In the present article, the monthly average daily diffuse solar radiation on a horizontal surface is calculated first, using 12 new hybrid models. A standard isotropic model is then used to estimate the global solar radiation on inclined surfaces. Finally, the monthly, seasonal, and yearly optimum slope angles to gain the maximum global solar radiation are suggested. The monthly optimum tilt angle varies from 0 degrees (May, June, and July) to 60 degrees (December). The seasonal optimum tilt angle is between 4 degrees (spring) to 53 degrees (autumn) and the optimum yearly slope angle is 29 degrees, which is close to the latitude of Kerman.