Peak sun hours in Illinois
Illinois averages 4.2 peak sun hours a day across the year, 31st of the 50 states. At that figure a 400-watt panel produces about 1.29 kWh a day after real-world losses, and covering the 900 kWh average US bill takes about 23 panels. Chicago, the state's biggest city, sits a little under the statewide figure at 4.1 hours, so 4.2 is the midpoint of a spread that leans away from the largest population centre.
modest sun: 4.2 of a possible ~6.5 hours
Panel counts for Illinois bills
Computed at Illinois's 4.2 hours with 400W panels and the 0.77 derate; the US-average column shows what the same bill takes at 4.5 hours, so you can see what your state's sun is worth in hardware.
| Monthly usage | Illinois | System size | US average |
|---|---|---|---|
| 500 kWh/mo | 13 panels | 5.2 kW | 12 panels |
| 750 kWh/mo | 20 panels | 8 kW | 18 panels |
| 900 kWh/mo | 23 panels | 9.2 kW | 22 panels |
| 1,000 kWh/mo | 26 panels | 10.4 kW | 24 panels |
| 1,500 kWh/mo | 39 panels | 15.6 kW | 36 panels |
| 2,000 kWh/mo | 51 panels | 20.4 kW | 48 panels |
What 4.2 hours means in practice
Illinois runs 7% below the national solar average, 31st of 50 at 4.2 hours. That is workable sun, not bad sun: the 900 kWh bill takes 23 panels here versus 22 nationally, 1 more. Germany built the world's densest solar fleet on worse.
How a Illinois year is shaped
The 4.2-hour figure is an annual average, and no month is average. These are the twelve monthly means for Illinois from NASA's 2001-2020 solar climatology, each as a share of the state's own annual mean, with that share applied to the 4.2-hour planning figure. June and December are marked.
| Month | Share of the annual average | Peak sun hours (est.) |
|---|---|---|
| January | 47% | 2.0 |
| February | 68% | 2.8 |
| March | 92% | 3.8 |
| April | 121% | 5.1 |
| May | 137% | 5.8 |
| June | 156% | 6.5 |
| July | 155% | 6.5 |
| August | 137% | 5.8 |
| September | 114% | 4.8 |
| October | 79% | 3.3 |
| November | 54% | 2.3 |
| December | 40% | 1.7 |
Illinois's curve tops out in June at 156% of the annual average and bottoms in December at 40%, a 3.9-to-one spread with 49% of the year's light landing in four months. In hours: 6.5 a day at the peak, about 1.7 in December.
Half the Illinois calendar sits above the annual average and half below: April through September are the six months that beat it, carrying 68% of the year's light. April is already worth about 5.1 hours a day, so the productive half of the year starts earlier than most people expect.
Illinois homes are light users by national standards, 693 kWh a month on EIA's 2024 average against 863 nationally, 20% below. That is only 18 panels at the annual figure, which is why modest arrays go a long way here. Carrying the same house through December on its own would take 45, 2.5 times more.
Illinois is more uniform than most. The 16 grid cells sampled across the state span 10%, sunniest in the south and dimmest in the northeast, which scales to about 4.4 hours against 4.0. Enough to notice on a production estimate, not enough to change which system you buy. Chicago sits inside that band, not at the edge of it.
What that sun is worth on a Illinois bill
| EIA, 2024 residential averages | Illinois | United States |
|---|---|---|
| Average home's monthly use | 693 kWh | 863 kWh |
| Price per kilowatt-hour | 15.87¢ | 16.48¢ |
| Average monthly bill | $109.99 | $142.26 |
| One 400W panel, a year's output | 472 kWh | 506 kWh |
| What that output displaces | $75 | $83 |
At 15.87¢ a kilowatt-hour Illinois sits 4% below the national 16.48¢, 18th of the fifty states on price. One 400-watt panel offsets roughly $75 of Illinois power a year. Mid-table on rates means the project lives or dies on the quote and the net-metering terms rather than on either the sun or the tariff.
Illinois lands in much the same place on both rankings, 31st for sun and 28th for what a panel earns, so its position in the sun table is a fair guide to its position on value. That is not automatic. Rhode Island gets less sun (4.1 hours to Illinois's 4.2) and pays 28.65¢ against 15.87¢, so one panel is worth about $57 less a year in Illinois than in Rhode Island.
Two warnings on those dollar figures. They are displaced electricity and nothing else: no install cost, no incentive, no financing, and no assumption about how your utility settles a surplus, which is the single biggest unknown in the sum. And a state average hides the utility you are actually on; 15.87¢ is every Illinois residential customer averaged together, and neighbouring territories can differ by more than the gap between two states in the ranking.
Getting a number for your address
For a location-exact figure, run your address through NREL's free PVWatts; it uses measured weather for your grid cell and will also model roof tilt and direction, which statewide numbers cannot. Then compare its annual kWh against a quote's promise before you sign anything.
Questions people ask
How many solar panels do I need in Illinois?
About 23 400-watt panels (9.2 kW) for the 900 kWh a month an average US home uses, at Illinois's 4.2 peak sun hours. For the average Illinois home specifically, which EIA measured at 693 kWh a month in 2024, it is 18. Your own bill is the variable that matters, and the calculator takes any figure.
Is 4.2 peak sun hours good for solar?
It is below the 4.5-hour national average but far from disqualifying. Panel counts run about 7% higher here for the same bill, and the December trough measured above deserves respect in any off-grid plan. Grid-tied systems ride through it on net metering.
How much is one solar panel worth a year in Illinois?
About $75. One 400-watt panel at Illinois's 4.2 peak sun hours makes roughly 472 kWh a year after the 0.77 derate, and Illinois residential electricity averaged 15.87¢ a kilowatt-hour in 2024 (EIA), which puts Illinois 28th of the 50 states on this measure against 31st on sunlight alone. That figure is displaced electricity only. It says nothing about install cost, incentives, or how your utility settles a surplus.
Do peak sun hours change with the seasons in Illinois?
Yes, by a factor of 3.9 here. NASA's 2001-2020 climatology puts Illinois's June at 156% of the annual average and December at 40%, so the 4.2-hour figure on this page is the midpoint of a curve running from roughly 6.5 hours a day down to 1.7. Grid-tied homes bank the summer surplus as credit; off-grid designs have to size for the trough, and in Illinois that means 45 panels for an average local bill instead of 18.