Because of a concern that in developing transitional energy systems the endpoint system requirements should be kept in mind, this paper focuses on storage in a renewable energy system that uses no fossil fuels. Based largely on the current seasonal patterns of consumption and wind and solar energy generated, it is estimated that the energy storage capacity that would be required to supply the electrical energy for the United States for a year given that the source of the electricity is from solar, wind, or a combination of the two, is in the order of 10%–20% of the total annual demand. While the uncertainty within and between published estimates of biomass availability is quite large, a partial review of the literature indicates that the global biomass primary energy potential could satisfy seasonal energy demands in a sustainable manner. The storage volumes required for biomass and hydrogen, another storage possibility, to meet seasonal storage needs are considerably smaller than that required for compressed air or elevated water.