What are the limits to renewable energy?

It looks like renewable energy is going to be of increasingly greater importance. How much of it can we use, compared to our current energy usage? Are there any downsides? It seems to me that wind turbine sites take up a lot of land. If we use solar power, will we be taking sun away from plants?

I do not wish to appear downbeat, but I was just wondering how much use we can realistically expect to make of renewable energy sources.