220 ILCS 5/9-210
The Commission shall have power to ascertain the value of the property of every public utility in this State and every fact which in its judgment may or does have any bearing on such value. In all proceedings before the Commission, initiated by the Commission upon its own motion, or initiated by an application of such public utility, in which the value of the property of any public utility or utilities is an issue, the burden of establishing such value shall be upon such public utility or utilities. In making such valuation the Commission may avail itself of any information, books, documents, or records in the possession of any officer, department or board of the State or any subdivision thereof. The Commission shall have power to make revaluation from time to time and also to ascertain the value of all new construction, extensions, and additions to the property of every public utility.
For purposes of establishing the value of public utility property, when determining rates or charges, or for any other reason, the Commission may base its determination on the original cost of such property.
220 ILCS 5/9-210