Energy efficiency of a single solid-oxide fuel cell (SOFC) has been studied in various experimental conditions and directly correlated to the character of the cell. A microtubular cell, which has the exact geometry for stack/module application, is used for this study. The energy efficiency simply improves by increasing the operating temperature, and reaches over 40% (lower heating value: LHV) at the operating temperature of over 700 degrees C in flowing 20% H-2-Ar fuel inside the tube. Impedance analysis has shown that the gas transport is the limiting factor for improving the energy efficiency at lower operating temperatures. (C) 2011 Elsevier B.V. All rights reserved.