Is solar worth it in Illinois? We modelled 46 real homes to find out
A study, not a sales page. We drew a random sample of detached homes across Ameren Illinois territory, ran every roof through the same model behind our free calculator, and report what came back — including the parts that do not help us sell anything.
The short answer
Of 46 homes, every one broke even inside 25 years and 44 of 46 were worth doing on net present value. Median payback 8.4 years, median return 10.8% a year, median 25-year net position $37,582. That is a good result — and it is not mainly because of the roofs. It is mostly policy, and policy can change.
1. Question
Not "does solar work in Illinois." You can get a yes to that from anyone selling it. The question we set was narrower and answerable: for a typical detached home in Ameren Illinois territory, buying at today's prices under today's policy, what is the payback period — and how much does it vary from roof to roof?
2. Method
We sampled 54 detached houses at random from OpenStreetMap across 17 central and southern Illinois cities, four per city, with a fixed random seed so the sample is reproducible rather than curated. We did not pick the homes. Picking them is how a study like this quietly gets its thumb on the scale.
Eight were then dropped, because resolving each home's actual electric utility showed they were not Ameren customers: four in Springfield are served by City Water, Light & Power, a municipal utility charging 10.6¢/kWh — half Ameren's rate — and four near Effingham by Norris Electric Cooperative. That left 46 homes across 13 cities.
| Held constant | Value | Source |
|---|---|---|
| Household consumption | 8,316 kWh/yr | Illinois residential average, 693 kWh/month — EIA Form EIA-861 (2024) |
| Electricity rate | 21.35¢/kWh | Ameren Illinois DS-1 Residential, filed tariff via OpenEI URDB |
| Installed cost | $3.30/W | Illinois modelled benchmark, DOE PVSCB Q1-2025 + BLS labour |
| Federal tax credit | 0% | Section 25D expired for expenditures after 31 December 2025 |
| Illinois Shines SREC | $67.50/MWh | IPA block price, paid as a lump sum on 15 years of projected output |
| Export credit | 100% of retail | Illinois mandates full retail net metering |
| Maintenance | $34/kW/yr | NLR Annual Technology Baseline, residential PV fixed O&M |
| Escalation / degradation / discount | 2.5% / 0.5% / 5% | Planning assumptions — all three swept in section 4 |
Every roof was modelled individually: Google's 3-D building data for plane geometry, per-plane shading measured from Google's annual flux raster (242 planes sampled; median shade factor 0.95, worst 0.54), and one NLR PVWatts® simulation per plane at its own tilt and azimuth. Systems were sized to roughly offset each home's own consumption, because Illinois net metering requires it. Median system: 7.0 kW producing 1,228 kWh per kW per year.
What “modelled individually” means
Three of the 46, chosen to span the shading range rather than to flatter the result. The left column is Google's aerial photograph. The right column is the same roof as Google's annual flux raster measures it — one reading of annual sunlight for every 10 cm of roof — with the panels our model actually placed drawn on top. Nothing in the right column is an artist's impression; it is the input the financial model runs on.
What the roof looks like
What the model measures
Peoria — open roof, five planes used. Google resolves fifteen separate roof planes here. The model puts panels on five of them, all facing 135–153°, where the raster reads 1,559–1,607 kWh/kW/yr. The north and west faces stay empty. Mean shade factor 0.88 · 1,358 kWh per kW per year · 6.4 kW (16 panels) · payback 7.2 years.
Decatur — nine planes, three used. Every panel sits within 9° of due south. The darkest patch, near the top of the frame, is a steep north-facing plane, and the model placed nothing on it — that darkness is orientation, not shade. Mean shade factor 0.78 · 1,190 kWh per kW per year · 7.6 kW (19 panels) · payback 8.9 years.
Decatur — one plane, heavy canopy. A single face at 182° and a 41° pitch, reading 1,169 kWh/kW/yr against roughly 1,800 unshaded. The trees take better than a third of it, and the house still breaks even inside eleven years. Mean shade factor 0.64 · 1,052 kWh per kW per year · 6.4 kW (16 panels) · payback 10.7 years.
Figure 1. Each frame is 28 m across; the scale bar is 5 m. Faded roofs are the neighbours' — Google measures the whole scene, and we keep only this building. The shade factor is the measured flux divided by what an unshaded plane at the same tilt and azimuth would make, so 0.64 means this roof loses just over a third of its sunlight to obstruction. Across all 46 homes the mean shade factor runs from 0.62 to 0.89; these three sit at the 2nd, 33rd and 93rd percentiles of that spread, which is what “span the range” means here. A dark plane is not automatically a shaded one — a north face is dark because of where it points. Dividing by an unshaded plane at the same tilt and azimuth is what separates the two, and getting that wrong is the single most common way a shading model flatters or punishes a roof by accident. Homes are identified by city only.
3. Results
Figure 2. Every home broke even, most between seven and ten years. The slow tail is shading and awkward orientation — not a different price or policy.
| Metric | Worst | Median | Best |
|---|---|---|---|
| Payback period | 14.9 yrs | 8.4 yrs | 7.2 yrs |
| Monthly saving, year 1 | $42 | $133 | $153 |
| Return (IRR) | 4.9% | 10.8% | 12.8% |
| Net present value | −$204 | $12,722 | $15,435 |
| 25-year net savings | $11,608 | $37,582 | $41,991 |
A 10.8% annual return, tax-free, on an asset bolted to your roof. The S&P 500 has returned roughly 10% a year since 1928 — before tax.
