Here's a handy little map from the Land Art Generator Initiative - charting the total global area required to power the world using solar power, and with zero carbon emissions.
Seems OK to me.
Of course, InformationIsBeautiful has to take it a step further and redraw the map using offshore wind power and... erm... Matrix-esque human battery power. I guess we're better off with windmills than with Neo. I wonder, though, which would be better: huge windmills in the oceans, or massive solar panels in the middle of nowhere.