The potentially damaging effects of climate change make it imperative to develop zero-carbon energy systems and societies based on renewable energy sources that do not negatively affect the environment However, these systems are often criticized for their intermittency, and the present paper proposes a method to analyze the true minimum capacity factor that can be expected from such a system based on a historical hourly estimation of the electricity produced by a given solar-wind generating mix. A simulation was carried out to show how much energy could be produced for a sample future group of scenarios encompassing a variety of solar and wind mixes, and the results show that, with a 1:2 mix of solar to wind energy, the system will always operate at least at 10% capacity from 10:00 to 16:00, as calculated using the meteorological conditions of the year 2001. This study also analyzes the land requirements necessary to implement such a solar-wind energy system, highlighting the vast areas that would be necessary to be covered with wind turbines and solar panels if such a system were to supply the majority of the electricity demand in Japan. (C) 2010 Elsevier Ltd All rights reserved