guide7 min read

Your Illinois Solar Output Falls About 45% by December — Here's the Month-by-Month Curve

Ryan Cook•August 28, 2026•Updated Sep 16, 2026

Every year around the middle of September, my phone starts doing the same thing.

It's a homeowner who went solar in the spring, watched the meter spin backwards all summer, and just opened a monitoring app showing numbers well below August. The question is always some version of the same one: did something break?

Almost never. What broke is the assumption that a solar array produces the same amount every month. It doesn't, and in Illinois the swing is bigger than most people are told before they sign.

The actual curve, in kilowatt-hours

Here's what the fall looks like for an estimated 7.5 kW system in two different parts of the state. These are estimates built from NREL's monthly solar resource data for each town, at a roof tilt near local latitude and with typical system losses, so your roof will land somewhere near them rather than exactly on them.

Month	Belleville area (Ameren Illinois), est.	Chicago area (ComEd), est.
July	~852 kWh	~800 kWh
August	~858 kWh	~754 kWh
September	~797 kWh	~720 kWh
October	~701 kWh	~568 kWh
November	~554 kWh	~418 kWh
December	~476 kWh	~367 kWh

From the summer peak to December, that's a fall of roughly 45% downstate and closer to 54% up north.

Pay attention to where the drop happens, because this is the part that catches people. September is still a good month — usually within about 5% to 7% of your summer numbers. October holds most of it. The floor drops out between October and November, when Illinois loses daylight faster than any other stretch of the year.

A little softening in September is just the calendar. A cliff in September is early, and it's worth a look.

Your roof pitch is doing more work than you think

Here's the number that surprises people, and it's genuinely reassuring.

Measured on a flat surface, Illinois sunlight in December is down roughly 72% to 77% from the June peak, depending on where you are. That sounds brutal. But a properly tilted array only loses about 44% to 54% over the same stretch.

The angle is the reason. Optimal tilt in Illinois runs about 39 degrees near Belleville and about 42 degrees around Chicago and Rockford — steep, and pointed right at where the winter sun actually sits. A tilted array catches low-angle light that a flat one mostly misses. Your roof is quietly holding about half the winter line for you.

Cold weather helps a bit too. Panels run more efficiently at lower temperatures, which is the opposite of what most people assume. Heat is the enemy of panel output, not cold — a December panel is working in better conditions than a July one, it just has far less light to work with.

Illinois is one state in June and two states in December

This one's a downstate-versus-Chicagoland fact I've never seen anyone write down.

In June, the solar resource across Illinois is nearly uniform — Belleville and Chicago land within about 4% of each other. By December they're about 23% apart, with the southern half of the state well ahead.

Practically, that means a Metro East or southern Illinois homeowner has a noticeably gentler winter than someone in the northern suburbs, and the two shouldn't be comparing monitoring screenshots with each other in January. If you're comparing your system to a friend's, compare against someone in your own half of the state.

How to tell normal from actually broken

Four checks, in the order I'd run them.

Compare the same month last year, not last month. This is the big one. Month-over-month always looks like a crash in the fall, because the season is crashing. Year-over-year removes the calendar. Within about 10% of last October is a healthy October.

Look for a flat line, not a slow slide. Real faults tend to be abrupt. A dead inverter or a tripped breaker reads as zero or near-zero on a sunny day, not as a gentle taper. Weather tapers. Hardware stops.

Check whether one section reads differently. If your app breaks output down by string or by panel, a single group lagging the others points at hardware or shading rather than at the season.

Check the shape of one clear day. A healthy array in December still draws a smooth arc — just a shorter, lower one. A curve with a bite out of the middle is usually shade; a curve with gaps is usually a fault.

If two or more of those look wrong, that's the point to call whoever is contractually responsible for service. On a lease, that's the system owner, not you.

What it does to your bill — and what it doesn't

Your summer surplus is what covers the winter. That's the design. Long June days produce more than the house uses, those credits bank with Ameren or ComEd, and November and December draw them back down. It's why systems here get sized against a year of usage instead of a month.

Two Illinois specifics matter. Residential systems interconnecting after December 31, 2024 are on supply-only net metering, so credits offset the supply side of the bill rather than all of it — and under those rules, unused credits don't expire. If you connected before 2025, you're on legacy net metering and your unused credits reset once a year, April or October, whichever your installer picked, so check that date before winter. I broke down how the 2026 net metering changes work if you want the mechanics.

If your winter bill reappears, that's usually the credits doing their job, not the panels failing at theirs.

