The United States is on the verge of a solar boom that could provide 4.3 percent of the nation's electricity by 2020, according to a new report from Bloomberg New Energy Finance.

There's just a 12-figure catch: Investors need to put $100 billion into the solar industry to keep the generation of solar electricity growing by 42 percent a year for the next decade to expand capacity from the current 1.4 gigawatts to 44 gigawatts.

"Policy measures such as tax credits, capital expenditure grants, generation incentives and renewable electricity credits will remain a key driver of solar uptake in the U.S. for at least the next three years," according to the report from Bloomberg New Energy Finance, a research and consulting firm. "The current drop in solar costs is taking place just as such policies are being implemented by the federal and various state governments, which is expected to lead to rapid growth in commercial, utility and residential solar power."

Over the past two years, solar module prices have plunged by 50 percent as low-cost Chinese manufacturers expanded production and entered the U.S. market.

"Policy, rather than sunshine, will remain the U.S.'s greatest solar resource for the next few years," Milo Sjardin, Bloomberg New Energy Finance's head of U.S. research, said in a statement. "By the middle of this decade, however, the U.S. retail solar market will be driven by fundamental, unsubsidized competition, which should transform the U.S. into one of the world's most dynamic solar markets."

Exhibit A for such a phenomenon is Germany. With about as much sunshine as Maine, the European nation became the world's solar stronghold through policies that rewarded homeowners, businesses, and farmers for generating their own electricity.

Such policies are needed in the U.S., according to the report, given that solar electricity remains four times as expensive to generate than coal-fired power.

Of course, the failure of Congress to pass national climate change legislation and the current attempt to kill California's global warming law shows that progress on green energy issues is not guaranteed in the U.S. And Congress' habit of offering short-lived tax incentives for renewable energy and then dithering about extending them when they expire has played havoc with the industry and investors.

Bloomberg New Energy Finance predicts photovoltaic panels will account for 30 gigawatts of the 44 gigawatts of solar electricity generation by 2020, with 14 gigawatts coming from solar thermal power plants. Solar thermal farms deploy huge arrays of mirrors to heat liquids to create steam that drives electricity-generating turbines.

That might be a conservative estimate, if the California and federal officials' rush to green light big solar projects in recent weeks is any indication. On Monday, for instance, Interior Secretary Ken Salazar approved a 1,000-megawatt solar thermal power plant to be built in the Southern California desert.

By year's end, nearly four gigawatts of solar thermal projects are expected to be licensed. Just 10 gigawatts to go until 2020.

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