The performance of a new acid water electrolyzer system for hydrogen production is investigated, based on semi-empirical equations of a phosphoric acid water electrolyzer. The circulating electrolyte concentrations under differently operating temperatures are optimized so that the minimum input voltages of the electrolyzer are determined for other given conditions. The optimum electrochemical characteristics of the electrolyzer are revealed. Moreover, it is expounded that the Joule heat resulting from the irreversibilities inside the electrolyzer is larger than the thermal energy needed in the water splitting process. The general performance characteristics of the phosphoric acid water electrolyzer system are discussed, from which the lower bound of the operating current density is determined. The upper bound of the operating current density is further determined by introducing a multi-objective function including the system efficiency and hydrogen production rate. Consequently, some optimum design strategies of a phosphoric acid water electrolyzer system are obtained and may be chosen according to different practical requirements. (C) Copyright 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.