Renewable sources of power, wind, solar, wave and tidal power have been criticized because they do not produce power at a consistent or predictable rate. "By mixing between sites and mixing technologies you can markedly reduce the variability of electricity produced by renewables, Graham Sinden, of Oxford's Environmental Change Institute was quoted as saying in the May 12th edition of The Guardian. Sinden went on to say, "And if you plan the right mix, renewable and intermittent technologies can even be made to match real-time electricity demand patterns. This reduces the need for backup, and makes renewables a serious alternative to conventional power sources." He found that by combining a diversity of geographical locations and a diversity of technologies, renewables combined with domestic combined heat and power could ultimately make the the following contributions to Britain's total energy supply: wind 35%, wave and tidal 15%, combined heat and power 15%, and solar 5-10%.
A more detailed report can be found at the University of Oxford's Environmental Change Institute's web site. The Cambridge-MIT Institute has a related paper "Diversity and Security In UK Electricity Generation: The Influence of Low Carbon Objectives"
The conclusion that diversity reduces variability is rather intuitive. These studies put some numbers on the required mix. I would assume that similar findings would apply to other regions of the world. If eventually more than 50% of our electricity could be produced from renewables that would be an enormous contribution. Add another 10% for hydro and 40% from a combination of bidiesel made from algae and syngas made from biomass and we have total independence from fossil fuels.