Renewable energy systems are often criticized because of their intermittency and need for substantial amount of backup in terms of other energy sources or storage. The present paper proposes a method to estimate the required amount of storage backup for a mostly solar and wind system that uses also biomass and hydroenergy as minor components of the electricity mix. An hourly simulation was carried out to determine the amount of electricity that could be produced based on the meteorological conditions of year 2001 in Japan, and this was compared with the maximum electricity demands imposed in the system for each month of the year. The system thus proposed has 100% chance of meeting around 40% of the electricity demand between 11:00 and 18:00, and the optimum scenario obtained (a 2:1 mix of solar to wind energy) required around 40 TW of storage to balance the extra electricity demand that occurs during the summer in Japan. It appears unlikely that the batteries in EV in vehicles will be able to meet this storage requirement in the future, though the system is able to adequately meet the electricity demand during the majority of the year, and hence showcases the viability of renewable energy. (C) 2012 Elsevier Ltd. All rights reserved.