ComEd posted four different supply prices between June 2025 and August 2026. The wholesale price of the electricity behind that supply changed every single month, and over the same fifteen months it ranged from 2.397 cents per kWh to 7.104. This page is the record of both series side by side, month by month, so the difference in how each behaves can be read rather than described.
What the two numbers are, and why they cannot be subtracted
They are not the same quantity, and the most important sentence on this page is the one that says so.
The Price to Compare is what ComEd charges for supply when you have not signed with a third party supplier: the electric supply charge plus the transmission services charge. It also recovers capacity, line losses and uncollectible accounts. ComEd does not profit on this energy; it buys power through state supervised procurement and passes the cost through.
The settled locational marginal price is the price of energy alone in the PJM market for the ComEd zone, in the hour it was delivered. It contains no capacity, no transmission, no losses and no uncollectibles.
So the distance between the two columns below is not a margin, not a markup and not a profit, and this page never subtracts them. What the two series can honestly be compared on is movement: how often each one changes, and by how much.
The record, month by month
Every row is the ICC's posted Price to Compare and its Purchased Electricity Adjustment for that month, beside the mean settled energy price for the same calendar month in the ComEd zone. The hour count is printed so the average can be checked; a month is on this page only if its hour count equals the calendar hours for that month.
| Month | Posted Price to Compare | PEA | Mean settled energy | Hours | Hours below zero |
|---|---|---|---|---|---|
| June 2025 | 10.028 | +0.527 | 5.023 | 720 | 3 |
| July 2025 | 10.028 | -0.446 | 4.881 | 744 | 0 |
| August 2025 | 10.028 | -0.029 | 3.835 | 744 | 0 |
| September 2025 | 10.027 | +0.022 | 3.963 | 720 | 5 |
| October 2025 | 9.689 | -1.469 | 3.227 | 744 | 61 |
| November 2025 | 9.689 | -1.721 | 3.741 | 720 | 20 |
| December 2025 | 9.689 | -0.849 | 3.621 | 744 | 7 |
| January 2026 | 9.660 | +0.357 | 3.251 | 744 | 94 |
| February 2026 | 9.660 | +0.889 | 3.881 | 672 | 39 |
| March 2026 | 9.660 | -0.151 | 3.191 | 744 | 51 |
| April 2026 | 9.660 | +1.159 | 2.397 | 720 | 104 |
| May 2026 | 9.660 | +8.166 | 3.078 | 744 | 42 |
| June 2026 | 10.399 | +0.230 | 3.423 | 720 | 12 |
| July 2026 | 10.399 | +0.247 | 7.104 | 744 | 3 |
| August 2026 | 10.399 | -0.142 | 4.165 | 744 | 3 |
All figures in cents per kWh. The full record, including the source URL on every row, is downloadable as CSV.
The posted price changed four times. The wholesale price changed fifteen.
Read down the first column and it is nearly a staircase: 10.028 for four months, 9.689 for three, 9.660 for five, then 10.399 from the June 2026 reset. Four values across fifteen months, and three of those four changes fall on a scheduled procurement window rather than on anything that happened in the market that month.
Read down the fourth column and no two months are alike. The mean settled energy price fell to 2.397 cents in April 2026 and rose to 7.104 cents in July 2026, nearly three times as much, and moved every month in between.
The sharpest single instance is July 2026. The mean settled energy price rose 3.681 cents from June, the largest month over month move in the record. The posted price did not change at all that month, because it had been set at the June reset and the next scheduled reset had not arrived.
That is what a fixed supply price is. It is not a claim about whether the price is high or low; it is a description of what the product does. A posted price holds still while the thing it pays for moves, and the reason a household on the default supply arrangement never sees a month like April 2026 is the same reason it never sees a month like July 2026.
What the adjustment does in between
The posted price is not the last word on a bill's supply cost. The Purchased Electricity Adjustment is a monthly credit or charge that trues up the difference between what the posted rates collected and what the electricity actually cost. In this record it is a credit in six of fifteen months, reaching -1.721 cents in November 2025, and a charge in nine, reaching +8.166 cents in May 2026.
That May 2026 figure is not an error and has been checked against the ICC's posted workbook: for that one month the adjustment was most of the supply cost. Any month's supply arithmetic on a real bill is the posted rate plus that month's adjustment, which is why a bill can print an all-in supply cost some distance from the headline number and still be exactly right.
Hours below zero
The last column counts the hours in each month when the settled energy price in the ComEd zone was negative, meaning the market paid to have power taken. There were 104 such hours in April 2026, one hour in seven, and 94 in January 2026.
A negative hour does not make anyone's bill negative, and a household on the posted Price to Compare experiences none of them: that is the same fixed price described above, seen from the other side. ComEd's Hourly Pricing program is the arrangement under which those hours reach a customer at all.
Where these numbers come from
The posted Price to Compare and the Purchased Electricity Adjustment are the Illinois Commerce Commission's own publication, Plug In Illinois, Historical Prices to Compare, retrieved August 31, 2026. The full nine year posted series and its reset calendar live on the Price to Compare record, which also carries that series as a chart and a downloadable dataset.
The settled prices are PJM's, from Data Miner, for the ComEd zone, at hourly settlement. The fifteen months here hold 10,968 hours and every month is complete; the generator refuses a month whose hour count does not match the calendar, which is why September 2026 is absent rather than shown as a low month on four hours of data.
Two limits are worth stating rather than leaving to be discovered. Months are grouped on the timestamp in UTC, so a month boundary is not local midnight; at an hourly grain that moves a handful of hours across a boundary and cannot move a monthly mean enough to change anything here. And a monthly mean of hourly prices is not what any particular customer paid, because customers do not consume equally in every hour; it is the price of the commodity over the month, which is the comparison this record is for.
What this record does not hold
It does not hold delivery. Delivery is the other half of a ComEd bill, it is set in an entirely different proceeding, and it behaves differently from supply: across the filed years the residential delivery charges have not fallen in any year on file, while the supply price in this record fell in October 2025 and January 2026.
It does not hold anything about whether a supplier offer is a good one. It holds what ComEd's own posted price was and what the energy behind it settled at, which is the record a person needs before that question can be asked honestly.
It does not extend before June 2025, because that is where the settled hourly series on file begins.
References