The seasonal swing lands differently on a lease

Worth knowing if you're still shopping.

Most solar leases in Illinois aren't a flat monthly fee — they're priced on what the system actually produces, as low as $0.10 per kWh. So a December that makes about 45% less power also bills about 45% less. The payment tracks the season down, then back up in the spring.

A solar loan doesn't do that. The payment in December is the payment in July, whatever the roof did that month. That's not an argument against owning — ownership has a long tail a lease doesn't, and I laid out both sides of that trade honestly. It's just a difference nobody mentions in August, and one that homeowners notice in their first winter.

Ask any company you're talking to whether the payment is tied to production or fixed, and get the full rate schedule in writing before signing.

The short version

Your panels are almost certainly fine. Illinois takes roughly 45% of your output away between August and December, most of it in a single hard step from October into November, and gives it back on the same schedule in the spring. The tilt on your roof softens it. Your banked credits cover it. And a production-priced lease bills less while it's happening.

If you want to see the estimated seasonal numbers for your own address instead of a range, run them in the calculator or check what your roof can actually hold.

Still not sure whether what you're seeing is the season or a service call? Reach me at (618) 217-2001 or through the contact page. I'd rather talk you out of a service call than watch you pay for one you didn't need.

Frequently Asked Questions

Because the sun sits lower and the days get shorter, not because anything failed. For a typical 7.5 kW system near Belleville, estimated output goes from roughly 858 kWh in August to about 797 in September, 701 in October, 554 in November and 476 in December. That's a drop of around 45% from the August peak. Up in ComEd territory the swing is steeper — a comparable Chicago-area system estimates near 800 kWh in July and about 367 in December, closer to a 54% fall. The important detail is where the real drop happens. September usually still lands within about 5% to 7% of your summer numbers, and October holds most of it. The steep part is October into November, when daylight shortens fastest. If your production fell off in September, that timing is early for weather alone and worth a closer look.

Compare the same month to last year, never this month to last month. Month-over-month always looks alarming in the fall because the season itself is falling. Year-over-year takes the calendar out of it. If October is within roughly 10% of last October, your system is fine. Then check three things in your monitoring app. First, look for a flat line on a sunny day — a real fault usually shows as zero or near-zero output, not a gentle slide. Second, check whether one string or one panel group reads differently from the rest, which points at hardware rather than weather. Third, look at the shape of a single clear day; a healthy array still makes a smooth curve in December, just a shorter and lower one. Estimates vary by roof, so treat these as guides and call your service contact if something looks genuinely off.

Yes, and better than the raw sunlight numbers suggest. Across Illinois, flat-surface sunlight drops roughly 72% to 77% from its June peak to December. But a properly tilted rooftop array only falls about 44% to 54% over that same stretch. The difference is your roof pitch. Optimal tilt in Illinois runs around 39 degrees down near Belleville and about 42 degrees up around Chicago and Rockford, and that angle is aimed squarely at the low winter sun. Cold actually helps a little too — panels are more efficient at lower temperatures, which partly offsets the shorter days. What genuinely stops production is snow sitting on the glass, and in Illinois that usually clears on its own within a day or two as the panels warm and the snow slides off a pitched array. Numbers here are estimates and vary by roof.

On a production-priced lease, yes — that's how the structure works. Most solar leases in Illinois aren't a flat monthly fee. They're priced on what the system actually produces, as low as $0.10 per kWh, so a December that makes roughly 45% less power also bills roughly 45% less. The payment follows the season down and back up again. That's a real difference from a solar loan, where the payment stays the same in December as it was in July regardless of what the roof did. It's worth asking about directly when you compare offers, because not every plan is built that way. Ask to see the full rate schedule in writing before you sign, and ask plainly whether the payment is tied to production or fixed. Individual results depend on your system, your roof and your contract.

They're what carries you through the short months, which is the whole design. Your array makes more than the house needs on long summer days, and that surplus builds credits with Ameren or ComEd. When production falls in November and December, you draw those banked credits back down. That's why solar in Illinois is sized against a full year of usage rather than any single month. Two things worth knowing. Illinois moved to supply-only net metering for residential systems interconnecting after December 31, 2024, so credits offset the supply portion of the bill rather than the whole thing, and under those rules unused credits don't expire. If your system connected before 2025, you're a legacy customer and unused credits reset once a year, in April or October depending on what your installer picked. Check which date is on your account before your first winter, not after the credits are gone.