Solar in Williamson County, Illinois: what to know first

GSM Solar can help a home in Williamson County if it fits three ways. You own the home. It is in the Ameren Illinois service territory. And the roof gets good sun. Some homes here get power from an electric co-op instead, and GSM Solar can't help those. The name at the top of your power bill tells you which you have.

Which power company serves your home in Williamson County?

Start with your power bill. The company named at the top is your utility. That one name decides whether GSM Solar can help, because GSM Solar works only with homes in the Ameren Illinois service territory. GSM Solar is not affiliated with Ameren Illinois, any co-op or any city utility, and none of them runs or backs its offer.

Two towns say on their own websites who serves them. The City of Marion, the county seat, says Ameren Illinois provides its residents' electric utility service. Carterville's city utilities page lists Ameren Illinois as the town's electric company.

A city's page speaks for homes inside the city. If your address says Marion or Carterville but the house sits outside city limits, don't assume. Check the bill.

For the other towns in the county, we did not find a page from the town or from its power company that says who serves it. We'd rather leave a town off this page than guess. If you live there, your bill is the answer.

What if an electric co-op sends your bill?

A co-op is a power company owned by the members it serves. Three of them say on their own websites that they reach into Williamson County:

• Egyptian Electric Cooperative Association serves members in parts of ten counties, Williamson among them.
• SouthEastern Illinois Electric Cooperative provides electric service to members in parts of Williamson County.
• Southern Illinois Electric Cooperative names Williamson as one of the southern Illinois counties where it has members.

The names are close, so read your bill carefully. SouthEastern and Southern are two different co-ops. None of them says it serves the whole county. A county name tells you nothing about one house on one road.

If one of these co-ops sends your bill, GSM Solar can't help with solar right now. That limit comes from where GSM Solar works. It says nothing about your roof.

Co-ops also write their own rules for solar. Illinois Shines says co-ops and city utilities may limit net metering or credit power a different way. SouthEastern Illinois Electric Cooperative, for one, posts its own net billing policy and its own application for member-owned generation. The rest of this site is written for the Ameren Illinois service territory, so its rules on credits may not match yours. If a co-op serves you, ask it how it handles solar on a home before anyone quotes you a system.

How much sun does a roof in Williamson County get?

Two units matter here. A kW is the size of a system, how much power the panels can make at one moment. A kWh is an amount of energy, the unit your power bill counts.

We ran PVWatts, a solar model from a national lab, for Marion. We asked it about one kW of panels on a roof that faces south, tilted at a moderate angle, with no shade. It models about 1,407 kWh a year per kW. Tap the number to see every input we used.

Scale it to your system size. Twice the kW gives about twice the kWh. The model also shows the seasons: short winter days make less, long summer days make more.

Keep in mind what the model can't see. It doesn't know about the trees on your lot, the vent pipes on your roof or the house next door. A roof that faces east or west makes less than the south roof in the model. Before you sign anything, ask for a custom estimate for your own system, from a solar installer. The questions to ask about your own roof, its age and its shade are in is your home a fit for solar.

Who issues the building permit in Williamson County?

That depends on where the house is.

Inside Marion or Carterville, the city handles it. In Marion, the city's Building and Code Services issues permits for development within the city. In Carterville, the city sets its own building permit rules and lists them on its building permits page.

Outside city and village limits, start with the county. Williamson County posts the county's building permit application and its building permit ordinance on the page of its Supervisor of Assessments. That is the county office that handles property assessments, and the permit forms are posted there too. The same page links a Green Energy Ordinance and a Green Energy Permit. Ask the office whether a rooftop system on a house needs any of these forms.

Ask the company you talk to who will apply for the permit, and ask for a copy once it is issued. Keep it with your contract. If you sell the house later, the papers a buyer will likely need include contracts, warranty papers, maintenance records, system diagrams and permits, and the interconnection agreement. See selling a home with solar.

What should you read before you talk to a solar company?

In the Ameren Illinois service territory, net metering is a credit on your bill for the power your panels send to the grid. A system that starts net metering on or after January 1, 2025 gets credit for supply charges only. With solar, you stay a utility customer, responsible for a utility bill. Our guide to net metering in Ameren Illinois explains what that credit covers, and our utility bill guide explains why the bill stays.

Illinois Shines is a state program that provides an incentive for solar projects. How Illinois Shines works walks through what it means for a homeowner.

Before you sign with anyone, read how to check a solar company in Illinois. It shows where to look a seller up. Every number on this page is listed with its source on our data page.

Quick answers

Can GSM Solar help my home in Williamson County?

It may, if you own the home, it is in the Ameren Illinois service territory, and the roof gets good sun. GSM Solar can't help homes that get power from an electric co-op.

Which electric co-ops serve Williamson County?

Three co-ops say they serve homes in the county: Egyptian Electric Cooperative Association, SouthEastern Illinois Electric Cooperative and Southern Illinois Electric Cooperative. The name at the top of your power bill tells you if one of them serves you.

Who issues a building permit outside city limits?

Williamson County posts its building permit application and ordinance on its Supervisor of Assessments page. Inside Marion or Carterville, the city handles permits.

Is the PVWatts number for Marion what my roof will make?

No. It is a model for a south-facing roof with no shade. Ask for a custom estimate for your own roof before you sign anything.