Google's push to convince the U.S. government to subsidize renewable energy sources is designed to reduce the country's greenhouse gas emissions and its dependence on foreign oil in the next two decades.

That's good for the general population, as CEO Eric Schmidt said during a question and answer session after his speech to the Commonwealth Club in San Francisco.

"Why not work on the important problems of the world." 

But what about for Google's interest?

Schmidt made it clear this week that while the company wants to make a difference globally, bringing down the cost of energy is also in Google's best interest.

"Clearly, it's in our interest to save money, and to get prices down. We're a big end-user" of electricity, Schmidt said.

Server costs, along with labor costs, are among the company's biggest expenses. Already, Google has taken steps that help its data centers use one-fifth the power of average ones of the same size.

It's also planning ahead.

Schmidt was asked about a patent that Google recently applied for regarding a floating, tidal-powered server farm, and whether that meant it was planning for a post-global warming world where water levels rise up and cover major cities.

"When global warming kicks in, we don't want data centers under water," he half-joked. Of course, "by then, the users will have to be on boats."

Sounds like the Google folks have bought into Kevin Costner's vision of the future.