I.How the agency buys the power
The buyer is the Illinois Power Agency, a state body that procures electricity for the default-service customers of ComEd in northern Illinois and, downstate, Ameren Illinois. It runs scheduled block procurements (bulk purchases of power for future months), and the Illinois Commerce Commission reviews the plan and the results. An independent procurement administrator runs the actual auctions, and the winning prices are published. The utility then passes the cost through to your bill with no markup, which is why ComEd prints a figure it calls the Price to Compare.
Because the power is bought ahead and laddered across several procurements, the rate you pay in any given month reflects a blend of past auctions rather than today's wholesale market. The agency typically holds procurements in the spring and the fall, so the supply rate changes on a schedule, not minute to minute. That is the honest reason a supply charge can hold steady for months while grid headlines swing: you are paying off purchases made, roughly, a year or two back.
II.Three legs, not one
An Illinois electric bill runs on more than one engine, and the supply charge is only the first. The second is delivery, the regulated cost of the poles, wires, and substations, which the Commerce Commission sets in rate cases and which has nothing to do with the auction. The third is capacity, the cost of keeping enough power plants on standby. For ComEd, capacity is bought through PJM's capacity auction (the yearly process where the regional grid operator pays plants to promise they will be available), a separate mechanism whose clearing prices have swung sharply in recent cycles. Ameren Illinois sits in a different grid, MISO, so the downstate math is not the same.
There is also hourly pricing. Customers who opt into ComEd's hourly program pay the real-time wholesale price instead of the agency's blended rate, which can be cheaper on mild days and painful during a summer heat wave off the lake. For the default customer who did nothing, though, the supply line traces back to the Illinois Power Agency's procurements, not to the capacity auction or the hourly market. Knowing which engine set your rate is the difference between reading your statement and guessing at it.
III.Check it against your own statement
This is the part a reader can verify without taking anyone's word for it. The agency posts its procurement results publicly, and ComEd publishes the Price to Compare that those results feed into. Find the supply section of your bill, note the per-kilowatt-hour supply rate, and set it beside the agency's published figures and the utility's current Price to Compare. If you are on default service and never switched, the numbers should line up closely.
If an alternate retail supplier has been pitching you a lower teaser rate, this is also where to be careful. The state runs a plain comparison site, Plug In Illinois, where the Commerce Commission lists licensed suppliers and their offer terms. Many of those offers reset after a few months or carry exit fees, so a rate that beats the Price to Compare today may not beat it next summer. It depends on the contract's fine print, and that is worth reading before you sign.
IV.Worth watching this month
1. Watch the Illinois Power Agency's site for the next published block procurement results, a routine posting that nonetheless sets what default customers pay in coming months.
2. Watch the Commerce Commission docket for its review of the agency's annual procurement plan, largely a routine step but the place where the buying strategy is actually decided.
3. Watch for the next scheduled change to ComEd's Price to Compare, since that is the number a default customer is measured against.
4. Watch PJM for the results of its next capacity auction, which genuinely matters because those capacity costs eventually flow into what you pay.
5. If you are weighing an alternate supplier, check Plug In Illinois for current licensed offers before committing to any contract.