Warren Buffett's MidAmerican Energy Holdings made its second foray into solar energy in as many weeks, saying on Friday it will buy a 49 percent stake in an Arizona power plant from NRG Energy Inc (NRG.N). MidAmerican, the utility affiliate of Buffett's holding company Berkshire Hathaway Inc (BRKa.N), said it will take a stake in the 290-megawatt Agua Caliente project in Yuma County, Arizona. The plant is being built by solar power company First Solar Inc (FSLR.O) and is supported by a $967 million loan guarantee from the U.S. Department of Energy. The announcement comes a little over a week after MidAmerican said it would buy First Solar's 550 MW Topaz Solar Farm power plant in California.