Demand is growing for equity in green technology companies, because demand is growing for green technology products, according to a report on this evening's Marketplace.

Cities and builders, for example, are looking more closely at new energy-efficient products, reports NPR's Sam Eaton. A new building in Denver will have an air filtration system that will save US$50,000 a year in electricity bills.

"Energy bills are skyrocketing," says builder Bob Padgett. "That's making people take a hard look at energy consumption."

And that has the investment community taking a hard look at the manufacturers of energy-efficient and clean-energy products.

Strion Air makes that energy-efficient filter, and just completed its Series B financing of $11 million, bringing total equity investment in the company to $16 million.

Nanosolar Inc., a developer of solar electric cells, closed $20 million in Series B funding in June, 2005, to support the company's move into commercial production. The round was led by Mohr, Davidow Ventures ($10.5 million), and includes Mitsui, Japan's largest distribution company.

"Investment really is driven because it's an attractive place to allocate capital today," Ira Ehrenpreis, cleantech investment firm Technology Partners, told Eaton. "The underlying markets of cleantech are among the largest markets in the world."

Indeed, the target markets in the U.S. are large, but that's just the beginning.

"India and China represent 40 percent of the world's population, and they're undergoing the type of industrialization that we experienced in the twentieth century," Ehrenpreis noted. "The energy demands that will cause is a tremendous opportunity."

Compared to the dot-com boom's pet foods and search engines, cleantech won't be as easy for consumers and ordinary investors to grasp. Most of the technologies are intended for business-to-business markets, where they are built into other products -- or buildings.

Some venture capitalists are serious about their investment in these technologies, Eaton says, but cautious investors might be more comfortable with the clean energy initiatives of larger players, such as GE, Sharp and Toyota.