The data of solar irradiance is very important to give information that one area is suitable or not to develop a photovoltaic (PV) power generation. Normally, the data of solar irradiance is recorded by government weather station. But some time, it is due to the limitation of equipment installation, financial support or error reading, thus the data of solar irradiance is missing. This paper presents an estimation of clear sky solar irradiance as potential of electrical power generation in Semarang, Indonesia. A mathematical modeling is applied to obtain the beam, diffuse, reflected and clear sky global solar irradiance using Matlab software. The electrical power generation of PV module is simulated based on the reported annual temperature in Semarang and the estimated beam solar irradiance. The results show that minimum, maximum and average clear sky global solar irradiance are 991.8 W/m², 1120 W/m² and 1061 W/m², respectively. It indicates that the sky in Semarang is very clear and suitable to be developed a PV power generation. Based on the reported annual temperature of 26.9⁰C and the beam solar irradiance of 968.3 W/m² in Semarang, if 250 W YGE PV module is simulated using PSpice, thus it will generate the electrical power of 243.2 W. It has dimension of 1.65 m x 0.99 m or 1.64 m². An area of 1 km² in Semarang can be installed around 610, 000 units of 250 W YGE PV modules. The total of 250 W YGE PV modules will generate the electrical power of 148.352 MW.