Ameren Illinois MISO Capacity $424.30/MW-day

What MISO's auction cleared for Ameren territory

In April 2026, MISO announced results of its Planning Resource Auction for June 2026 through May 2027 [cub-ameren-capacity-2026]. For Ameren Illinois, located in MISO Zone 4, the summer capacity price cleared at $424.30 per MW-day [cub-ameren-capacity-2026].

That is lower than the distorted $666.50 per MW-day summer price from the 2025 auction, which included a software coding error that later produced customer credits [cub-ameren-capacity-2026]. It is still about 14 times the $30 per MW-day summer price from 2024 [cub-ameren-capacity-2026].

MISO capacity is seasonal. Prices drop sharply after summer in Zone 4 [cub-ameren-capacity-2026]. Ameren's default supply rate reflects that pattern, which is why downstate operators feel whiplash between June and October resets.

How capacity shows up on an Ameren bill

Capacity is payment for reserve margin: generators commit to be available when demand spikes [cub-ameren-capacity-2026]. Most Ameren customers do not see a separate capacity line. Instead, capacity is rolled into the price to compare, which also includes energy, transmission, and a supply cost adjustment [cub-ameren-capacity-2026].

CUB estimates capacity is roughly 20 percent of the supply price for typical customers [cub-ameren-capacity-2026]. Participants in Ameren's Power Smart Pricing hourly program do see capacity as its own line item [cub-ameren-capacity-2026].

For a cannabis facility, three bill layers still apply:

Layer	Who sets it	Shoppable?
Supply (includes capacity)	MISO auction + procurement	Yes, retail supplier
Delivery energy	Ameren tariff	No
Demand (kW)	Ameren tariff	No

Switching suppliers changes how capacity is priced in your contract, not Ameren's delivery demand charge. Read understanding your commercial utility bill before comparing quotes.

Ameren's summer 2026 price to compare

Ameren's summer supply rate, also called the price to compare, is 11.326 cents per kWh through September 2026 [cub-ameren-capacity-2026]. That is slightly below last summer's 12.18 cents but 39 percent higher than summer 2024 [cub-ameren-capacity-2026].

CUB translated the year-over-two-summers gap into usage terms: about $16 more per 500 kWh compared with summer 2024 [cub-ameren-capacity-2026].

Worked example (inputs we chose):

Input	Value
Monthly kWh	120,000
Summer PTC	11.326¢/kWh [cub-ameren-capacity-2026]
Supply charge	120,000 × $0.11326 = $13,591/month
vs summer 2024 at ~8.15¢*	120,000 × $0.0815 = $9,780/month
Extra supply cost vs 2024	$3,811/month

*Summer 2024 implied from CUB's 39 percent comparison [cub-ameren-capacity-2026].

Delivery and demand would stack on top. A Springfield or Metro East cultivator on Ameren Illinois should pull actual tariff demand charges from bills, not this supply-only sketch.

Why Ameren feels MISO more than ComEd feels PJM

Most MISO utilities are vertically integrated: they generate power and recover capacity outside the auction [cub-ameren-capacity-2026]. Ameren Illinois does not generate electricity and relies heavily on MISO's auction to cover its capacity needs [cub-ameren-capacity-2026]. That structural difference is why downstate default supply swings with MISO results while northern Illinois swings with PJM.

CUB cites several drivers of elevated MISO prices [cub-ameren-capacity-2026]:

• Data-center load growth feeding demand forecasts
• MISO's Reliability-Based Demand Curve pricing methodology
• Interconnection delays averaging about three and a half years
• Summer capacity mix still weighted to coal and gas (over 60 percent in the last auction CUB cited)

Illinois passed the Clean and Reliable Grid Affordability Act in 2025 to expand efficiency and storage after the prior summer spike [cub-ameren-capacity-2026]. CUB also supports the POWER Act to hold data centers accountable for energy costs [cub-ameren-capacity-2026].

Contract timing for downstate cultivators

MISO's planning year runs June 1 through May 31. Ameren's supply price resets October 1 to a lower seasonal level [cub-ameren-capacity-2026].

If you shop retail supply:

1. Compare the all-in fixed rate to 11.326 cents for summer months [cub-ameren-capacity-2026].
2. Ask whether capacity is embedded or passed through separately.
3. Align contract start with October if winter/summer spreads matter to your budget.
4. Review Illinois switching steps and fixed vs index contracts.

Northern Illinois operators facing ComEd's PJM-driven summer rate should read our ComEd summer 2026 supply post for the parallel PJM story.

What this means operationally

The $424.30/MW-day summer capacity price is a market fact from MISO's April 2026 auction [cub-ameren-capacity-2026]. It flows into default supply unless you contract differently. Ameren does not mark up supply; generators and marketers capture the margin [cub-ameren-capacity-2026].

You cannot shop away delivery demand. You can decide whether default supply or a retail fixed contract fits your load profile through the next seasonal reset. Model both summer and October prices before locking a 36-month term that treats every month like July.

