ComEd vs. Ameren Illinois

ComEd and Ameren Illinois are the two investor-owned electric utilities in Illinois, and for a business the important difference is not the company but the grid: ComEd's northern Illinois territory, including Chicago, is inside PJM, while Ameren Illinois's central and southern territory is inside MISO. Both deliver power under the same state deregulation law, both allow commercial customers to buy supply from a retail electric supplier, and both offer a default supply price procured by the Illinois Power Agency. But capacity costs, the timing of the capacity year, default supply pricing and demand tariffs all differ, and a business with sites in both territories is effectively buying in two markets.

We manage accounts across both and the questions are always the same: why is the capacity line so different, why did one site's default price move when the other's did not, and can the two be put on one contract. Here is the comparison, written for a controller or facilities manager rather than for a regulator.

The territories

ComEd serves roughly the northern fifth of Illinois: Chicago, the collar counties, Rockford and the area out to the Iowa border, about four million customers. It is a PJM utility, which puts it in the same wholesale market as Pennsylvania, New Jersey, Maryland, Ohio and the District of Columbia.

Ameren Illinois serves most of the rest of the state: Peoria, Springfield, Champaign, Decatur, Bloomington and the Metro East across the river from St. Louis, about 1.2 million electric customers. It is a MISO utility, sharing a market with Missouri, Indiana, Michigan, Wisconsin, Minnesota and the states south to Louisiana.

A handful of smaller utilities and municipal systems cover the gaps, and customers of municipal utilities and cooperatives generally cannot choose a supplier at all. For the two large territories, retail choice has been in place for commercial customers since the early 2000s and the market is mature.

Capacity: the biggest difference

Both grids pay generators to be available at peak, and both pass that cost to retail customers through a capacity charge based on the customer's draw at the grid's peak. The mechanics differ:

• ComEd (PJM). The capacity year runs June through May. The price is set three years ahead in PJM's base residual auction, and the ComEd zone often clears at its own, higher, price because of transmission limits into northern Illinois. The 2025-26 delivery year cleared at $269.92 per MW-day RTO-wide and 2026-27 at $329.17, the auction cap. A customer's tag is its average draw during the five highest PJM system hours of the prior summer. Our note on PJM capacity charges has the detail.
• Ameren Illinois (MISO). The planning year also runs June through May, but MISO's planning resource auction is held each April for the year starting that June, so the price is known only two months ahead rather than three years. MISO prices had been low for years and then rose sharply: the 2025-26 auction produced a summer price many multiples of the previous year's, which arrived on Ameren customers' bills with very little warning. The tag is based on the customer's draw at MISO's coincident peak.

The practical consequence is that a ComEd customer can see its capacity cost for the next three years and decide whether to fix it in the supply contract; an Ameren customer cannot, and has to decide whether to pay a supplier to take that uncertainty. For loads in Ameren territory we have increasingly recommended fixing capacity inside the supply price for exactly that reason, and for ComEd loads the decision depends on where the known auction prices sit relative to the supplier's premium.

Default supply: the Price to Compare

A commercial customer that does not choose a supplier is served at a default price procured by the Illinois Power Agency through periodic auctions. ComEd publishes it as the Price to Compare; Ameren Illinois publishes the equivalent for its rate zones. Both change at least twice a year, in June and October, and both move with the wholesale market and the capacity price. For small commercial accounts the default price is often competitive and the right choice. For larger accounts, those above the utility's hourly-pricing threshold, default service is hourly: the customer pays the real-time wholesale price every hour, which is a floating exposure most businesses do not want. That threshold is the point at which a fixed supply contract stops being optional.

Delivery and demand tariffs

Delivery, which the utility bills regardless of supplier, is structured differently. ComEd's commercial rate classes are defined by demand bands, with demand charges and distribution facilities charges that reward a flat load profile. Ameren's rate structure, with its own demand and distribution components, has historically been somewhat simpler for mid-sized accounts but carries different seasonal pricing. For a business with both, the same operational change, say shifting a process off the afternoon peak, produces different savings in each territory, and the rate class review has to be done separately for each.

Supply markets and suppliers

Most large retail suppliers are licensed in both territories, and the Illinois Commerce Commission licenses them statewide. But the wholesale product they are hedging is different: PJM West hub and ComEd zone prices for the north, Indiana hub or MISO Illinois prices for the south. A supplier strong in PJM is not automatically competitive in MISO and vice versa, which is why a same-day bid for a mixed portfolio should go to suppliers active in both and should allow the two territories to be awarded separately if that produces the lower total.

A portfolio in both territories

Take a distributor with a warehouse in Joliet (ComEd) and one in Decatur (Ameren). The right approach is usually one contract, one supplier, two prices: a ComEd price and an Ameren price on one piece of paper with one end date, so the renewal is managed once. The capacity treatment can differ by site. The term can be aligned so both end in a shoulder month. And the usage bandwidth should be set per site, because a change at one warehouse should not breach the band for the other. A supplier that insists on a single blended price across the two zones is simplifying its own hedge at the customer's expense.

Our notes on Illinois energy deregulation and Chicago business electricity rates cover the state and the ComEd market in more depth. The point of this comparison is narrower: in Illinois the utility name tells you which grid you are in, and the grid is what sets the strategy.