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ST. LOUIS COUNTY — Ameren won state regulatory approval Wednesday to build a $900 million natural gas-fired power plant in south ӣ����Ƶ County and agreed to take some steps that could result in more use of wind and solar energy.

The Missouri Public Service Commission, meeting in Jefferson City, unanimously OK’d an agreement worked out by the ӣ����Ƶ-based electric utility and a critic of the new plant — Renew Missouri, a nonprofit advocating for increased use of renewable energy.

“This order is a wonderful compromise because it moves toward balancing our energy portfolio with both traditional sources and renewable sources,” commission member John Mitchell said at the meeting.

PSC members also said that the new Castle Bluff plant, which will replace Ameren’s former coal-fired Meramec Energy Center, is needed to deal with potential shortfalls of electric power in the ӣ����Ƶ area and other parts of the state.

“Absent this action, customers in that area would incur significant additional costs,” Mitchell said.

Ameren also has said the project is needed to deal with increased power demand, particularly as data centers and other energy-intensive commercial activities grow in importance.

“Castle Bluff is an investment in our future,” said Jeff Moore, director of combustion turbine generator plants for Ameren Missouri, after Wednesday’s decision.

“It’s going to provide essential electric reliability for our customers, especially during extreme weather events” when demand is greatest.

He said construction will begin in the next two to three weeks at the site, at the confluence of the Mississippi and Meramec rivers. He said the plant is to start operating in late 2027.

The agreement calls for Ameren to study the possibility of buying power from Grain Belt Express, a long-planned electric transmission line designed to send power from wind turbine farms in Kansas across northern Missouri to the Indiana border. Grain Belt also is a party to the agreement approved Wednesday.

The agreement says Ameren will make ‘’best efforts” to install a battery energy storage system at one or more of its former coal plant sites, such as Castle Bluff, by 2027. The utility also will study the possible construction of another battery system by 2034.

James Owen, an attorney for Renew Missouri, said such systems often are used to store renewable energy, such as wind and solar power, though other energy also can be stored.

Owen said because it had appeared likely that the state commission would approve the natural gas-fired plant, Renew Missouri believed it would be good to try to get some advances for renewable energy as part of the package.

He said it would “help us stomach what we think is ultimately a bad decision by Ameren” to build a natural gas-fired plant.

At a virtual public hearing last month on the issue, residents and advocacy groups complained about the plant’s cost, the volatility of natural gas prices, the reliability of the fuel source in extreme conditions, impacts on local air quality and how it could exacerbate climate change.

Jason Holsman, one of the PSC members, said at Wednesday’s meeting that while he supports “future technology clean energy,” the state has a “resource capacity issue right now” and renewable energy is produced intermittently.

He also pointed out that the new Ameren plant would be built where there already is infrastructure in place. And, he added, natural gas is cleaner than coal.

Ameren previously has said it aims to invest billions in wind and solar projects over the next two decades, as costs of the technologies drop and as the utility strives to trim and eventually eliminate carbon emissions.

The agreement approved Wednesday also includes other requirements, such as details on how the costs would be included in Ameren’s rate base.

An attorney for another party to the agreement, the Midwest Energy Consumers Group, could not be reached for comment Wednesday. That organization advocates for large commercial and industrial power customers.