Authors have proposed the Totalized Hydrogen Energy Utilization System(THEUS) for load leveling and stabilizing the grid system. THEUS consists of fuel cells, water electrolyzers, hydrogen storage and their auxiliary machinery. The basic operation for load leveling is as follows: In the nighttime, it produces hydrogen through water electrolysis by using low-cost electric power and stores the hydrogen. In the daytime, it conducts fuel cell generation using the stored hydrogen to meet the demand. Hydrogen storage equipment is the core of THEUS. In this paper, the metal hydride tank was chosen as hydrogen storage. The hydrogen desorption process is endothermic reaction. It is possible to use the cool heat for air conditioning. It is necessary to develop a useful numerical simulation code for not only the operation method but also the optimization of the structure of the tank. We developed the simulation code. The validity of the code has been improved by comparing with experimental results. Concretely, the following points were considered. (1)The thermal capacity of the tank. (2)Heat loss to the outside. (3)The copper plate effect. The simulation results become in good agreement with experimental results. And to achieve hydrogen high flow rate, new design tank performance was investigated.