Is solar worth it in Illinois? We modelled 46 real homes to find out

We ran the SIA calculator (officially the SIA Solar Design Studio) on 46 randomly selected houses spread across Ameren's entire service territory in Illinois. Two things changed for Illinois buyers inside eighteen months: the federal ITC expired for systems placed in service after 31 December 2025, and net metering stopped crediting delivery charges for anyone connected after 2024. What is left is Illinois Shines and a smart inverter rebate. We wanted to find out whether that is still enough.

1. Research question

Is solar a good financial investment for homeowners in the Ameren territory of Illinois? To answer it we ran the SIA calculator over 46 randomly selected rooftops — the same free tool anyone can point at their own address — designing an optimal layout for each and assessing its payback period, monthly and lifetime savings, return on investment, and net present value.

2. Method

We sampled 54 detached houses at random from OpenStreetMap across 17 central and southern Illinois cities, four per city, using a fixed random seed, so anyone can draw the same 54 houses again and check our work. Randomly selecting the homes was necessary to avoid introducing bias into this analysis.

Eight were then dropped, because the calculator resolves each address to its actual electric utility, and theirs was not Ameren: 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)
Average monthly electric bill	$148	693 kWh/month at the DS-1 rate below; excludes the $14.01 fixed charge, which solar does not offset
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	53% of retail	11.326¢/kWh — Ameren's Price to Compare, which is the supply charge plus the transmission service charge. Since 1 January 2025, CEJA credits exports on the supply section only; delivery charges are no longer offset
Smart inverter rebate	$300/kW-DC	Paid on interconnection, introduced to offset the loss of delivery credits — Ameren Illinois
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, the largest size Illinois net metering credits; section 5 tests smaller ones. 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 in the SIA Solar Design Studio runs on.

What the model is actually adding up

Payback is the last step of a long arithmetic, and the inputs to it are worth seeing. A house does not use electricity evenly, a roof does not make it evenly, and the two peaks do not line up. Everything below is the model's own input, not an illustration.

3. Results

Metric	Worst	Median	Best
Payback period	17.8 yrs	10.1 yrs	8.3 yrs
Monthly saving, year 1	$41	$92	$101
Return (IRR)	3.2%	8.6%	10.9%
Net present value	−$4,649	$6,633	$9,064
25-year net savings	$11,026	$25,120	$28,003

Throughout this study a home is judged on the same bar the calculator holds a design to, not on whether it merely breaks even: good means payback inside ten years and more than $15,000 saved over 25; marginal means inside fifteen years and more than $5,000; anything else is poor. The worse of the two axes decides, so a quick payback on a tiny system does not pass.

Returns ran from 3.2% to 10.9%, with half the houses between 7.0% and 9.5%. Price, electricity rate and household usage were identical for every home, so that spread is the roof and nothing else. An 8.6% 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, and with single years that lost 40%.

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.

What each support is actually worth

The tornado ranks the variables; this puts money on them. We re-valued all 46 houses with each support removed in turn, changing nothing else — same roofs, same production, same $3.30/W, same 21.35¢/kWh.

5. What your electricity bill actually changes

Consumption enters the model twice: it sets how big a system Illinois lets you build, and it sets how much of your own output you use rather than export. Since 2025 those two are worth different amounts, so both matter.

A bigger household does not install the same system: Illinois lets you size up to your own consumption and no further. So the honest test re-sizes the array for each household rather than holding it fixed, and it separates two things people routinely conflate:

Household usage	Median system	Payback	NPV	25-year net
5,000 kWh/yr	4.4 kW	9.9 yrs	$4,327	$15,439
8,316 kWh/yr (study baseline)	7.2 kW	10.4 yrs	$6,303	$25,228
12,000 kWh/yr	10.4 kW	10.8 yrs	$8,125	$34,242
16,000 kWh/yr	14.0 kW	11.4 yrs	$8,237	$42,096

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 1.5 years.

That flatness is what lets us hold consumption at one figure for the main sample without the answer depending on it, and it is just as well, because the figure is conservative. The 8,316 kWh we use is Illinois' all-residential average, which includes apartments and condominiums. This study sampled detached houses, and a detached house uses more — NREL's ResStock puts the Illinois detached stock nearer 12,000 kWh, though its modelled level runs high against metered EIA totals. The band above spans both, and the payback answer moves by half a year across that part of it. A household at 12,000 kWh gets a larger system and about a third more money, on much the same timeline.

