In order to benefit from the high electrochemical efficiency of solid oxide fuel cells (SOFC), a detailed balance of plant (BOP) has to be developed. An energetic and economic analysis of a decentralized natural gas-fuelled SOFC-power plant in the range of 200 kW capacity is carried out, All calculations start from a basic plant concept with a simple flowsheet and a basic parameter set of SOFC operation and economic data. Changes in costs of electricity (COE) and plant efficiency are determined for the variation of cell operation parameters. This includes the influence of air temperature increase in the stack, degree of internal reforming, cell voltage and fuel utilization. The results indicate two classes of cell parameters. Cell voltage and fuel utilization show a cost optimum characteristic, whereas the other parameters have a uniform influence on efficiency and costs of electricity.