Futurist Ray Kurzweil that made a number of accurate predictions about future says "We are only 8 doublings away from it [solar enegery] meeting our energy needs". In this TED talk he explains that 8 doublings is not too far considering the exponential growth of many technologies. Below is a screenshot of logarithmic Megawatts of photovoltaic production over 1975-2007.

 

http://www.youtube.com/watch?v=QJsHRltEVBc