Most renewable energy sources cannot provide stable, continous supply (ie: Solar, Wind, Current, Tidal Wave etc).
The solution is energy storage.
There are many technology for energy storage from phase change, superconductor ring, nanotechnology for Hydrogen storage, Hydrogen Peroxide, Compressed Air, Flywheels, Bactery/Algae, Metal/Zinc/Vanadium/Engineuity, pumping hot water into underground caves etc.
http://dmoz.org/Science/Technology/Energy/Storage/In Alaska there are giant geodesic domes with insulator and special glass for efficient energy storage based on greenhouse principle.
Which method is the most efficient and low cost for residential/home?
Which method is suitable for large scale power generators?