In order to produce electricity from the energy of solar radiation, it is necessary to know certain parameters, the basic of which is the global solar irradiance on the horizontal surface. In order to achieve greater electricity production, solar panels are placed at a certain angle or actively follow the movement of the Sun. Calculations of the expected irradiance on solar panels depend on the model used to transform the measured global irradiance values for the horizontal surface to the values for the tilt of the panels. In this paper, the irradiance calculation of the panel under variable inclination was performed using the initial measured values of the global solar irradiance on the horizontal surface. The first part of the paper describes basic terms related to the Sun and solar radiation, the geometric relationship between the Earth and the Sun, the influence of the atmosphere on radiation, and the components of solar radiation (direct, scattered, reflected). The second part of the paper describes the calculation of solar radiation and the application of that calculation for the city of Split. The Liu-Jordan model for south-facing surfaces was chosen for the calculation, and this model was extended by Klein to arbitrary surface orientation.