Use "PV Watts" to assess the solar capacity of a given location.
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. http://www.illinoisattorneygeneral.gov/environment/netmetering.html
This link addresses Comed Net Metering https://www.comed.com/customer-service/rates-pricing/interconnection/Pages/net-metering.aspx
This link addresses City of Chicago Build Code matters pertaining to roof top photo voltaic panels in Chicago. http://www.cityofchicago.org/city/en/progs/env/solar_in_chicago.html