A story in the Guardian tells about an estimate made concerning energy from the sun:
Gerhard Knies, a German particle physicist, was the first person to estimate how much solar energy was required to meet humanity’s demand for electricity. In 1986, in direct response to the Chernobyl nuclear accident, he scribbled down some figures and arrived at the following remarkable conclusion: in just six hours, the world’s deserts receive more energy from the sun than humans consume in a year. If even a tiny fraction of this energy could be harnessed – an area of Saharan desert the size of Wales could, in theory, power the whole of Europe – Knies believed we could move beyond dirty and dangerous fuels for ever.
Germany is leading a project called Desertec with the goal of providing 15% of Europe’s electricity by 2050 through a vast network of solar and wind farms.