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.

The interaction is small but no longer absent. Under the full retail netting that applied before 2025 it genuinely vanished — a kilowatt-hour was worth the same whether you burned it or sold it. Supply-only netting brings the question back: a household that uses more of what it makes keeps more of the value, which is part of why the largest systems here pay back slowest.

Smaller than your usage is allowed, and it pays back sooner

Illinois caps a system at your consumption; it does not require you to reach it. Since 2025 the choice pulls both ways. An exported kilowatt-hour earns only the supply part of the rate, which rewards a system small enough for the house to use most of its output. Illinois Shines pays on every kilowatt-hour a system makes, exported or not, and the smart inverter rebate pays per kilowatt, which reward size. We held the household at the state average and varied only the array, taking the best roof positions first as the calculator does.

Array size	Upfront, after rebate	Payback	Return	25-year net
50% of usage, 3.6 kW	$10,800	8.1 yrs	11.2%	$17,740
70%, 4.8 kW	$14,400	9.0 yrs	9.9%	$20,815
85%, 6.0 kW	$18,000	9.8 yrs	9.0%	$23,245
100%, 7.2 kW	$21,600	10.4 yrs	8.3%	$25,228

In Illinois, size is a choice between speed and total. A system covering half your usage pays back 2.3 years sooner and returns 11.2% instead of 8.3%, on half the outlay; one covering all of it saves about $7,500 more over 25 years. Net present value barely moves, so neither is the wrong answer.

That is not how it came out in PG&E territory, where net present value falls by about a tenth between a system covering 70% of usage and one covering all of it. Here it falls by about 3%, because Illinois Shines and the rebate keep paying for the panels a smaller system would leave off. One caution runs the other way: the model prices every size at the same $3.30 a watt, and a small system usually costs more per watt, because permits, design and the inverter do not shrink with the array. A real half-size quote will narrow the payback gap, so ask for both sizes priced.

6. How you pay changes what you keep

A lease or PPA now does cost the typical household money, which was not true a year ago. It is the clearest single consequence of the two policy changes: the fund's payment is fixed at signing against what your roof generates, while the value of that generation fell when exports stopped earning delivery charges. Whatever is left goes to whoever owns the system. On the median home the fund behind the PPA still earns 21.8% while the household ends up −$9,760 over 25 years. Buying the same roof outright keeps $25,120. The gap is the SRECs and the smart inverter rebate, which follow ownership — and since 25D expired, a third-party owner can still claim a commercial credit that you no longer can.

If you do not have $23,760 in cash

Most people do not, so the loan column is the realistic one. On these 46 roofs a 10% deposit at 7% over fifteen years means a median $2,310 down and a $187 monthly payment, against a median first-year bill saving of $92. That leaves the household about $95 a month out of pocket for fifteen years — roughly $17,000 — before the loan ends and the system runs free for the remaining decade. Median payback is 18.2 years against 10.1 for buying outright, and the 25-year position is $12,301 against $25,120. On four of the 46 the loan never pays back at all inside 25 years.

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. The case for storage has actually improved and is still not enough. Under the old full retail rule an exported kilowatt-hour already earned full price, so storing it gained nothing and lost about 8% to round-trip efficiency. Supply-only netting gives a battery a real job — a kilowatt-hour kept at home is worth 21.4¢ against 11.3¢ sold — but about 10¢ a kilowatt-hour of arbitrage does not repay a battery inside its warranty at today's installed prices.

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

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 municipal bond yield quoted in the summary is the approximate 25-year AAA tax-free yield to maturity, FMSbonds, 7 September 2026. The full methodology documents every model, default and data source, with citations.

The dataset is archived alongside the model: the 54 homes as sampled, all 46 valued under five policy regimes, every held-constant input with its source, and the scripts that produced them — with the commit the model was at when it ran, because these numbers only mean anything against a specific version of it. A test suite recomputes every figure quoted on this page from that archive and fails if the page and the data ever disagree.

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.