California, like other states that are in the midst of carrying out their green energy goals, is constrained by the amount of space it has available on its transmission system.

FORBES: Lobbyists Try to Halt State Green Energy Standards

"These draft designations set us on the path to modernize our constrained and congested electric power infrastructure, " says Energy Secretary Samuel Bodman.

FORBES: Magazine Article

They are also constrained indirectly by the influence of water on food, energy, transportation, and industry.

UNESCO: Natural Sciences

The future energy picture for the U.S. or the planet is not constrained by the availability of supplies, either fossil or non-fossil, but by efficiency gains in generation and consumption.

WSJ: Nansen G. Saleri: Our Man-Made Energy Crisis