Missouri regulators approve Ameren's 545 MW of batteries, 400 MW of solar

Ameren Missouri can build 545 MW of batteries at three sites and buy two solar projects totaling 400 MW, after the Missouri Public Service Commission approved a settlement on October 21, 2026 that hangs fire-safety, cost and reporting conditions on the certificates. The order, in case EA-2026-0183, takes effect November 20. It decides that Ameren may build; who pays, and how much, is left for later proceedings.

The batteries are Castle Bluff, 95 MW at the retired Meramec coal plant in St. Louis County; Huck Finn, 200 MW at Ameren's Audrain Energy Center in Audrain County; and Millcreek, 250 MW in Lincoln County, which also gets a new 161 kV switching station, Quinn, to tie it to the transmission system. The solar comes through build-transfer agreements: Tom Sawyer, 175 MW in Audrain County, from EDF power solutions, and Ringer, 225 MW in Stoddard County, from Invenergy, per Ameren's release. Castle Bluff shares its site with the Castle Bluff gas turbine plant now under construction, so a former coal plant becomes a gas plant with a battery beside it.

Ameren's May 22 application has construction on all three batteries starting in the first quarter of 2027 and service in the second quarter of 2028. Huck Finn and Millcreek are four-hour lithium iron phosphate systems, which by our arithmetic makes them 800 MWh and 1,000 MWh of energy behind their megawatt ratings (the power-versus-energy distinction). Tom Sawyer is due in 2028, Ringer in 2029. Ameren plans to connect Millcreek through MISO's Expedited Resource Addition Study, which the application says can produce an interconnection agreement in as little as three to four months and spares the project the risk of a standard queue position stretching its timeline (the wider backlog is on our interconnection reference page).

What the conditions require

Millcreek drew the most specific conditions, and battery safety runs through them. Under the agreement, Ameren must meet the Office of the Public Counsel (OPC) and commission Staff to discuss the results of Tesla Megapack 3 safety testing, and must file a public pleading reporting the results, with the supplier's confidential test report attached for Staff and OPC, at least 90 days before batteries are installed at the Millcreek site. It must meet OPC, Staff and Renew Missouri to discuss expanding the Millcreek project's online FAQ to cover NFPA 855 compliance, Megapack 3 safety, nearby water and gas pipes, and rural versus urban siting, and at a minimum expand the FAQ and file a notice affirming that the batteries are meant to comply with NFPA 855 and will be safety-tested before installation.

For every battery site, Ameren has to file a site-specific hazard mitigation analysis or plan, and whatever fire-response containment procedures its environmental team develops, after the facility goes into service. The agreement also adds a performance check: within 90 days of service, Ameren must report how long each battery takes to discharge from 100% to below 5% at two rates between 25% and 75% of its maximum, on days without MISO capacity advisories, conservative operations or max-gen events. A rated duration, in other words, gets measured.

The money conditions are reporting rather than caps. Ameren must notify the commission if any project's total cost moves by more than 15% of its base or risk-adjusted estimate, explain why and say how it tried to mitigate the increase, and file quarterly construction reports covering permits, costs, milestones and the effects of tariffs, tax credits and Foreign Entities of Concern rules. It also agreed to account for prolonged winter stress and consider longer-duration storage, including 8-hour batteries, in its first integrated resource plan under section 393.1900.

The bill comes later

The order says nothing in it is a finding on the reasonableness of the expenditures or on the properties' value for ratemaking, and it reserves ratemaking treatment for later proceedings. The agreement spells it out: approval leaves open, among other questions, prudence, cost allocation (including whether costs go to large-load customers), rate recovery and the amount, if any, that reaches customer rates. The public version of the application blacks out the projects' cost estimates. It says the batteries should qualify for the 30% investment tax credit, Castle Bluff for up to 50% with bonus credits, and Huck Finn and Millcreek for up to 40%.

Ameren's case for need cites its 2025 preferred resource plan, which calls for 1,000 MW of battery storage by 2030, and signed electric service agreements for 2.8 GW of large load. The OPC did not sign the agreement and did not object to it. In its response, it said it will likely argue in a future rate case that the large-load customers who created the need for the projects should bear their full cost. The memorandum attached to it, by OPC chief economist Geoff Marke and policy analyst Jordan Seaver, says "the investments contemplated in this docket are alarmingly cost-prohibitive, provide little to no winter capacity accreditation, and have been met with a high degree of skeptical and resistant local public outcry." It puts their default winter accreditation at 8%.

Lincoln County objected; the rules counted parties

The case docket lists 340 public comments, and the OPC memo, counting 321 on October 6, says most concerned Millcreek. The commission held five local public hearings, the last in Lincoln County in August. The concerns, per the OPC, include battery safety, water runoff, the effects of fires, property values and local roads, and accusations that Ameren is overpaying to secure property.

Three Lincoln County residents asked to intervene out of time on September 30 and October 2, arguing in part that only a party may object to a settlement. The commission denied them on October 19, finding their interests no different from the general public's, which the OPC represents. Two days later it treated the agreement as unanimous, because no party had objected within seven days of its filing.

Lincoln County's objections may still matter in the rate case. The OPC memo says that, as its authors understand it, concerns like these do not typically affect the commission's certificate analysis, "but they may form the basis of a potential prudence disallowance in a future rate case." It put Ameren on notice that the OPC "may support future prudence disallowances should information arise in future investigations that Ameren acted imprudently in constructing or operating these Projects." The order says the commission weighed the opposition, Millcreek's in particular, and pointed to the pre-installation safety testing and NFPA 855 compliance; its release calls that balancing the landowners' "interests", where the order says "objections."

The three batteries follow Big Hollow. On February 11 the commission approved Big Hollow, an 800 MW gas plant in Jefferson County paired with a planned 400 MW battery, per Ameren. Add the three batteries approved October 21 and the two cases come to 945 MW by our count, against the 1,000 MW Ameren plans by 2030.