4. What actually decides it
We re-ran all 46 homes changing one variable at a time. This is the section that matters, because it separates the questions worth asking an installer from the ones that are noise.
Figure 3. A longer bar means the variable moves the answer more. Two policies sit at the top. The roof itself is fifth.
Figure 4. The worst-shaded quarter of roofs still paid back in a median 11.4 years, 91% of them worth doing. A mediocre roof in Illinois beats a good roof in a state with worse export rules.
5. What your electricity bill actually changes
We ran the sweep twice, and the two answers together are the interesting part.
Holding the system size fixed, consumption made no difference at all. Five thousand kilowatt-hours a year or sixteen thousand, payback came out at 8.4 years either way. That looked like a bug until we traced it, and it is not one: under full retail net metering every kilowatt-hour is worth the retail rate whether you use it or export it. Where the electron goes stops mattering.
But a bigger household does not install the same system — Illinois net metering lets you size up to your own consumption. Re-running with the system re-sized to each household is the honest test, and it separates two things people routinely conflate:
Figure 5. Twenty homes, each re-sized four times. A threefold increase in usage nearly triples the money and barely moves the payback period.
| Household usage | Median system | Payback | NPV | 25-year net |
|---|---|---|---|---|
| 5,000 kWh/yr | 4.4 kW | 8.2 yrs | $7,949 | $23,138 |
| 8,316 kWh/yr (state average) | 7.2 kW | 8.7 yrs | $12,607 | $38,508 |
| 12,000 kWh/yr | 10.4 kW | 9.0 yrs | $16,055 | $53,088 |
| 16,000 kWh/yr | 14.0 kW | 9.8 yrs | $18,245 | $66,466 |
Your bill decides how much there is to win, not whether you win. Triple the usage and the money nearly triples — but the payback period moves by only 1.6 years.
The payback drifting up as systems grow is real and worth understanding. Panels are placed on the best roof planes first, so each additional panel on a larger system goes somewhere slightly worse — a shadier plane, a poorer angle. Big systems are still worth building; they simply have a marginal panel that earns less than the first one did.
One caveat on all of this: it holds because Illinois credits exports at the full retail rate. Under a 25% export credit, a 5,000 kWh household falls to an 18.5-year payback and a negative NPV while a 16,000 kWh household holds at 10.9 years. The policy and the household interact everywhere else. Illinois currently removes the interaction.
Figure 6. The same 46 roofs, same prices, same incentives. Only the export rule changes. It is the biggest lever in the study and the one no homeowner controls.
So the honest headline is not "Illinois roofs are good"
They are ordinary. Illinois yields about 1,228 kWh per kW — less than Arizona, more than Seattle. What makes these numbers work is full retail net metering plus Illinois Shines. Remove the SRECs and median payback goes from 8.4 to 12.6 years. Move to net billing and it goes to 15. Both are policy, both have been changed in other states, and neither is guaranteed to last. That is an argument for acting while they exist, not an argument that the roof is special.
6. How you pay changes what you keep
Figure 7. Every route leaves the homeowner ahead. None of them leaves the homeowner equally ahead.
A lease or PPA is not a bad deal in the sense of costing you money — all 46 homes came out positive on both. It is a deal where most of the value goes to whoever owns the system. On the median home the fund behind the PPA earns 21.8% while the household keeps $5,967. Buying the same roof outright keeps $37,582 and earns 10.8%. The gap is the tax credit and the SRECs, which follow ownership — and since 25D expired, a third-party owner can still claim a commercial credit that you no longer can.
7. Batteries: not yet, and for a specific reason
We tested all five batteries in our catalogue against every home. None paid for itself on any of the 46. The best case still left the household about $14,300 worse off over 25 years. It is the same net-metering rule working against storage: when an exported kilowatt-hour already earns full retail, storing it to use later gains nothing and loses about 8% to round-trip efficiency.
That is a statement about money, not about batteries. Backup power in an outage is a real reason to buy one, and roughly six hours of essential loads is a real benefit. It is simply not a financial one in Illinois today.
8. Limitations
- Forty-six homes across thirteen cities describes a distribution. It is not enough to compare cities against one another, and you should not read city-level differences into it.
- Roof condition is not modelled. A roof needing replacement in eight years adds a removal-and-reinstall cost that can invert any verdict here.
- The main sample holds consumption at the state average. Section 5 sweeps it, but at four levels across twenty homes rather than continuously across all 46.
- A 25-year horizon outlives most homeownership. Median US tenure is about 12–13 years, and selling early changes the answer.
- Homes were drawn from OpenStreetMap, which may under-represent newer subdivisions with less complete mapping.
- Modelled production carries the usual PVWatts uncertainty, and shading is measured from imagery rather than surveyed on the roof.
9. Check it yourself
Every number here came from the free calculator on this site, running public data: NLR PVWatts® for production, Google Solar for roof geometry and shading, OpenEI URDB for the Ameren tariff, and the Illinois Power Agency's published block prices for the SRECs. The full methodology documents every model and default — including the nine defects we found in our own arithmetic and fixed.
The point of publishing the method is that you should not have to trust the conclusion. Put your own address in and see whether your roof looks like the median one. No phone number, no sales call, and if the numbers do not work at your address we will say so.
Study run 7 September 2026. 54 homes sampled with seed 20260907; 46 in Ameren Illinois territory after utility resolution. Informational estimates, not installation quotes.