An innovative, flexible modular concept for a fuel cell inverter system with optimized overall efficiency for a domestic heater is presented and compared with conventional and fully bidirectional solutions. Three types of bidirectional insulated DC/DC converters are discussed. The design should take into account the asymmetrical flow of power from the fuel cell to the mains and back to the fuel cell for the supply of the auxiliary equipment. An integrated auxiliary power supply turned out to be best, which is fed during normal operation directly by the fuel cell, and, during start-up by the mains. A loss analysis for both operation modes has been carried out as support for the final design decision