Illinois Solar Production & Irradiance Data
3.89 peak sun hours/day · 6,828 kWh/year from a 6kW system · below national average
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Illinois receives an average of 3.89 peak sun hours per day, which translates to a Global Horizontal Irradiance (GHI) of 3.89 kWh/m²/day. When solar panels are tilted at the optimal angle for Illinois's latitude, irradiance increases to 4.5 kWh/m²/day — a 16% boost over flat-mounted panels.
This puts Illinois below the national average of 4.29 peak sun hours per day. While Illinois receives less sunlight than sun-belt states, solar is still a sound investment — especially when electricity rates and local incentives are factored in. Germany, with similar sun hours, is one of the world's largest solar markets. These figures come directly from the NREL Solar Resource API using TMY (Typical Meteorological Year) data averaged from 1998-2020.
Expected Annual Output by System Size
| System Size | Annual Output | Avg Monthly | Annual Savings | 25-Year Savings |
|---|---|---|---|---|
| 4 kW (12-16 panels) | 4,552 kWh | 379 kWh | $805/yr | $20,125 |
| 6 kW (18-24 panels) | 6,828 kWh | 569 kWh | $1,208/yr | $30,200 |
| 8 kW (24-32 panels) | 9,104 kWh | 759 kWh | $1,610/yr | $40,250 |
| 10 kW (30-40 panels) | 11,380 kWh | 948 kWh | $2,013/yr | $50,325 |
Based on 4.5 kWh/m²/day tilt irradiance, 14% system losses, and 17.69¢/kWh electricity rate. Actual output varies by installation specifics.
Month-by-Month Solar Production in Illinois
Estimated monthly output for a 6kW system:
Solar production in Illinois peaks in June when a 6kW system generates approximately 888 kWh — about 13% of annual production in a single month. The lowest production month is December at approximately 239 kWh (3.5% of annual output). This 3.7:1 seasonal ratio is significant — battery storage or time-of-use rate plans can help smooth out this variation.
How Illinois Compares Nationally
Tips for Maximizing Solar Production in Illinois
While Illinois's 3.89 peak sun hours is below the national average, solar is still viable and cost-effective. Maximizing panel tilt angle and ensuring zero shading are critical to getting the most from your system.
Consider adding battery storage to your Illinois solar system. A 10-13 kWh home battery lets you store excess daytime production for evening use, maximizing your self-consumption and reducing grid dependence. This is especially valuable for backup power during outages.
Solar Irradiance Technical Data
Global Horizontal Irradiance (GHI): 3.89 kWh/m²/day. GHI measures the total solar radiation received on a horizontal surface, including direct sunlight and diffuse radiation from clouds and atmospheric scattering. This is the baseline measurement for solar resource assessment.
Tilt Irradiance: 4.5 kWh/m²/day. When panels are tilted at the optimal angle for Illinois's latitude, they receive 16% more solar radiation than flat-mounted panels. This tilt optimization is one of the easiest ways to boost your system's output.
System Output Estimation: The annual production figures use the NREL PVWatts methodology: Tilt Irradiance × 365 days × System Size × (1 − System Losses) × Inverter Efficiency. We use 14% system losses (wiring, soiling, degradation, mismatch) and 96% inverter efficiency, matching PVWatts defaults.
Illinois Solar Production FAQ
How many peak sun hours does Illinois get?▼
Illinois averages 3.89 peak sun hours per day, which is below the national average of 4.29 hours. Peak sun hours represent hours of full-intensity sunlight equivalent — a key factor in estimating solar panel output. This data comes directly from the NREL Solar Resource API.
How much electricity will a 6kW solar system produce in Illinois?▼
A 6kW solar system in Illinois produces approximately 6,828 kWh per year, or about 569 kWh per month on average. Production peaks in June (approximately 888 kWh) and is lowest in December (approximately 239 kWh). At Illinois's average rate of 17.69¢/kWh, this saves you about $1,208/year.
What is the best month for solar production in Illinois?▼
June is typically the best month for solar production in Illinois, with a 6kW system producing approximately 888 kWh. The long daylight hours and high sun angle combine for maximum energy generation. The worst month is December, producing about 239 kWh — roughly 27% of the best month's output.
How does Illinois's solar potential compare to other states?▼
Illinois's average of 3.89 peak sun hours per day puts it below the national average of 4.29 hours. A 6kW system in Illinois produces 6,828 kWh/year, compared to the national average of 7,562 kWh/year — that's -734 kWh/year (-10%). Combined with Illinois's above-average electricity rate of 17.69¢/kWh, solar is especially financially attractive here.
What affects solar panel output in Illinois?▼
Key factors affecting solar output in Illinois include: roof orientation (south-facing is ideal), tilt angle (latitude tilt of 36° is optimal), shading from trees or buildings, panel efficiency rating, and local weather patterns. Illinois's GHI (Global Horizontal Irradiance) is 3.89 kWh/m²/day, but panels tilted at the optimal angle receive 4.5 kWh/m²/day — a 16% improvement over flat-mounted panels.
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Illinois Solar Data
- ☀️ Peak Sun Hours: 3.89 hrs/day
- 📊 GHI: 3.89 kWh/m²/day
- 📐 Tilt Irradiance: 4.5 kWh/m²/day
- ⚡ 6kW Output: 6,828 kWh/yr
- 📈 Best Month: June
- 📉 Worst Month: December
- 💡 Electricity Rate: 17.69¢/kWh
- 💰 Annual Savings (6kW): $1,208
- ✅ Data: NREL API Verified