Illinois is served by two large electric utilities. Roughly the top corner of the state is ComEd territory and most of the rest is Ameren Illinois. If your business has sites in both, or if you are choosing where to put one, you will eventually be handed a savings number from one territory and asked to apply it to the other.
Do not. The short answer is that the physics transfer, the method transfers, and the money does not.
What transfers between the two
Your load shape. A bakery runs like a bakery in Peoria and in Joliet. The pattern of when you draw power is a property of your operation, and it is the input to every calculation that follows.
The method. Take the interval record, apply the tariff's own rule for converting it into billed quantities, and check the result against the bill. That procedure is utility independent. It is what separates a measurement from an estimate, and it works anywhere a bill and a meter both exist.
The questions. Is there a charge in dollars per kilowatt. Over what window and what interval length is it measured. Does anything carry forward from a prior month. How far is your top interval above your second. Those four questions are the right ones in any territory, and the answers are what differ. Worked through for a downstate account, they become whether Ameren Illinois has peak hours you actually pay for and, if your bill moved, which part of it changed.
Illinois statute. Some things are set by state law rather than by a utility, so they apply across both territories. Net metering is the clearest example: the current Illinois arrangement credits exports against the supply portion only, which means a demand charge is never offset by exported solar. That is why a solar payback model built on offsetting a demand charge is wrong on both sides of the territory line.
What does not transfer
The rate. Obviously, and it matters less than people think, because a rate is a multiplier and multipliers are easy to swap out.
The determinant. This is the one that actually breaks a transferred quote, and it is the one nobody mentions. The determinant is the rule that turns your meter record into the billed number. In ComEd's standard commercial delivery classes, we have reconciled it to the cent: billed demand is the highest half hour recorded inside a weekday window running 9 a.m. to 6 p.m., and nothing from a previous month carries forward. Overnight and weekend load is therefore free of demand charge, and one bad month costs one month.
Change any one of those and the same operation produces a different bill from the same meter data. A window running all hours makes your night shift expensive. A sixty minute interval smooths a brief surge that a thirty minute interval would have billed you for. A ratchet makes one bad afternoon expensive for a year. All of those structures exist, and which of them exists on an Ameren Illinois account is a question we have not answered.
How multiple meters combine. On the ComEd commercial bills we have reconciled, demand is site coincident, meaning the peaks of separate meters are summed on the clock rather than each meter's own maximum being added. Those two calculations produce different numbers from identical data, and which one applies decides whether splitting or combining meters is worth anything.
Why we will not give you the Ameren side of the comparison
We reconstruct ComEd commercial bills and reconcile them to the cent against the interval data behind them. We have not reconciled an Ameren Illinois bill.
We could read Ameren's filed tariff and write a comparison table from it this afternoon. It would look authoritative, and we would have no way of knowing whether it was right. Tariff text explains mechanics that reconciliation confirms; on its own it has been wrong often enough in ComEd territory, where we do check, that we will not publish an unchecked version for a utility where we do not.
So the comparison stays half finished, in public, until someone brings us an Ameren Illinois bill with its interval data. That is the honest state of it. The same limit governs everything else we have written about that territory: which rate zone you are in, whether business hourly supply is available the way ComEd's Rate BESH is, and how to request your interval data are all method and no Ameren numbers, deliberately.
What to ask before a portfolio decision
If you are comparing sites, or deciding where to place load, ask the person who produced the number these four things.
Which rate schedule did you price this against, and which utility filed it? A statewide average is not a rate schedule.
What determinant did you assume? Window, interval length, and whether anything carries forward. If they cannot state all three, they did not model a bill, they modelled a multiplier.
Did you use my metered intervals or a class profile? A class profile assumes you look like your neighbours, which is exactly the assumption a portfolio comparison is trying to test.
Show me the gap between my top interval and my second. That number caps what peak reduction can recover, and its absence from a proposal is the most reliable sign that no measurement happened.
Those four questions work in both territories, which is more than can be said for the savings number.
Is Ameren Illinois the same as ComEd?
No. They are separate utilities serving different parts of Illinois, each with its own tariff filed with the Illinois Commerce Commission. Advice written about ComEd's billing mechanics does not automatically apply to an Ameren Illinois account.
Can I compare an electricity savings quote across the two utilities?
Not directly. Your load shape and the analysis method transfer; the rate and, more importantly, the determinant that converts your meter record into billed quantities do not. A quote priced on one utility's determinant can be wrong by a wide margin on the other.
What is a determinant and why does it matter more than the rate?
It is the tariff rule that turns your interval record into the billed kilowatts or kilowatt hours. It sets the quantity, and changing it changes the bill from identical meter data. Rates are easy to substitute; determinants are not.
Does Illinois net metering work the same in both territories?
Net metering is set by Illinois statute rather than by each utility, so the framework applies across the state. Under the current arrangement exports credit against the supply portion only, which means a demand charge is not offset by exported generation in either territory.