To establish environmentally friendly small-scale CHP units, a concept with serial connection of two SOFC stacks and an electrical efficiency of up to 50 % was developed. The underlying idea of the STAGE-SOFC concept is to couple a small exothermal CPOX stage with a larger endothermal steam reforming stage. This complete new system layout was validated by system modelling, CFD simulations of single components, experimental investigation of reformer performance and testing of a complete 5 kW prototype system for more than 600 hours under various parameters and loads. On basis of these results an improved second prototype was designed for long-term tests under real operating conditions.