Solar Answers → Your Home & Roof
How many solar panels do I need for my Illinois home?
A typical Illinois home lands around 6 to 7 kW — roughly 15 to 18 panels at 400 watts each — to match a year of its electricity use on paper. That comes from Illinois' average of 693 kWh a month (EIA, 2024) and production of about 1,240 kWh per kW per year. Your own last 12 months of usage decides your real number.
Independent public sources we cite and link — The Day Company is not affiliated with or endorsed by any of them.
How many solar panels does the average Illinois home need?
About 6 to 7 kW — roughly 15 to 18 panels at 400 watts — covers a year of average Illinois household use. The U.S. Department of Energy notes that NREL's analyses use an average residential system size of 7.15 kW, with a 3 to 11 kW range. Illinois homes sit toward the middle of that range, because Illinois households use less electricity than the national average.
That last point is why national panel-count answers tend to run large. The average Illinois home used 693 kWh a month in 2024 — about 8,316 kWh a year — according to EIA's state-level residential data. The U.S. average that same year was 865 kWh a month. An answer built on the national figure oversizes an Illinois roof by roughly a quarter before anyone has looked at your bill.
So treat 6 to 7 kW as the shape of the answer, not the answer. The next three sections give you your own number.
How do I find my actual usage on my ComEd or Ameren bill?
Look for kilowatt-hours, not dollars. Your bill states the kWh used for that billing period, and your online utility account keeps at least 12 months of history. Add twelve consecutive months together and you have the only input that really matters for sizing — your true annual usage, seasonal swings included.
Twelve months matters more than it sounds. A single July bill in an air-conditioned home will size you too big; a single April bill will size you too small. Illinois usage swings hard in both directions, and a system designed off one month is designed off noise.
If the dollar figure on that bill is what brought you here, the reason it moves is a separate question — why is my ComEd bill so high? breaks the charges apart. For sizing, ignore the dollars entirely and write down the kWh.
How much electricity does 1 kW of solar produce in Illinois?
Roughly 1,170 to 1,310 kWh per year for every kW of panels, using the University of Illinois Extension's method. The Extension puts Illinois at 4 to 4.5 peak sun hours a day and applies a 0.80 factor covering panel efficiency loss over time, cloud and snow shading, and weaker output on very hot days. This page uses about 1,240 kWh per kW as the midpoint.
That 0.80 factor is deliberately conservative — it accounts for efficiency decline across the panels' lifetime, not just year one. A number built this way will read slightly lower than a first-year production model, which is the right direction to be wrong in when you are making a decision.
For a figure tied to your specific address, roof pitch and orientation, the public tool is NREL's PVWatts Calculator. Any installer worth signing with will run it — or an equivalent model — for your actual roof, and should show you the inputs they used.
How do I calculate my own system size?
Divide your annual kWh by annual production per kW. Using the Extension's figure, that is your yearly usage divided by about 1,240. A home using 12,000 kWh a year works out to roughly 9.7 kW. The Extension also suggests starting with a goal of 50 to 75 percent of your usage rather than 100 percent, since usage varies by season and time of day.
System size (kW) × 1,000 ÷ 400 = approximate panel count
Worked example. Twelve months of bills add up to 12,000 kWh. Divide by 1,240 and you get 9.7 kW. Multiply by 1,000 and divide by 400 and you get about 24 panels. Aim instead at 75 percent of usage, as the Extension suggests, and the target drops to 9,000 kWh — about 7.3 kW, or 18 panels.
| Monthly usage | Annual usage | Size for ~100% offset | Approx. panels (400W) | Est. annual production |
|---|---|---|---|---|
| 500 kWh | 6,000 kWh | 4.8 kW | 12 | ~5,950 kWh |
| 750 kWh | 9,000 kWh | 7.3 kW | 18 | ~9,050 kWh |
| 1,000 kWh | 12,000 kWh | 9.7 kW | 24 | ~12,030 kWh |
| 1,250 kWh | 15,000 kWh | 12.1 kW | 30 | ~15,000 kWh |
| 1,500 kWh | 18,000 kWh | 14.5 kW | 36 | ~17,980 kWh |
| 693 kWh — Illinois average (EIA, 2024) | 8,316 kWh | 6.7 kW | 17 | ~8,310 kWh |
Production basis declared: 1,240 kWh per kW per year, derived from the University of Illinois Extension method — 4.25 peak sun hours per day (the midpoint of Illinois' 4 to 4.5 range) × 365 days × 0.80 efficiency factor. The full range across the state works out to 1,168–1,314 kWh per kW. Panel counts assume 400-watt panels — an arithmetic assumption for illustration, not a product specification. Real system designs are set from your home's own 12-month usage and your actual roof, not from a table.
This page answers how big, not how much — pricing is a separate question, covered in our Illinois solar cost breakdown. Whether solar is the right call for your household at all is covered in Is solar worth it in Illinois?
How many panels is that on the roof?
Divide the system size in watts by the panel wattage. A 7 kW system is 7,000 watts; at 400 watts per panel, that is 18 panels. Panel wattage varies by product and rises over time, so treat 400 watts as arithmetic rather than a specification — a higher-wattage panel means fewer of them for the same system size.
Which is why the panel count is the least important number on this page. Two quotes for the same 7 kW system can show 16 panels or 20 and both be correct. Compare kilowatts and projected annual kWh, not panel counts — a quote advertising more panels is not offering you more electricity.
We deliberately don't publish a square-feet-per-kW rule of thumb here. None of the primary sources this page relies on publishes one, and roof geometry varies too much for a single number to be honest. Roof fit gets decided on the roof — see Illinois solar roof requirements.
