Solid oxide fuel cells (SOFC) can generate electricity and heat with minimal negative impact on the environment and with high electrical efficiency (∼60%). SOFC stacks are the key component of cogeneration units in the range from several kW up to hundreds of kW. Larger power output of a system is achieved by multiplication of number of separate modules which are usually based on 1,000 W class stacks. In the design of the SOFC stack the following parts are distinguished: two terminating plates (top and bottom), interconnectors, separators, gaskets, electrical connections, fuel and oxidant delivery lines, and fuel cells. Geometric features, physical and chemical property of the above mentioned components affect the performance and mechanical strength of the SOFC stack. Two different approaches for air supply can be used and were investigated, namely the internal and external manifolding. Comprehensive analysis of the geometric features of the stack together with inserts, the supply and discharge piping of reaction gases was performed in order to determine the optimal operating conditions on the latest 3rd generation of the SOFC stack which is under development in the Institute of Power Engineering (IEN). Article includes the results of laboratory tests and numerical studies. Obtained results indicate the set of parameters which influence the performance and lifetime of SOFCs. Among others, one of the most important variables are the type of fuel, operating conditions, homogeneity of temperature distribution and uniformity of gas flow.