Quick answer
If your system was installed before 2025, be careful: you keep full retail net metering unless the change is big enough to need a new ComEd interconnection agreement or more than doubles capacity in Ameren territory, and taking the $300/kW utility rebate on the addition can end that status. A small, panel-only addition often keeps it; ask your installer and utility before signing.
Will adding panels cost me my old net metering?
| Change | ComEd | Ameren Illinois |
|---|---|---|
| Small expansion | Keeps full net metering unless it requires a new interconnection agreement | Keeps full net metering unless capacity increases more than 100% |
| Taking the generation rebate | Ends full net metering | Ends full net metering |
| Legacy term | 30 years from permission to operate | 30 years from permission to operate |
Systems installed before 2025 earn credits on the supply, delivery and tax parts of the bill, worth noticeably more than the supply-only credit newer systems get. The Citizens Utility Board warns that pre-2025 owners lose that status if they take the distributed generation rebate. On an expansion, the rebate can cost far more than it pays.
Do I need a new interconnection application to expand?
Usually yes, because the utility needs to approve the larger system. Most home systems fall in Illinois's Level 1 review for inverter-based systems of 25 kW or smaller. In ComEd territory, the trigger for losing legacy net metering is a change big enough to need a new interconnection agreement, so whether your addition changes the system's AC size, the inverter capacity, matters a lot.
With microinverters, or an existing inverter with spare capacity, panels can sometimes be added without raising the AC size. Ask your installer exactly how the utility will classify the change, in writing, before you sign.
Can added panels get the rebate or Illinois Shines?
Systems connected since 2025 can get the ComEd or Ameren rebate of about $300 per kW with a smart inverter. For an expansion, confirm with your utility whether the added capacity qualifies and, for pre-2025 systems, what it would do to your net metering.
Illinois Shines treats an expansion as a new project added to an existing contracted system at the same interconnection point. Its pricing for expansions has been set against the combined size, which can make the incentive for the added panels small. Ask your installer for the actual REC value on the disclosure form.
Can new panels be mixed with older ones?
Yes, with care. Panels wired together in a string should have similar electrical characteristics, or the weakest one drags the string down. Microinverters or optimizers avoid most of that mismatch, and a separate string or inverter for the new panels is another common fix.
Before work starts, ask the original installer and the panel and inverter makers to confirm in writing that the expansion won't affect your existing warranties. See solar warranties explained.
Is adding panels or a battery better?
For a pre-2025 system, extra panels add credits at the full retail rate as long as you keep legacy status. For a newer system, exported power earns only the supply rate, so a battery that stores midday solar for evening use, with the $300/kWh storage rebate, can be worth more than extra panels. See the Illinois battery rebate.
Sources
- Illinois Shines: FAQs related to changes in net metering in Illinois (July 2024)
- Ameren Illinois: Changes to net metering
- Citizens Utility Board: Solar and storage rebates fact sheet (2025)
- Citizens Utility Board: Illinois net metering for systems installed before 2025
- Illinois Administrative Code: Distributed generation interconnection (83 Ill. Adm. Code 466)
- ComEd: Distributed generation rebate
- Illinois Shines: Expansions, co-locations and size changes
- ComEd: Distributed generation and storage rebates
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.