Del. Code tit. 26 § 302
The Commission may, from time to time, ascertain and determine the rate base of any public utility whenever, in the judgment of the Commission, it is necessary so to do for the purpose of carrying out this chapter, and in making such determination the Commission may have access to and use any books, documents, or records in the possession of any department, board, commission or agency of this State or any political subdivision thereof. In ascertaining and determining the rate base, the Commission may determine every fact, matter, or thing which, in its judgment, does or may have any bearing thereon.
If a water utility is not, pursuant to § 122(3)c. of Title 16, under review concerning its water system's ability to provide adequate service to its customers under its present certificates of public convenience and necessity or subject to a review by the Commission of the appropriate rates to be charged by the water utility in light of the quality of service being provided to its customers, the Commission will include in the utility's rate base, treat as used and useful utility plant, and, accordingly, allow to be fully recovered in the utility's rates without imputation of revenues, all costs which are incurred by the water utility, in the exercise of its good faith business judgment, in constructing facilities (including without limitation supply, treatment and transmission facilities) to serve the needs of existing customers or of persons who are reasonably anticipated by the water utility to be its customers within 3 years from the date used by the Commission to recognize rate base in the rate proceeding. The number of customers reasonably anticipated to be added within that 3-year period will consist of customer projections which are relied on by the utility and are generated by professional engineers or planners, governmental or regulatory agencies, officials or authorities, or the water utility itself, and which are not arbitrary and capricious. If the water utility does not, by the end of the 3-year period after the date used by the Commission to recognize rate base in the rate proceeding, reach at least 75% of the total number of customers originally anticipated to be served by the facilities, the Commission may only then require the water utility to impute revenues and then only to the extent of the number of customers it originally anticipated to be served by the facilities but who have not, as of the end of the 3-year period, been added.
26 Del. C. § 302