Quick answer
A typical Illinois home with a $150 monthly bill needs a system of about 8.0 kW, around 20 panels of 400 watts, in ComEd territory, and slightly fewer downstate where panels produce more. Size from your annual kWh use: divide it by roughly 1,200 to 1,300 kWh per kW, depending on how far south you live.
How many solar panels do I need for my electric bill?
| Monthly bill | Yearly use | System size | Panels | Roof space |
|---|---|---|---|---|
| $75 | 4,235 kWh | 3.6 kW | 9 | ~190 sq ft |
| $100 | 6,000 kWh | 5.2 kW | 13 | ~270 sq ft |
| $150 | 9,529 kWh | 8.0 kW | 20 | ~420 sq ft |
| $200 | 13,059 kWh | 10.8 kW | 27 | ~570 sq ft |
| $250 | 16,588 kWh | 14.0 kW | 35 | ~740 sq ft |
| $300 | 20,118 kWh | 16.8 kW | 42 | ~880 sq ft |
| Monthly bill | Yearly use | System size | Panels | Roof space |
|---|---|---|---|---|
| $75 | 4,235 kWh | 3.6 kW | 9 | ~190 sq ft |
| $100 | 6,000 kWh | 4.8 kW | 12 | ~250 sq ft |
| $150 | 9,529 kWh | 7.6 kW | 19 | ~400 sq ft |
| $200 | 13,059 kWh | 10.4 kW | 26 | ~550 sq ft |
| $250 | 16,588 kWh | 13.2 kW | 33 | ~690 sq ft |
| $300 | 20,118 kWh | 16.4 kW | 41 | ~860 sq ft |
Yearly use is estimated from the bill at about 17¢/kWh after fixed charges. Your actual kWh, printed on your bill, gives a better answer.
How do I calculate the right solar system size?
- 1
Add up 12 months of kWh
Your ComEd or Ameren bills or online account show monthly usage. Add any planned EV or heat pump.
- 2
Divide by your area's output
About 1,200 kWh per kW a year in northern Illinois, up to about 1,330 in far southern Illinois, less with shade.
- 3
Divide by panel wattage
System kW × 1,000 ÷ panel watts gives the panel count. With 400 W panels, a 7 kW system is about 18 panels.
- 4
Check roof space
Allow roughly 21 square feet per panel, plus setbacks from roof edges that some towns require.
Should I install more panels than I need?
Usually not by much. For systems connected since 2025, extra power you export earns roughly the supply rate, about 10.1¢ per kWh with ComEd for October 2026–May 2027, not the full price. A system sized to your yearly use, or slightly above if you're adding an EV or heat pump, usually pays back fastest.
Enter your bill in the solar savings calculator to see size, cost and payback together.
Sources
This guide is general information, not tax, legal or financial advice. Incentive programs change, so confirm current terms with your installer, your utility and a tax professional.