Theoretical energy density for electrochemical capacitors with intercalation electrodes is applied to carbon electrodes having the ability for electrochemical intercalation of ions. Energy density theory was applied to these capacitors and demonstrated that energy densities are 70-114 Wh/kg based on electrode material only, 14-30 Wh/kg based on electrode material and electrolyte, 8-20 Wh/kg based on electrode material, electrolyte, and separator paper, and 7-15 Wh/ kg for a packed capacitor. The volumetric energy densities are also estimated. The influences of capacitor operational voltage, electrode material specific capacitance, and electrolyte ion concentration were discussed. (c) 2005 The Electrochemical Society.