Price is What You Pay, Value is What You GetThe first thing to know is that Buffett tries to never overpay for anything. He only buys when he's sure that an asset is undervalued and is likely to have bottomed in market price. So this could be a sign that after all the softening prices and bankruptcies, that the solar industry is on its way to better days (or at least, it won't get much worse). Or at least that at today's prices, you get more value than what you pay for when you invest in solar.
Another thing about Buffett is that he thinks long-term. He's not looking to flip assets, he wants to own them forever if possible. So that makes his very conservative about going into unpredictable industries (that's why he almost never invests in technology companies, he can't predict what the field will be like in 10-20 years). This tells us that Buffett feels that the long-term future of solar looks rosy, and that even though natural gas prices have been low recently, that in the long-term, solar is one of the the places to be. Of course, any TreeHugger ready could have told you that, but it's always nice to be on the same side as the greatest living investor (and one of the top philanthropists, along with Bill Gates, it must be noted).