Renewables have the power to transform not just the world's energy markets, but global economics and geopolitics. But wind and solar alone cannot deliver a world of clean and free fuel. Both are, by their very nature, variable, so to realise their true potential other technologies need to be harnessed. Improving connectivity to other countries is one relatively simple solution, but in a world where governments are becoming increasingly preoccupied with energy security, its attractions are somewhat limited. Managing demand more effectively using smart grids and appliances is another. But the technology with the most revolutionary potential is energy storage.