Exxon Mobil is big, all right

DAVIS, Calif.--You read a lot about renewable energy and the coming opportunities, but there's still a lot of money to be made in gas.

Mark Henwood, managing director at Camino Energy, which analyzes alternative fuel stocks, recently ran a math problem to see just how big the clean tech market is. (We ran into him near the buffet line at the GoingGreen conference this week.)

He totaled up the market cap of all of the clean energy companies now traded on the public exchanges. The list includes solar companies, ethanol refiners and clean coal manufacturers. It does not include agribusiness companies with interests in fuel, however.

Henwood found 314 companies with a cumulative market cap of $210 billion as of September 9. That's about 40 percent of the size of Exxon Mobil, which hovers at around $465 billion.

Henwood's observation dovetails with a comment from Justin Adams, director of long term research at BP. There are many discussions at energy conferences, but not many on existing energy sources. Still, we will have to come to grips with issues like coal gasification and sequestration, among other topics, Adams added.