Solar and windpower are currently inadequate, but that doesn't always have to be the case, especially if we can develop grid-scale liquid metal batteries like this:
Here's a link I've posted before displaying the current amount of energy produced by the various sources. The amount of land that we'd have to devote to solar to make a meaningful contribution would be unsustainable.
I'm not sure, I've often seen maps with areas required to power the world with solar* and while they are big, they aren't impossible, especially if done smartly (big solar farms in deserts connected via HCDC transmission lines, but also solar PV and solar water heaters on rooftops close to users).
http://www.ted.com/talks/donald_sadoway_the_missing_link_to_...