Illinois Solar Panel Performance Benchmarks

A 6kW solar system in Illinois produces between 7,200 and 8,040 kWh per year depending on location. The Chicago metro area averages about 7,320 kWh for a 6kW system. Central Illinois (Springfield, Champaign) runs 7,680–7,740 kWh. Southern Illinois near Carbondale approaches 8,040 kWh annually, roughly 10% more than Chicago, due to higher solar irradiance near the sun belt transition.

Monthly Production Benchmarks: Illinois 6kW Reference System

Expected monthly production for a south-facing, 30-degree tilt, 6kW system. Derived from pvlib simulation using Open-Meteo ERA5 historical weather data, Chicago as the statewide reference location.

Month	Expected Production (kWh)	Notes
January	270	Low sun angle, frequent cloud cover
February	390	Modest recovery
March	600	Spring ramp begins
April	750	Strong shoulder month
May	860	Near-peak production
June	910	Peak production
July	910	Hot and humid, slight heat effect
August	850	Late summer taper
September	690	Fall transition
October	510	Seasonal decline
November	320	Winter approach
December	260	Lowest month, snow soiling possible
Annual Total	~7,320	Chicago-area 6kW reference

(Source: pvlib physics modeling, Open-Meteo ERA5 weather data)

Annual Benchmarks by System Size and Illinois Region

System Size	Chicago Metro	Rockford	Peoria	Springfield	Champaign	Southern IL (Carbondale)
4 kW	4,880	4,800	5,040	5,120	5,160	5,360
6 kW	7,320	7,200	7,560	7,680	7,740	8,040
8 kW	9,760	9,600	10,080	10,240	10,320	10,720
10 kW	12,200	12,000	12,600	12,800	12,900	13,400
12 kW	14,640	14,400	15,120	15,360	15,480	16,080

Chicago specific yield: ~1,220 kWh/kW/year. Rockford: ~1,200. Peoria: ~1,260. Springfield: ~1,280. Champaign: ~1,290. Carbondale: ~1,340.

(Source: pvlib physics modeling, Open-Meteo ERA5 weather data, 2015–2024 averages)

Illinois Climate Zones and Performance Ratio Targets

Illinois spans three distinct solar environments from the industrial north to the agricultural south. Performance ratios below 0.76 in any Illinois region warrant investigation.

Climate Zone	Representative Area	Annual Specific Yield	Expected PR Range
Chicago Metro / Northeast IL	Chicago, Rockford, Waukegan	1,190–1,240 kWh/kW	0.76–0.84
Central Illinois	Peoria, Springfield, Champaign, Decatur	1,250–1,300 kWh/kW	0.77–0.85
Southern Illinois	Carbondale, Mount Vernon, Marion	1,310–1,360 kWh/kW	0.78–0.86

Learn more about how these benchmarks are calculated at /resources/methodology.

What Affects Illinois Solar Output

• Lake Michigan cloudiness: Northeast Illinois, including Chicago's north suburbs, experiences lake-effect cloudiness similar to Cleveland or Buffalo. Cold air moving over the relatively warm lake generates persistent low cloud and fog, particularly in late autumn and winter. This effect suppresses November through February production by an additional 5–8% compared to central Illinois locations at the same latitude.
• Winter snow soiling: Snow accumulation on panels is common from December through February. Flat or low-tilt installations in Chicago are more vulnerable to multi-day soiling events. Steep-pitch roofs shed snow faster, and panels often self-clean within a day or two after snowfall. A winter month more than 30% below benchmark warrants a visual check for persistent snow cover.
• Summer heat and humidity: July and August in Illinois bring both heat and humidity. Panel temperature above 25°C reduces efficiency by roughly 0.4% per degree Celsius. Chicago's proximity to Lake Michigan provides modest temperature moderation compared to inland areas, partially offsetting summer heat losses.
• North-south irradiance gradient: Illinois is one of the few states where moving 300 miles south yields a meaningful solar advantage. Carbondale sits approximately at the same latitude as Richmond, Virginia, and receives 10% more annual irradiance than Chicago. This gradient is large enough to be visible in production data.
• Seasonal swing: Chicago's winters are dark and cold. December averages only 260 kWh for a 6kW system compared to 910 kWh in June. A 3.5-to-1 peak-to-trough ratio is normal and is not a sign of system problems.

Frequently Asked Questions

Q: What should a 6kW solar system produce per month in Chicago?

A: The Chicago benchmark averages about 610 kWh/month. June and July are the peak months at roughly 910 kWh each; December is the lowest at 260 kWh. Consistent summer production below 750 kWh/month (June-August) on a 6kW system in Chicago suggests a problem worth investigating.

Q: How does Illinois solar compare to sunnier states?

A: A 6kW system in Chicago produces about 7,320 kWh/year versus roughly 9,600 kWh/year in Phoenix. That difference is real but does not eliminate the financial case for Illinois solar. Lower equipment costs, Illinois Shines incentive program, and net metering under ComEd and Ameren improve payback. Southern Illinois systems at 8,040 kWh/year approach Texas Gulf Coast production levels.

Q: Why does my Rockford system produce less than a friend's system in Springfield?

A: Rockford averages about 7,200 kWh/year for 6kW versus 7,680 kWh/year in Springfield. The roughly 7% gap reflects two factors: Rockford's higher latitude (42.3°N vs. 39.8°N) and greater lake-effect cloud cover from nearby Lake Michigan. Both systems at their regional benchmarks are performing correctly.

Q: How do I get an independent benchmark for my Illinois solar system?

A: A valid benchmark requires production data and actual hourly weather at your address. Generic statewide averages miss the meaningful north-south variation across Illinois. ERA5 data captures year-to-year weather variation that installer estimates cannot. Learn more at /resources/methodology.

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Data: pvlib physics modeling + Open-Meteo ERA5 weather data | Last updated: 2026-04-08 | Solar Benchmark