How rooftop solar actually works.
If a salesperson hasn't already walked you through this, here's the simple version — without the jargon — before any of the math or incentive stuff matters.
- 1
Sunlight hits the panels.
Solar panels are roughly 18 to 22 square-foot rectangles, each one filled with silicon cells. When sunlight hits a cell, electrons get knocked loose. That movement of electrons is electricity. There's no engine, no moving parts, no noise — just light in, electrons out. A panel keeps producing for 25 to 30 years.
A typical Illinois home installs eight to eighteen panels. The exact number depends on your roof size, the angle, how shaded it is, and how much electricity you use.
- 2
The inverter converts that electricity for your house.
Panels produce electricity in a form your house can't use directly (called DC). An inverter — usually a flat box mounted somewhere near your electrical panel — converts that into the same form of electricity that comes from the wall outlet (AC). Now it's usable.
Some systems use one big inverter for the whole roof. Others use small "microinverters" attached to each panel. Both work; they have different tradeoffs around shading and monitoring.
- 3
Your house uses what it needs. The rest goes back to the grid.
On a sunny afternoon, your panels usually make more electricity than your house is using. That extra power flows back out to the grid — the wires running along your street. ComEd (or whatever utility serves you) tracks it through your meter.
At night, or on cloudy days, your panels don't make enough. You pull power from the grid like a normal house. The grid is acting like a giant battery — taking your excess during the day, returning it during the night.
- 4
An optional battery can hold your own power instead of sending it to the grid.
A home battery (Tesla Powerwall, Enphase IQ, etc.) is a separate piece of hardware — usually wall-mounted in the garage or basement. With a battery, your daytime excess charges the battery first; only what the battery can't hold goes to the grid. At night, you draw from the battery first; only when it's empty do you pull from the grid.
Batteries are optional. They add to the cost. They're worth it if you want backup during outages or if your utility pays significantly less for excess power than it charges for power you import.
- 5
Your bill shows a credit for what you sent back.
ComEd tracks both directions. Your bill shows the total electricity you imported, minus a credit for the excess you exported. Depending on the rules in effect, that credit might be at the same rate you pay for power, or it might be less. The credit can roll forward month to month.
In Illinois there's also a separate program that pays you for the environmental certificate your system produces — that's a one-time payment, separate from the monthly bill credit. (More on that on the Illinois buyback page.)
So what does it cost, and what do I get back?
That's the part where the numbers actually matter — and where they depend on your specific house, your specific roof, and your specific utility. We have a page for every Illinois city we cover that walks through real installed-system costs, real production estimates, and real incentives.
Find your city → or Run the numbers for your specific house →