Less than a year ago, Southern Co. officials forged a political and financial deal to continue building a nuclear-power plant beset by cost overruns and delays. The company pledged that a new construction contractor would fix problems that had plagued the project.

On Wednesday, Southern said its costs to build the Alvin W. Vogtle Electric Generating Plant had risen again by more than $1 billion and said it would take a charge to its second-quarter earnings to cover the new cost overruns. Company officials said they still expected to have the two new power-generating units completed by 2021 and 2022.

The continuing difficulties finishing Vogtle highlight how difficult it would be to even think about a nuclear renaissance in the U.S., despite support for nuclear power from the White House. Existing plants are closing at a rapid clip because of competition from lower-cost natural gas. And Vogtle has turned from a showcase of a new generation of nuclear power into an object lesson in the financial risk of a private company trying to build a nuclear-power plant.

Southern said it wouldn’t ask Georgia regulators to pass along most of the cost increase to electricity customers. It said it would take a pretax charge of $1.1 billion to its earnings and issue $800 million in new equity before the end of the year. The company said that this stock issuance wouldn’t affect its 4%-to-6% earnings-per-share guidance.

The total projected cost of the new Vogtle reactors now tops $27 billion, more than double the initial estimates when they were approved in 2008. The 2021 startup for the first new unit is more than five years after it was initially scheduled to start.