The third-generation 1-kW(e)-class module was developed with an automatic control system. A conversion efficiency of 48% ac/lower heating value [ac/LHV] was achieved with an exhaust heat recovery unit. An endurance test using the third-generation 1-kW(e) module was done for over 1000 h and no degradation of the power generation performance was observed. In parallel, a single-cell unit, which includes one cell and two metallic separators, was tested for over 10000 h and the degradation rate of the terminal voltage was found to be 1-2%/ 1000 h. In the direction of scale-up, a triple-stack module of 3-kW(e) output was developed. A partial load as well as excess loads on the module were tested and the output power of 1-5 kW(e) was attained under thermally self-sustainable conditions. It was found that a high efficiency of 55% dc/lower heating value [dc/LHV] was obtained under stable operation. Ongoing research of the fourth-generation 1-kWe module has resulted in the conversion efficiency of 58% [dc/LHV]. (c) 2006 The Electrochemical Society.