Use "PV Watts" to assess the solar capacity of a given location.

Net Metering

This link defines Net Metering in the State of Illinois.  In general the law requires that Comed provide a one to one credit for electricity provided the the grid from small providers like residences.
This link addresses Comed Net Metering
ComEd's Net Metering program is for residential or commercial customers who own or operate an eligible renewable electric generator of up to 2,000 kW that generates electricity for their own use. Learn more about Distributed Generation (DG), Interconnection and Net Metering in this helpful video.
Across the country, there is growing interest in using rooftop solar panels and other small scale, on-site power sources known as distributed generation (DG). To encourage the introduction of these systems when they first came to market years ago, many states approved a billing system called net metering.

This link addresses City of Chicago Build Code matters pertaining to roof top photo voltaic panels in Chicago.