This paper presents a transient model of the energy system of the small Italian island of Ventotene. The model has been developed as a tool to validate the performance related to the integration of renewable energy technologies and energy efficiency measures into the island stand-alone power system, in presence of seasonal-dependent electric load equilibrium conditions. A hydrogen-based energy systsem has been proposed as an effective option for the seasonal buffering of the stochastic power contribution from renewable energy sources. The time-dependent behavior of the hybrid, diesel, and hygrogen-renewable, power generation system has been investigated by matching it with the electric load data with an hourly distribtuion over the period of one reference year. The technical feasibility and the behavior of the systems have been evaluated in terms of its electric and environmental performance.