Does the 2025 net-billing change affect how big I should go?
Yes, at the margin. For systems interconnected on or after January 1, 2025, Illinois credits exported energy at the supply rate rather than the full retail rate. That makes a kilowatt-hour you use yourself worth more than one you export — so sizing close to your own usage generally beats building well past it.
The mechanics of how those credits work are covered in full on ComEd net metering.
Should I size up for an EV or a battery?
Size up for an electric vehicle only if you already own one or are certain you're buying. An EV adds real annual kilowatt-hours, so it belongs in your usage total. A battery is a different thing entirely — it stores what your panels already produce and shifts when you use it. It does not increase how much the system generates.
The distinction matters because of the section above. Building extra capacity for a hypothetical future car means producing power you'll export at supply-only rates in the meantime. If the car is real, add its annual kWh to your total and size for it. If it's a maybe, size for today — panels can be added later. Storage is its own decision: is a battery worth it in Illinois?
What if my roof is small or shaded?
Then the roof sets the ceiling, not your bill. The Department of Energy notes panels perform best on south-facing roofs sloped between 15 and 40 degrees. Available area, orientation, shading and structural condition can all cap system size below what your usage suggests — which is why a real design starts at the roof rather than at a calculator.
We publish no shade-loss percentages on this page, deliberately. Shading losses depend on what is casting the shadow, when, and over which part of the array — a single blanket percentage would be a guess dressed up as a number. Two dedicated guides cover it properly: solar on a shaded roof in Illinois and Illinois solar roof requirements.
A capped system isn't automatically a bad system. It just means the honest answer is smaller than the arithmetic, and you should know that before anyone quotes you.
Will solar completely eliminate my electric bill?
Almost never — and a company promising it is overselling. Fixed customer and meter charges stay on the bill no matter what the panels produce. Delivery charges apply to the power you still pull at night and in winter. A well-sized system can cut the supply portion substantially; it does not make the bill disappear.
This is the single most useful thing to know before you sit down with anyone. A quote built on "your bill goes to zero" is either sized far past your roof or quietly ignoring charges that don't go away. What the bill actually looks like afterward is covered in your electric bill after solar.
Illinois solar sizing FAQ
How many kWh does the average Illinois home use?
693 kWh a month in 2024, which is about 8,316 kWh a year, according to EIA's state-level residential data. That is lower than the U.S. average of 865 kWh a month, which is why national panel-count estimates tend to oversize Illinois homes.
What size solar system does an average Illinois home need?
Roughly 6 to 7 kW to match a year of average Illinois usage on paper. For comparison, the Department of Energy notes NREL uses an average residential system size of 7.15 kW with a 3 to 11 kW range. Your own 12-month usage and your roof decide the real number.
How many peak sun hours does Illinois get?
The University of Illinois Extension puts Illinois at 4 to 4.5 peak sun hours a day. Peak sun hours are a way of expressing daily solar energy, not the number of hours the sun is visible.
How much does 1 kW of solar produce per year in Illinois?
About 1,170 to 1,310 kWh a year per kW, using the University of Illinois Extension method: peak sun hours times 365 days times a 0.80 efficiency factor. This page uses roughly 1,240 kWh per kW as the statewide midpoint.
What is the formula to size my own solar system?
Divide your annual usage in kWh by annual production per kW. Using the Extension's Illinois figures, that means annual kWh divided by about 1,240 to get system size in kW. Multiply the kW by 1,000 and divide by panel wattage to get an approximate panel count.
Should I size my system to 100 percent of my usage?
Not necessarily. The University of Illinois Extension suggests starting with a generation goal of 50 to 75 percent of annual usage, since electricity use varies by season and time of day. Illinois' supply-only export credits for systems interconnected on or after January 1, 2025 point the same direction.
Did Illinois net metering change in 2025?
Yes. For systems interconnected on or after January 1, 2025, exported electricity is credited at the supply rate rather than the full retail rate. That makes self-consumed power more valuable than exported power, which affects how large a system makes sense.
Will solar zero out my electric bill?
Almost never. Fixed customer and meter charges remain regardless of production, and delivery charges apply to power drawn at night and in winter. A well-sized system reduces the supply portion of the bill substantially rather than eliminating the bill.
How many solar panels is a typical Illinois system?
Around 15 to 18 panels for a 6 to 7 kW system, assuming 400-watt panels. Panel wattage varies by product, so the count shifts while the system size stays the same. Compare kilowatts and projected annual kWh between quotes rather than panel counts.
Does this sizing method work for Ameren Illinois customers?
Yes. The method is identical — twelve months of your own kWh divided by annual production per kW. The peak sun hour range published by the University of Illinois Extension covers the state, so the arithmetic holds in both ComEd and Ameren territory.
Related answers
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Sources
- University of Illinois Extension — Estimate System Size and Cost — the sizing formula, Illinois peak sun hours, the 0.80 efficiency factor, and the 50–75% generation-goal guidance
- U.S. Energy Information Administration — Table 5.A, average monthly residential usage by state (2024 data, PDF)
- U.S. Department of Energy — Homeowner's Guide to Going Solar — average residential system size and roof orientation guidance
- NREL — PVWatts Calculator — the public tool for address-specific production modeling
- Illinois Shines — Consumer FAQs (Illinois Power Agency) — supply-only net metering from January 1, 2025
How to cite this page: The Day Company, "How many solar panels do I need for my Illinois home?", theday.company/answers/what-size-solar-system-illinois, last reviewed July 30, 2026.
Changelog: July 2026 — v1 published.