How many solar cells it would take to supply the entire earth's electricity needs?
A picture representing the surface of the Earth that would have to be covered in solar cells to generate all the world's electrical needs was posted as part of a post rebutting Superfreakonomics, here. The image itself is here. The author includes all of his calculations and assumptions in the post.
Remember that when you hear people talking about how infeasible it would be to wean ourselves from a coal economy. Coal is almost exclusive used to generate electricity in the US, and coal power plants produce about half of the electricity the US consumes (which itself is about 40% of the energy used by the US per year). It's even more realistic when you consider wind power and efficiency improvements.