A kW class all-vanadium redox-flow battery (VRB) stack, which was composed of 14 cells each with an electrode geometric surface area of 875 cm(2), with an average output power of 1.14 kW, at the charge-discharge current density of 70 mA cm(-2), was successfully assembled by filter press type. Then, a 10 kW class VRB stack was manufactured with a configuration of 4 x 2 (serial x parallel) of the improved aforementioned kW class stack modules, which produced a direct output of 10.05 kW (current density 85 mA cm(-2)). The energy efficiency of more than 80%, at an average output power of 10.05 kW, for the 10 kW class VRB stack was achieved, indicating VRB is a promising high efficiency technology for electric storage. (c) 2006 Elsevier B.V. All rights reserved.