The total amount of energy raining down from the sun could provide 6,000 times the globe's annual energy needs.
Greenpeace recently released its Solar Generation 2010 report, and the very first figure (above) provides a helpful perspective on the potential for solar power.
Those cubes represent our various sources of energy. As you can see, the annual solar irradiation to the earth, represented by that transparent box, just dwarves them all. According to
Wikipedia, the theoretical potential for solar power is 6,000 times global annual energy consumption.
Obviously we're not going to blanket the entire planet in solar panels. But with that amount of energy raining down from the sky, we don't need to. If those six black dots below were solar plants with only 8 percent efficiency, we'd generate enough energy for the whole world.
Being able to store and transport that energy—and making sure we have access to the metals we need to make solar panels in the first place—are still big challenges, of course.