Regulators gave Ameren Missouri permission to build a new natural gas and battery storage energy center in Jefferson County on Wednesday.
The Missouri Public Service Commission approved the Big Hollow Energy Center, which will be a simple-cycle natural gas plant — a type that can be turned on quickly to meet demand when a lot of people are using electricity — and a battery energy storage system.
The plant will be built on the site of Ameren's retired Rush Island coal plant and is already connected to the transmission system, so the utility will not need to spend extra money to connect the plant to the grid.
The natural gas plant will be able to generate 800 megawatts of power, which is about two-thirds the capacity of Ameren's Callaway nuclear power plant, while the battery storage system will store 400MW of power.
In documents requesting approval for the project filed in June, Ameren officials said it was necessary, in part, to power new large data centers the company expects to move to the region.
"The Big Hollow Projects are needed to provide reliable service to all of our customers, due to the likelihood that we will add customers with significant new large loads in just the next few years," wrote Ajay Arora, senior vice president and chief development officer.
In a January filing, Arora wrote that the company is at an advanced stage of signing electric contracts with companies for 2,000MW of new electric load, which would represent 2 ½ gas plants of electricity.
Arora also said in filings that the battery storage in particular would be appealing to data center customers with renewable energy goals.
"They have the ability to be charged by excess renewable energy on the electric system and thereby generate 24/7 hourly matching of carbon free energy that is important for large load customers," Arora wrote in June.
For example, Google is building large data centers and has publicly committed to 24/7 carbon-free energy.
Natural, or methane, gas produces less carbon than coal when it is burned, but it emits methane, which is a stronger planet-warming gas. During a virtual public hearing about the project, more than a dozen people said they opposed it, in part because of natural gas' environmental impact.
The potential cost of the Big Hollow project is redacted in the application before the Public Service Commission. Ameren's Senior Director for Regulatory Affairs Steve Wills did not share the estimated cost but said in a statement that the company is engaged in the bidding process and is keeping state regulators informed.
"As with all our projects, we are focused on managing costs responsibly and delivering value for customers," Wills said. "Any request to include costs associated with Big Hollow in rates would occur only after the facility is complete and reliably serving customers. The Commission would ultimately approve and ensure rates are fair and reasonable."
Another recently approved simple-cycle natural gas plant, Castle Bluff in south St. Louis County, had an estimated cost of $900 million in its application for approval that is now unredacted. It is also estimated to generate 800MW of power.
A new Missouri law allows utilities to use construction work in progress accounting, charging customers for power plants as they are being built instead of waiting until they are in place. In order to use that for this project, Ameren must request approval from the commission and provide documentation that the method would be less costly for consumers.
This article was updated to include comment on cost from Ameren.
Copyright 2026 St. Louis Public Radio
