Most of the small lines on a ComEd bill change once or twice a year. The Purchased Electricity Adjustment changes every single month, and roughly four months in ten it is a credit rather than a charge.
Its informational sheet is on its 212th revision. Nothing else in ComEd's book comes close.
What it cost on two real bills
| Bill | Usage | Filed factor | Arithmetic | Printed |
|---|---|---|---|---|
| July 2026 billing period | 953 kWh | 0.247 cents per kWh | 953 x $0.00247 = $2.35391 | $2.35 |
| June 2026 billing period | 1,518 kWh | 0.230 cents per kWh | 1,518 x $0.00230 = $3.49140 | $3.49 |
Two different months, two different factors. That is the defining feature of this line, and it is why comparing the Purchased Electricity Adjustment between two of your own bills tells you nothing about your usage.
Fifteen months of filed values
Every one of these is a filed value from a stamped ComEd informational filing with the Illinois Commerce Commission. Values in brackets are credits, following the tariff's own notation.
| Billing period | PEA factor, cents per kWh |
|---|---|
| June 2025 | 0.527 |
| July 2025 | (0.446) |
| August 2025 | (0.029) |
| September 2025 | 0.022 |
| October 2025 | (1.469) |
| November 2025 | (1.721) |
| December 2025 | (0.849) |
| January 2026 | 0.357 |
| February 2026 | 0.889 |
| March 2026 | (0.151) |
| April 2026 | 1.159 |
| May 2026 | 8.166 |
| June 2026 | 0.230 |
| July 2026 | 0.247 |
| August 2026 | (0.142) |
Six of the fifteen are credits. On a 1,000 kWh bill, November 2025 took $17.21 off the total and May 2026 added $81.66 to it.
The May 2026 value is real
It is not a typographical error and it is worth stating plainly, because anyone who sees 8.166 next to a column of values under 2 will assume a misplaced decimal point.
The figure appears on ComEd's stamped filing for the May 2026 monthly billing period, alongside an Hourly Purchased Electricity Adjustment of 8.032 cents per kWh for the same month. Two different factors, filed together, both around eight cents. A misplaced decimal in one would not reproduce itself in the other.
For one month, the adjustment line on a ComEd Rate BES bill was roughly the size of the entire supply charge.
The filed sheet states the factor. It does not state which term in the equation produced it, and we are not going to guess. What can be said from the tariff is that the equation contains terms capable of producing exactly this shape: an A term for correcting errors in previously applied factors, carrying interest, and an AB Automatic Balancing Factor carrying the cumulative debit or credit balance. Both may be amortised across up to twelve periods, and both may also be applied in one.
One number changes the sign of the whole period. Summing the fifteen filed factors gives 6.790 cents per kWh in net charges. Take May 2026 out and the remaining fourteen months sum to a net credit of 1.376 cents, an average of about 0.1 cents per kilowatt hour back to customers. That arithmetic is ours, done on the filed values above.
What the filed tariff says it is for
Rider PE governs how ComEd buys electricity for customers who take supply from ComEd. The Purchased Electricity Adjustment is the true up between what that supply actually cost and what customers were actually billed.
The filed equation, in words: accrued expenses, less accrued revenues, plus the automatic balancing factor, plus any ordered adjustment, optionally amortised, divided by forecast usage.
- AE, accrued expenses of procuring power for Rate BES customers.
- AR, accrued revenues recognised for those customers.
- AB, the Automatic Balancing Factor, the running debit or credit balance from applying past factors, including interest at the rate the ICC sets under 83 Illinois Administrative Code Section 280.40(g).
- A, an amount ordered by the ICC or determined by ComEd to correct errors in previously applied factors, also with interest.
- U, forecast usage.
The line on your bill is settling a month that is already gone
This is the part that is invisible from the bill and that changes how you should read it.
The tariff defines the effective period, the month in which a factor is applied, as the first monthly billing period beginning no earlier than fifteen calendar days after the final reconciliation of the PJM-conducted settlement process for the determination period.
PJM runs the wholesale market. Its settlement for a given month is not final for some weeks. So the Purchased Electricity Adjustment on this month's bill is not this month's cost. It is the settled difference on an earlier month, arriving once the wholesale market has closed its books and a fifteen day clock has run.
That is why the factor bears no relationship to what the weather did while you were using the electricity you are being billed for.
ComEd must file each revised factor with the ICC for informational purposes no later than the twentieth day of the month before it applies, with supporting work papers. The next month's factor is therefore public before the month begins, which is how the August 2026 credit above is already known.
Can it be avoided?
This one, unlike the other lines in this set, yes. With a caveat that matters.
The Purchased Electricity Adjustment applies to kilowatt hours provided to retail customers taking service under Rate BES, ComEd's Basic Electric Service. Rate BES is the default supply you get when you have not chosen a retail electric supplier. The filed note says so directly: the factor is designated on bills as the Purchased Electricity Adjustment pursuant to Rate BES.
If you buy your supply from a retail electric supplier, you are not taking supply under Rate BES and this line is not on your bill.
That makes it genuinely different from the delivery side riders, which follow the kilowatt hours regardless of who sells them to you. It is worth being precise about this, because the usual advice that switching supplier cannot touch your bill's small print is correct for those riders and wrong for this one.
The caveat is the table above. This factor is a credit about as often as it is a charge, and over the fourteen months excluding May 2026 it was a net credit. Leaving Rate BES does not only remove a charge. It removes a two way settlement that has, over most of the recent record, run in the customer's favour. Whether switching supply is a good idea is a question about the supply rate and your load shape, and this line is not the reason to do it either way.
What this page was checked against
Both, and unusually thoroughly. The mechanism is quoted from Rider PE, Sheets No. 324 through 328 of ComEd's filed Schedule of Rates. Every monthly factor in the table is a filed value from a stamped ICC informational filing. Two unrelated bills reconcile to the cent against the June and July 2026 factors.
The one thing on this page that is not filed is the arithmetic on the series, the 6.790 cent sum and the 1.376 cent excluding-May credit. Those are ours, computed from the filed values, and shown so you can redo them.
The other lines on the same bill are covered in every line on a ComEd bill, explained.
What is the Purchased Electricity Adjustment on my ComEd bill?
It is the monthly true up between what ComEd's electricity supply actually cost and what customers taking ComEd supply were billed for it, filed under Rider PE and applied to customers on Rate BES.
Why does it change every month?
Because it settles against the PJM wholesale market, which finalises each month separately. ComEd files a new factor with the ICC by the twentieth of the month before it applies. Informational Sheet No. 3 is on its 212th revision.
Why is my Purchased Electricity Adjustment negative?
Because the factor is a credit in months where ComEd over recovered against actual supply cost. Six of the fifteen filed factors from June 2025 to August 2026 were credits, including every month from October to December 2025.
Was the May 2026 figure of 8.166 cents per kWh a mistake?
No. It appears on the stamped filing for that month alongside an hourly factor of 8.032 cents per kWh. The filed sheet does not state which term of the equation produced it.
Can I avoid the Purchased Electricity Adjustment?
Yes, by taking supply from a retail electric supplier rather than under ComEd's Rate BES. But the factor is a credit roughly as often as it is a charge, so removing it removes a settlement that runs both ways, and it is a poor reason on its own to switch supply.