The concept and the design of a micro-solid oxide fuel cell system is described and discussed. The system in this study is called the ONEBAT system and consists of the fuel cell PEN (positive electrode -electrolyte - negative electrode) element, a gas processing unit, and a thermal system. PEN elements of free-standing multi-layer membranes are fabricated on Foturan (R) and on Si substrates using thin film deposition and microfabrication techniques. Open circuit voltages of up to 1.06 V and power of 150 mW cm(-2) are achieved at 550 degrees C. The membranes are stable up to 600 degrees C. The gas processing unit allows butane conversion of 95% and hydrogen selectivity of 83% at 550 degrees C in the reformer and efficient after-burning of hydrogen, carbon monoxide, and lower hydrocarbons in the post-combustor. Thermal system simulations prove that a large thermal gradient of more than 500 degrees C between the hot module and its exterior are feasible. The correlation between electrical power output - system size and thermal conductivity - heat-transfer coefficient of the thermal insulation material are shown. The system design studies show that the single sub-systems can be integrated into a complete system and that the requirements for portable electronic devices can be achieved with a base unit of 2.5 W and a modular approach. (c) 2007 Elsevier B.V. All rights reserved.