ComEd Interconnection: Will Your Commercial Solar Project Clear It?

Interconnection is the step that decides whether a commercial array exists at all, and it is the step a proposal is least likely to have priced. Everything else in a solar decision is economics. This one is permission.

The useful thing about the process is that the hard part is determined early. Which review level a project triggers follows from its export capacity and nameplate rating, and both are known before anyone produces a drawing.

The levels, and what puts you in one

Illinois runs interconnection through a tiered review under Part 466, and ComEd administers it. The tiers step up with size and with whether the system exports at all.

The fastest path is reserved for small inverter-based, lab-certified systems under both an export cap and a nameplate cap. Above that, projects move into a voltage-tiered review that reaches up to five megawatts, where the ceiling on your project depends on the voltage of the line you connect to rather than on your roof. A system configured to never export lands in its own category. Everything that fits none of these falls to the largest review, with the highest fee.

The practical consequence is that size is not a smooth dial. Crossing a threshold moves you into a different review with different engineering requirements and a different fee, and a project sized just over a line can cost materially more to permit than one sized just under it.

What you owe, and what the installer owes

This is worth being blunt about, because it is where buyers get surprised.

The engineering package is the installer's. One-line and three-line diagrams, relay settings, the site plan showing the point of interconnection, equipment data sheets, and a professional engineer's stamp above certain inverter thresholds. Those are due at application, not later, and they require a site visit and protection design that no data analysis can substitute for.

What the buyer can supply, and should, is the measured layer: what the building actually draws, when it peaks, and therefore what the system needs to be. That is the part that sets the size, and the size is what sets the review level. Getting it from the meter rather than from a rule of thumb is what keeps a project from being designed into an expensive tier for no reason.

The question worth asking first

Not "can we interconnect," which is nearly always yes at some price, but "which level does the size we actually need put us in, and is the system that clears the cheaper review still the system worth building."

That is answerable from your own interval data and your payback math before an application is ever filed.

The application also names the rider the system will operate under, and the name is informative: Rider POG and what it does and does not govern, including two charges people expect to appear that are not in ComEd's tariff at all.