Still, another problem for investors is that nearly all of the major alternative-energy pure-play firms are early-stage companies with relatively small, albeit rapidly growing, sales and no history of earnings or expectations of profitability in the near future.

So the prices for these stocks look frothy to say the least. Capstone Turbine, for example, has a market value of $347 million even though sales are expected to reach just $24 million next year. So it's trading at 14.5 times estimated revenue.

14.5x isn't that bad when compared with tech stocks or healthcare right now.