The global radiation on the inclined plane $GTI$ (global tilted irradiance) results to:
$$ GTI = BTI + DTI + RTI $$
The direct radiation to the horizontal $BHI$ (beam horizontal irradiance) can be calculated from the geometric conditions (cosine) over the elevation angle of the $\gamma_\text{M}$ to the inclined plane to the direct radiation to the inclined plane $BTI$ (beam tilted irradiance).
For more information on reflected radiation to the inclined plane $RTI$, see Calculation > Irradiation > Ground reflection.
For the calculation of diffuse radiation on the inclined plane $DTI$ (diffuse tilted irradiance) the following models are available:
In previous versions of PV*SOL® only this model was available