I.Two grids, one state line
ComEd, the utility for Chicago and most of northern Illinois, belongs to PJM Interconnection, the grid operator that runs the system across much of the Mid-Atlantic and parts of the Midwest. Ameren Illinois, which serves much of central and southern Illinois, belongs to MISO, the Midcontinent Independent System Operator. A grid operator (the entity that keeps supply and demand matched second by second and runs the markets that pay for power) sets the rules a project lives under. In Illinois, there are two of them.
The two systems meet at what engineers call a seam. PJM and MISO coordinate across it under a long-standing joint operating agreement, but they stay separate markets, with separate auctions, separate interconnection queues, and separate reliability studies. A project does not get to pick a side. Its location on the map decides which rulebook applies.
II.What changes when you cross the seam
Capacity is the clearest example. Both grids pay power plants, and increasingly batteries, to promise they will be available when demand peaks, through a capacity auction (the recurring process where the grid buys those promises a few years ahead). PJM and MISO run these auctions on different schedules and with different designs, and the prices that clear can diverge sharply. At PJM's most recent auction, capacity prices rose steeply from prior years, per PJM's posted results, while MISO's seasonal auction has told a different story. For a developer, that gap can decide whether a project pencils out.
Interconnection is the other big one. Before anything connects, it has to clear a study queue, and both PJM and MISO have spent recent years reworking backlogged queues into a grouped cluster approach that studies projects together rather than one at a time. The waits run into years on both sides, at last public report, but the exact process, cost allocation, and timeline differ. Where a project sits changes who studies it and how long the line is.
III.Who decides what
The Illinois Commerce Commission (the state body that regulates investor-owned utilities like ComEd and Ameren Illinois) sets retail rates and oversees distribution planning on both sides of the seam. What it does not do is run the wholesale markets or the capacity auctions. Those sit with PJM and MISO, which answer to the Federal Energy Regulatory Commission, the federal regulator for interstate power. A rooftop or community solar question in Illinois usually touches at least two levels of government at once.
State programs, by contrast, apply across the whole state. The Illinois Shines program, run through the Illinois Power Agency, buys renewable energy credits from distributed and community solar in both ComEd and Ameren Illinois territory under the state's Climate and Equitable Jobs Act. A community solar subscriber near Peoria and one in Cook County can draw on the same state incentive, even though their power clears in two different wholesale markets.
IV.Worth watching this month
1. Watch PJM's posted auction calendar for the timing and results of its next capacity auction, which can move capacity costs that eventually reach northern Illinois supply charges.
2. Watch MISO's planning resource auction schedule, usually resolved in spring, for how capacity prices land on the Ameren Illinois side.
3. Watch the Illinois Commerce Commission's docket listings for distribution and grid-planning proceedings, most of them routine but some setting multi-year spending.
4. Watch the Illinois Shines pages for application-window openings in the Adjustable Block Program, which open and close as blocks fill.
5. Watch FERC filings on PJM and MISO interconnection queue reform, a slow process that still decides how fast local solar and storage can connect.