220 ILCS 5/9-250
Whenever the Commission, after a hearing had upon its own motion or upon complaint, shall find that the rates or other charges, or classifications, or any of them, demanded, observed, charged or collected by any public utility for any service or product or commodity, or in connection therewith, or that the rules, regulations, contracts, or practices or any of them, affecting such rates or other charges, or classifications, or any of them, are unjust, unreasonable, discriminatory or preferential, or in any way in violation of any provisions of law, or that such rates or other charges or classifications are insufficient, the Commission shall determine the just, reasonable or sufficient rates or other charges, classifications, rules, regulations, contracts or practices to be thereafter observed and in force, and shall fix the same by order as hereinafter provided.
The Commission shall have power, upon a hearing, had upon its own motion or upon complaint, to investigate a single rate or other charge, classification, rule, regulation, contract or practice, or any number thereof, or the entire schedule or schedules of rates or other charges, classifications, rules, regulations, contracts and practices, or any thereof of any public utility, and to establish new rates or other charges, classifications, rules, regulations, contracts or practices or schedule or schedules, in lieu thereof.
220 ILCS 5/9-250