It has been a rough few weeks for solar energy ETFs such as Guggenheim Solar ETF (NYSEArca: TAN) and Market Vectors Solar Energy ETF (NYSEArca: KWT).

The sector ETFs have sold off sharply after a fast start this year. [Solar ETFs Recharging with Over 50% Rally in 2012]

Yet investors like Warren Buffet, Google (NasdaqGS: GOOG), KKR & Co. (NYSE: KKR), MetLife (NYSE: MET) and John Hancock Life Insurance have funneled more than $500 million into renewable energy in 2011, reports Christopher Martin for Bloomberg.

Solar projects now provide returns of 15%, attracting many eager investors.

“After tax, you’re looking at returns in the 10 percent to 15 percent range” for solar projects, Dan Reicher, executive director of Stanford University’s center for energy policy and finance, said in the article. “The beauty of solar is once you make the capital investment, you’ve got free fuel and very low operating costs.”

“A solar power project with a long-term sales agreement could be viewed as a machine that generates revenue,” Marty Klepper, an attorney at Skadden Arps Slate Meagher & Flom LLP, which helped arrange a solar deal for Buffett, said in the article. “It’s an attractive investment for any firm, not just those in energy.”

Solar power is viewed as a long-term investment, similar to some bonds. Additionally, the investments also function like infrastructure projects, with cash flows comparable to toll roads, bridges or piplines, Stefan Heck, a director at McKinsey & Co. said.