Supply-only credit, and why old advice misleads
As of 2025, new residential and small commercial net metering customers in ComEd territory moved from full retail rate net metering to supply-only net metering. An exported kilowatt-hour is credited against the electricity supply portion of your bill rather than against the full retail rate.
Your bill has two broad parts. Supply is the electricity itself; delivery is the network that brings it to the house. Power you consume as you generate it avoids both parts, because you never buy that kilowatt-hour at all. Power you export earns the narrower supply-only credit. So the same panel is worth different amounts depending on whether the household is using the output at the moment it is produced.
This is why a neighbour's numbers from a few years ago are not a guide. Someone who registered under the older arrangement is on different terms from someone registering now, and a payback estimate carried over from that period overstates what you would get. Ask your installer to confirm the current net metering tariff for your address in writing, and to show whether their model credits exports at supply-only rates.
What it means for how the system is designed
The first consequence is sizing. When exports earn less than self-consumption saves, the useful size is the one that matches your household's own use rather than the one that fills the roof. Bring twelve months of bills to the first conversation and ask the installer to size against them.
The second is timing. Shifting flexible load into daylight converts export credit into avoided purchase: running the dishwasher, the laundry or an EV charger while the array is producing is worth more than doing it after dark. That is free to do and it changes the economics of a system you already own.
The third is storage. A battery lets you use your own generation in the evening instead of exporting it at the narrower credit, which is a real argument rather than a sales one. Whether it pays depends on how much surplus your array actually produces and what the storage costs, so ask for that tradeoff to be modelled against your own usage rather than asserted.
Illinois rules have been moving, so date your information
Illinois net metering rules have been the subject of ongoing Illinois Commerce Commission proceedings as part of broader distributed generation policy updates, and credit rates and program terms can change as a result. The 2025 move to supply-only crediting is itself an example of that.
The practical habit worth adopting is to date every piece of information you rely on. A guide with no date on it, or one written before 2025, may be describing an arrangement that no longer applies to a new customer. That includes national solar websites, which update state pages unevenly.
Ask your installer to confirm the current tariff and any applicable charges for ComEd at the point you are quoted, not at the point they learned it. A company that installs regularly in the territory will have the current terms to hand.
Joliet roofs, and what they produce
Plan on roughly 1,259 kWh a year for every kW of panels as a screening figure. It is derived from satellite irradiance data with a standard performance ratio applied, not measured from local systems, so the site survey is what produces the number worth contracting on.
Single-detached houses are 69.5 percent of the housing stock here, and single-attached units another 9.4 percent, where the roof plane and property line need establishing on site rather than from an aerial image. Two-unit buildings hold 5.1 percent and three and four unit buildings 3.8 percent, and in those an array feeds one electrical service, so which meter it connects to decides whose bill it reduces.
Buildings of 20 or more units hold 6.1 percent, where a resident does not control the roof and the decision belongs to the owner or the association. On any of these, ask for production modelled per roof plane with the shading assumptions stated, so you can see which panels are actually earning.
The money side of a Joliet project
Illinois Shines, the state Adjustable Block Program administered by the Illinois Power Agency, pays Renewable Energy Credit money to Approved Vendors under its Distributed Generation track, and those vendors may pass the value on to you as reduced equipment or installation cost. Participation requires an Approved Vendor, and how much of the payment reaches you is the vendor's choice, so ask each one for the Disclosure Form that states it.
Illinois Solar for All is a separate income-based program covering homeowners, renters through community solar and multifamily building owners, structured so participating households pay less for electricity than they would have without solar. If money is the obstacle rather than the roof, check eligibility there before shopping quotes.
The 30 percent federal residential tax credit under Section 25D is no longer part of the arithmetic. It applied through December 31, 2025 and does not cover a purchased home system placed in service after that date, so a 2026 purchase in Joliet cannot claim it. A lease or power purchase agreement provider may claim the separate business credit under Section 48E and reflect some of it in your rate, which is a question for the provider and a tax advisor.