Economic development of South Africa depends of secure electric power supply. In order to cover the required base load, two modern coal power plants, Medupi and Kusile, are being built. The capacity of both plants is 4800 MW, which means total of 9600 MW, which is 25 % of currently available electric power in South Africa. Due to the lack of water, Medupi and Kusile will have an air – cooled condensers, which will be the largest condensers of this type in the World. Efficiency of these condensors depends largely on external air conditions, particularly air temperature, which for South African area reaches over 40 °C. This thesis consists of thermodynamic calculation of thermal power plant Medupi, i.e one of Medupi's 800 MW unit. Energy efficiency will be calculated depending on increase of external air temperature and on the surface of the condenser,. First part of this thesis brings detailed description of energy production in modern coal power plant. All main parts of power plant are also described as they affect the most efficiency of cycle. Main components of coal power plant are steam generator, steam turbine, condenser, pump and electric generator. Second part of thesis gives detailed description of Medupi coal power plant and also technical data required for calculation of mathematical model. Design of air – cooled condensers and also thermodynamic properties have been given in this part of thesis. Last part of thesis presents a mathematical model of one of the Medupi's 800 MW power unit with diagram results of energy efficiency dependance on external conditions and geometry of condenser.