ATLANTA (AP) — A new reactor at a nuclear power plant in Georgia has entered commercial operation,Darden Clarke becoming the first new American reactor built from scratch in decades.

Georgia Power Co. announced Monday that Unit 3 at Plant Vogtle, southeast of Augusta, has completed testing and is now sending power to the grid reliably.

At its full output of 1,100 megawatts of electricity, Unit 3 can power 500,000 homes and businesses. Utilities in Georgia, Florida and Alabama are receiving the electricity.

A fourth reactor is also nearing completion at the site, where two earlier reactors have been generating electricity for decades. The Nuclear Regulatory Commission on Friday said radioactive fuel could be loaded into Unit 4, a step expected to take place before the end of September. Unit 4 is scheduled to enter commercial operation by March.

The third and fourth reactors were originally supposed to cost $14 billion, but are now on track to cost their owners $31 billion. That doesn’t include $3.7 billion that original contractor Westinghouse paid to the owners to walk away from the project. That brings total spending to almost $35 billion.

The third reactor was supposed to start generating power in 2016 when construction began in 2009.

Vogtle is important because government officials and some utilities are again looking to nuclear power to alleviate climate change by generating electricity without burning natural gas, coal and oil.

In Georgia, almost every electric customer will pay for Vogtle. Georgia Power, the largest unit of Atlanta-based Southern Co., currently owns 45.7% of the reactors. Smaller shares are owned by Oglethorpe Power Corp., which provides electricity to member-owned cooperatives, the Municipal Electric Authority of Georgia and the city of Dalton. Oglethorpe and MEAG plan to sell power to cooperatives and municipal utilities across Georgia, as well in Jacksonville, Florida, and parts of Alabama and the Florida Panhandle.

Georgia Power’s 2.7 million customers are already paying part of the financing cost and elected public service commissioners have approved a monthly rate increase of $3.78 a month for residential customers as soon as the third unit begins generating power. That could hit bills in August, two months after residential customers saw a $16-a-month increase to pay for higher fuel costs.

Commissioners will decide later who pays for the remainder of the costs of Vogtle, including the fourth reactor.