CRGA Act and efficiency programs downstate

Illinois responded to the 2025 MISO spike with the Clean and Reliable Grid Affordability (CRGA) Act, expanding efficiency and battery storage programs CUB cites as consumer protection tools [cub-ameren-capacity-2026]. Ameren publishes no-cost and low-cost efficiency offers for commercial customers that can trim kWh even when supply ¢/kWh stays elevated.

Efficiency reduces energy charges first. It can lower demand if HVAC and lighting upgrades cut coincident peak. It does not replace reading your supply contract or comparing the price to compare on Plug In Illinois [il-plugin-illinois].

Lever	Affects supply ¢?	Affects demand kW?
LED retrofit	Lowers kWh	Often lowers peak
Staggered photoperiods	Same kWh	Lowers peak
Retail fixed contract	Locks supply benchmark	No effect on delivery
Community solar (credit)	Offsets supply $	No

Community power and municipal aggregation caution

CUB warns that municipal aggregation deals are not automatically cheaper than Ameren's supply price [cub-ameren-capacity-2026]. Illinois ICC maintains a list of community contracts; confirm expiration dates and add-on fees before enrolling a cultivation site.

If your city aggregation price floats with capacity, you may simply inherit MISO volatility with less transparency than default supply. Compare aggregation all-in to 11.326¢ summer PTC [cub-ameren-capacity-2026].

Document checklist before you sign

1. Twelve months of Ameren bills with supply, delivery, and demand separated
2. Interval data if available (Power Smart Pricing participants already have granular views [cub-ameren-capacity-2026])
3. Written pass-through table from any retail supplier
4. Expansion schedule aligned to October supply reset [cub-ameren-capacity-2026]
5. Confirmation your account is commercial and eligible for choice on Ameren Illinois

MISO Zone 4 geography and which grows sit in it

Ameren Illinois serves downstate Illinois outside ComEd's northern footprint. MISO assigns the territory to Local Resource Zone 4, where the April 2026 Planning Resource Auction cleared summer capacity at $424.30 per MW-day [cub-ameren-capacity-2026]. Springfield, Metro East, Champaign-Urbana, and Decatur cultivation clusters all share that auction result even though local delivery tariffs differ by substation.

Zone 4 summer capacity remains weighted to coal and gas at over 60 percent of the accredited mix in the auction CUB cited [cub-ameren-capacity-2026]. That fuel mix interacts with MISO's Reliability-Based Demand Curve, which sets prices based on reserve margin rather than a simple fixed adder. When accredited resources retire faster than new solar and storage interconnect, the curve steepens and auction prices jump.

The $666.50 software error and customer credits

The 2025 MISO auction produced a distorted $666.50 per MW-day summer price after a software coding error [cub-ameren-capacity-2026]. Regulators and utilities later issued customer credits to unwind part of the overcharge. The 2026 clearing at $424.30 is lower than that distorted peak but still roughly 14 times the $30 per MW-day summer price from 2024 [cub-ameren-capacity-2026].

Downstate operators who locked retail contracts during the spike should verify whether capacity true-ups flowed through on supplier invoices in 2025. If credits hit default supply but your contract passes through capacity at actual cost, you may have overpaid relative to neighbors on default service. Pull supplier settlement statements alongside Ameren price to compare history on Plug In Illinois [il-plugin-illinois].

ComEd PJM summer 10.399¢ versus Ameren MISO 11.326¢

Northern Illinois ComEd customers face PJM capacity embedded in a summer 10.399 cents per kWh supply rate filed May 2026 per our ComEd summer 2026 post. Downstate Ameren customers face MISO capacity in an 11.326 cents summer benchmark [cub-ameren-capacity-2026]. ComEd's vertically integrated structure buffers some capacity exposure; Ameren Illinois does not own generation and passes MISO auction results more directly [cub-ameren-capacity-2026].

Region	ISO/RTO capacity	Summer 2026 supply benchmark
ComEd (north)	PJM RPM	10.399¢/kWh
Ameren (downstate)	MISO PRA Zone 4	11.326¢/kWh

Illinois MSOs with Chicago veg rooms and downstate flower sites need two procurement playbooks, not one corporate mandate. Read our ComEd summer 2026 supply post for the northern parallel and Illinois commercial electricity rates for statewide context.

October 1 non-summer reset planning

CUB expects Ameren's supply price to fall October 1, 2026 when MISO summer capacity rolls off the default benchmark, though the exact non-summer figure was not finalized when CUB published its June 2026 Q&A [cub-ameren-capacity-2026]. Cultivators signing summer-only fixed retail should confirm automatic renewal language before October to avoid riding a variable index into winter without noticing [cub-ameren-capacity-2026].

If your harvest calendar concentrates kWh in Q3, summer 11.326 cents dominates cash even if October brings relief [cub-ameren-capacity-2026]. Model monthly kWh curves, not annual averages, when comparing 12-month fixed retail to semiannual default under Illinois switching guidance.