MISO said a yearslong software error caused it to clear more capacity than intended in past capacity auctions and resulted in an approximate $280 million impact to market participants in this year’s auction.
MISO said it uncovered the coding error — which had gone unnoticed since 2017 — in a third-party vendor’s work. The error caused MISO to clear additional capacity at higher auction clearing prices in the 2025/26 Planning Resource Auction (PRA), the RTO said.
That likely means the error produced higher prices and higher reserve margin requirements in MISO’s auctions all the way back to the 2018/19 planning year.
MISO said the error calculated its loss-of-load expectation (LOLE) using an “all-hours” methodology, rather than the tariff-defined “daily peak hour” methodology, leading this year’s auction to clear more capacity than intended.
MISO’s tariff defines LOLE as “the sum of the loss-of-load probability for the integrated daily peak hour for each day of the year.” As currently defined, a day with a loss-of-load event is counted in MISO’s LOLE calculations only if the event happens during the hour with daily peak load.
MISO said it discovered the error in June while running simulations of LOLE in preparation for the very change the software error induced. MISO wants to change its LOLE definition from one that’s expected only on the daily peak hour (the “daily peak hour” methodology) to one that could crop up at any hour in the day (the “all-hours” methodology). MISO has said that increasingly, generation emergencies can strike at any time.
MISO made a FERC filing in late August to transition from the daily peak hour to the all-hours LOLE methodology. It plans to use the approach formally beginning with the 2026/27 planning year if it receives FERC permission.
In MISO, the LOLE is the primary factor that determines demand curves in the capacity auction, which has a direct effect on clearing prices.
MISO said that while it won’t specify the exact number of additional megawatts that ended up clearing, the all-hours software approach led to an estimated 1 to 2% over-procurement of resources in the case of the 2025/26 auction.
In an email to RTO Insider, MISO said the $280 million financial impact from the over-procurement will extend to companies that entered the auction long or short on megawatts. That means if a market participant was paid based on auction results, then they must pay back a portion of their earnings to MISO. Market participants who were charged, on the other hand, can expect a refund from MISO.
MISO added that it will make only “paper adjustments” without financial impact for market participants that netted out their generation and load in the auction.
Settlement adjustments could affect any generator with accredited capacity in the 2025/26 PRA, MISO added.
MISO acknowledged that planning reserve margin requirements likely have been skewed since 2018 because of the software error. However, the RTO noted that its tariff limits evaluation of a continuing error to a one-year look-back period.
MISO’s seasonal planning reserve margin requirements for the 2025/26 planning year are 7.9% in summer, 14.9% in fall, 18.4% in winter and 25.3% in spring.
MISO said it will not retroactively alter 2025/26 capacity clearing prices to correct the error.
“MISO is not rerunning or resettling the PRA. We are not accepting new bids or establishing a new auction clearing price. Instead, adjustments will be made via settlements, not through price recalculation,” MISO said in a statement to RTO Insider.
The 2025/26 auction cleared at $666.50/MW-day in summer, $69.88/MW-day in spring, $33.20/MW-day in winter and $91.60/MW-day in MISO Midwest and $74.09/MW-day in MISO South for fall. (See MISO Summer Capacity Prices Shoot to $666.50 in 2025/26 Auction.)
MISO said it’s sending notices to generation companies about the financial impacts to their portfolios. It said it plans to “issue all adjustment statements or invoices” by Sept. 25.
MISO told RTO Insider that it would not disclose the vendor responsible for the error. The grid operator did not comment on whether it would continue to use the vendor’s services. MISO said at the time it discovered the error, the vendor “confirmed the software has never calculated LOLE based on the daily peak hour methodology since implemented in the 2018/19 PRA.”
MISO said it made a self-report with FERC and notified its Independent Market Monitor and Board of Directors. The grid operator also said it’s “working to strengthen validation and product testing for critical software.”
MISO leadership plans to discuss the software error and ongoing correction efforts with its board during a Sept. 16 meeting in Detroit, part of its quarterly Board Week.


