The invention relates to a fuel cell apparatus that includes a housing. The housing defines a substantially isothermal zone which integrates a fuel cell and a tail gas burner with the isothermal zone. The fuel cell and the tail gas burner are in thermal communication and share a common wall. In one embodiment, the housing further integrates a fuel reformer, where the fuel reformer is in thermal communication with the fuel cell. In one embodiment, the fuel cell and the tail gas burner can be arranged to produce a power density greater than or equal to about 2 W/cc. The fuel cell preferably is a solid oxide fuel cell.