NASA Glenn Research Center (GRC) has recently demonstrated a polymer electrolyte membrane (PEM) based regenerative fuel cell system (RFCS) that operated for five contiguous back-to-back charge/discharge cycles over a period of . The system operated continuously at full rated power with no significant reactant loss, breakdowns, or degradations from June 26 through July 1, 2005. It demonstrated a closed-loop solar energy storage system over repeated day/night cycles that absorbed solar electrical power profiles of and stored the energy as pressurized hydrogen and oxygen gas in charge mode, then delivered steady electrical power with product water during discharge mode. Fuel cell efficiency, electrolyzer efficiency, as well as system round-trip efficiency were determined. Individual cell performance and the spread of cell voltages within the electrochemical stacks were documented. The amount of waste heat dissipated from the RFCS was also reported. The RFCS demonstrated fully closed-cycle operation without venting or purging, thereby conserving reactant masses involved in the electrochemical processes. Smooth transitions between the fuel cell mode and electrolyzer mode were repeatedly accomplished. The RFCS is applicable to NASA’s lunar and planetary surface solar power needs, providing lightweight energy storage for any multikilowatt-electrical application, where an environmentally sealed system is required.