WASHINGTON, Dec 15 (Reuters) - The Federal Energy Regulatory Commission on Thursday approved a bid by a unit of Berkshire Hathaway Inc. (BRKa.N: Quote, Profile, Research) (BRKb.N: Quote, Profile, Research) to buy the PacifiCorp utility from Britain's Scottish Power Plc (SPW.L: Quote, Profile, Research) for $5.1 billion.

MidAmerican Energy, the utility arm of billionaire investor Warren Buffett's Berkshire Hathaway, agreed in May to buy Portland, Oregon-based PacifiCorp from Scottish Power for $5.1 billion, plus the assumption of more than $4 billion in debt.

FERC Chairman Joseph Kelliher said there was no overlap between the service territories of MidAmerican and PacifiCorp.

"It's a very straightforward matter to conclude there is no harm to competition," Kelliher said.

MidAmerican, based in Des Moines, Iowa, will gain power plants and add 1.6 million customers in six western states, increasing its customer base to 6.6 million.

The merger should spur needed investment in PacifiCorp's transmission grid because of MidAmerican's access to Berkshire's capital, FERC Commissioner Suedeen Kelly said.