The technical and economic feasibility of retrofitting existing nuclear or fossil-fueled steam power plants with feedwater thermal energy storage (TES) systems for peaking power applications was investigated. A major objective of the study was to determine if retrofitted thermal energy storage (RTES) systems could result in significant fuel savings in oil- and gas-fired peaking plants. From this study it was concluded that RTES require high capital expenditure, excessive plant downtime for installation (16 mo for fossil-fuel; 24 mo for nuclear), that retrofitting 17,000 MWe of coal and nuclear plants would result in only about 2 percent annual savings in oil consumed by the U.S. utility industry in 1974, and that the technical questions which remain could best be answered by retrofitting a relatively new reliable plant as a test facility. The utility industry is receptive to the TES concept but not to the RTES concept. It is recommended that no further effort be expended on RTES, that TES studies should concentrate on coal and nuclear plants, and that a TES Proof-of-Concept Facility should be designed and constructed. (LCL)