Purpose of specific study is computational simulation using software which has been developed in visual basic programming environment and calculates the average monthly intensity of solar radiation that receives a tilted surface photovoltaic or solar collector with certain tilt angle and optimal tilt angle accordance with which there are the maximum solar gains. In the first step the mathematical models that govern solar geometry and solar radiation will be described. In the second step the mathematical models are converted to programming language and the required graphical user interface is created in visual basic.