Most of the countries have increased the production of renewable energy to reduce pollution and their dependency on oil and natural gas. In case of Japan, solar power is increased rapidly, especially after the Fukushima Nuclear Accident. The load leveling and fluctuation absorption are the main bottlenecks to the integration of solar PV power into the future electricity system. Hydrogen is considered as an energy carrier in a future energy system based on renewable resources. Totalized Hydrogen Energy Utilization System (THEUS) consists of a unitized reversible fuel cell and a hydrogen storage tank. The main objective of this paper is to evaluate the THEUS operation and performance at different variations in solar photovoltaic (PV) power during a sunny day and a partly cloudy day and to characterize its dynamic response. Energy efficiency of the THEUS was evaluated in water electrolyzer and fuel cell mode operation.