i_h = sin b

Solar Intensity-- The solar intensity at a direction normal to the title surface (I_s) is the vector sum of the components along the line normal of the titled surface.

                  I_T = I_DN cos i_T

where  I_DN = the solar intensity irradiated on a surface normal to the sun's rays.

For a horizontal surface  S = 0^o , the solar intensity normal to the horizontal surface is:

                    I_h = I_DN cos i_h

For a vertical  surface  S = 90^o , the solar intensity normal to the vertical surface is:

                    I_v = I_DN cos i_v