65 ILCS 5/11-117-13
Any municipality, owning a public utility, shall keep the accounts for each public utility distinct from other municipal accounts and in such manner as to show the true and complete financial results of municipal ownership or ownership and operation, as the case may be. These accounts shall be so kept as to show (1) the actual cost of the municipality of each public utility owned; (2) all costs of maintenance, extension, and improvement; (3) all operating expenses of every description, in case of municipal operation; (4) the amounts set aside for sinking fund purposes; (5) if water or other service is furnished for the use of a public utility without charge, as nearly as possible, the value of that service and also the value of any similar service rendered by each public utility to any other municipal department without charge; (6) reasonable allowances for interest, depreciation, and insurance; and (7) estimates of the amount of taxes that would be chargeable against each public utility if owned by a private corporation.
The corporate authorities shall print annually for public distribution, a report, in the form specified in this Section, showing the financial results of the municipal ownership or ownership and operation. The accounts of each public utility shall be examined once each year by a licensed Certified Public Accountant permitted to perform audits under the Illinois Public Accounting Act who shall report to the corporate authorities the results of his examination. This accountant shall be selected in such manner as the corporate authorities may direct, and he shall receive for his services such compensation, to be paid out of the revenue from each public utility, as the municipality may prescribe.
65 ILCS 5/11-117-13