A fuel cell system with planar stack configuration is studied with a view of portable applications. A mathematical model to estimate the discharge characteristics of the system is presented in this research to clarify the effect of the number of segmented cells on the output power and energy losses generated in the system. In the isothermal case of this analysis, we revealed that output power of a planar stack SOFC increases with increasing the number of cell segments because the current distribution in the electrolyte becomes more uniform by increasing the number and this results in a reduction of energy losses in the system.