In less than two weeks, MidAmerican Energy Holdings Co. has gone from zero to 840—at least in terms of solar megawatts. This time it purchased a 49 percent stake in NRG Energy’s (NYSE: NRG) 290-megawatt Agua Caliente solar project in Yuma County, Ariz., slated for completion in 2014.

MidAmerican Energy is controlled by billionaire investor Warren Buffett’s Berkshire Hathaway. It is the nation’s largest purveyor of wind among rate-regulated utilities. And by the end of 2011, 28 percent of all its generation will come from renewable and non-carbon sources, according to a press release.

But this is only the second instance where the Iowa-based company has invested in a solar project. On Dec. 7, it purchased the 550-megawatt Topaz Solar Farm in California that’s being developed by First Solar, Inc. (NASDAQ: FSLR).

Agua Caliente also will use First Solar modules and is being built by the company. While MidAmerican purchased a large stake in the project, it will not control the project.

“NRG will remain the majority owner,” said NRG spokesperson Lori Neuman.

This $1.8 billion Agua Caliente project is supported by a $967 million loan guarantee from the U.S. Department of Energy. It’s under a 25-year, power-purchase agreement with Pacific Gas and Electric.

The company will use the proceeds from MidAmerican’s purchase for general corporate purchases.

“Such as further reinvestment in our business, further solar developments, capital allocation in the form of share repurchases or debt pay-down,” Neuman said.

NRG is looking to sell other solar projects in its portfolio.

“As mentioned over the past few months, and more recently during our quarterly earnings, we’re continuing to pursue other opportunities to sell-down additional solar projects. But we haven’t talked about any specific companies,” Neuman said.

This is the first project NRG has worked on with MidAmerican.

NRG has a history of both wind and solar project development. It has 485 megawatts of wind power in the U.S. It also has more than 1,900 megawatts of solar generation in various stages of development, with 900 megawatts of solar generation operating or under construction.