Global radiation received on an inclined surface is composed of beam and diffuse radiation as well as reflected radiation from the surroundings. The main subjects of the analysis on global radiation on a tilted surface is
1. daily optimum tilt angle
2. monthly optimum tilt angle
3. yearly optimum tilt angle
Models available in literature to calculate the global radiation on an inclined surface are similar except for the diffuse radiation component. Among these approaches the simplification of the distribution of diffuse radiation in the atmosphere to be isotropic is the simplest and has a minor effect on the accuracy of prediction of the global radiation. because the diffuse radiation contributes 20% of global radiation in clear skies.
A computer programmed is constructed on mathematical formulation to find the hourly, daily and monthly average values of optimum tilt angle, optimum orientation and maximum total radiation on the tilted surface of the collector. The daily and monthly average values of optimum tilt angle are calculated, so that the average values of the maximum total radiation will be near to its hourly values.
The following results have been drawn :
1. The optimum slope is found to depend on Several parameters, namely ) W,A
Z,C, , ,n,,f(=pioptργφβ the first three of these parameters are the most important; the effect of the remaining parameters is not so strong. Since diffuse radiation decreases optβ and reflected radiation increases optβ combined they tend to neutralize each others effect, resulting in surprisingly good estimates for optβ .  2. the daily optimum tilt angle, for a flat plate collector facing south and concentrating, changes throughout the year with its minimum value in June and maximum value in December. 3. The yearly average of the daily optimum tilt angles is found to be 32.63° and 54.19° for a flat plate collector facing south and a collector with the optimum orientation, respectively,