PORT GIBSON (AP) — State utility regulators have approved a plan by Entergy Mississippi to increase the output from its Grand Gulf Nuclear Power Plant in Port Gibson.

Approval by the Mississippi Public Service Commission came Monday. Federal regulators also must approve the proposal.

Haley Fisackerly, president/CEO of Entergy Mississippi, says Grand Gulf’s capacity would rise to 1,443 megawatts from 1,265, a 13 percent increase.

Fisackerly, in a statement released Wednesday, says the total cost, estimated at $510 million, will be shared among the joint owners of Grand Gulf. He said costs to Entergy Mississippi are estimated at $151 million.

He says Entergy expects the additional production to offset initial project costs, resulting in little-to-no-initial affect on customers’ electricity bills. He says savings on bills expected to appear within a couple of years after project implementation.

The project’s completion is scheduled for 2012. Most of the work will be accomplished during regularly scheduled maintenance outages.

Entergy officials said the project will be one of the largest expansions of a nuclear power plant in U.S. history and will make Grand Gulf the largest single-unit nuclear power plant in the country. Fisackerly said Grand Gulf can produce electricity more cheaply than the company can buy power from the open market.

‘‘This is a cost-efficient way of meeting Mississippi’s increasing need for more electricity,’’ Fisackerly said. ‘‘We’ve worked closely with the MPSC to bring the benefits of the Grand Gulf upgrade to our customers.’’

Grand Gulf powered up in 1985 after an 11-year construction period. It employs about 750 people and is located in Port Gibson in Claiborne County.

In its order, the MPSC said the costs and benefits of the uprate are expected to effect all Entergy’s subsidiaries in Mississippi, Louisiana, Arkansas and one for New Orleans. Customers of South Mississippi Electric Power Association in Hattiesburg, which owns 10 percent of the plant and receives 10 percent of the energy, will see a $1.4 million savings starting in 2013, according to the MPSC order.

Most jobs created by the upgrade will be temporary, with up to 1,000 extra jobs expected six months after the first design phase starts, Entergy Nuclear spokesman Suzanne Anderson said.

‘‘They will be in engineering, technical, construction, everything,’’ Anderson said.