220 ILCS 5/9-212
No new electric utility generating plant or gas production facility, or significant addition to existing facilities or plant, shall be included in a utility's rate base unless and until the utility proves, and the Commission determines, that such plant or facility is both prudent and used and useful in providing utility service to the utility's customers. For purposes of this Section, "prudent" means that at the time of certification, initiation of construction and each subsequent evaluation of any construction project until the time of completion, based on the evidence introduced in any hearings and all information which was known or should have been known at the time, and relevant planning and certification criteria, it was prudent and reasonable to conclude that the generating or production facility would be used and useful in providing service to customers at the time of completion. If the Commission has issued a certificate of public convenience and necessity for the completed facility, and to the extent that the Commission approves continued construction upon reevaluation subsequent to certification, such actions shall constitute prima facie evidence of the prudence of construction. If the Commission determines as a result of reevaluation during construction that the facility should not be completed, such determination shall constitute prima facie evidence that subsequent construction expenditures were imprudent.
A generation or production facility is used and useful only if, and only to the extent that, it is necessary to meet customer demand or economically beneficial in meeting such demand. No generation or production facility shall be found used and useful until and unless it is capable of generation or production at significant operating levels on a consistent and sustainable basis. Any pollution control devices for the control of sulfur dioxide emissions installed or used in accordance with, and up to the cost specified in, an order or supplemental order of the Commission entered pursuant to subsection (e) of Section 8-402.1 shall be deemed prudent and shall, upon being placed into operation on a consistent, sustainable basis by the public utility, be deemed used and useful.
220 ILCS 5/9-212