In 1969, SRP initiated its Hydroelectric Expansion and Frequency Unification (HEFU) program to increase hydroelectric generating capacity at facilities on the Salt River. This program included the installation of pumped storage units at Mormon Flat Dam in 1971 and at Horse Mesa Dam in 1972. The HEFU program also provided for converting the conventional hydroelectric generating facilities at the dams on the Salt River from the outmoded 25-hertz (Hz) to the modern frequency of 60 Hz and in 1973, a new 60-Hz, 36-MW generating unit was installed at Theodore Roosevelt Dam, which replaced the existing 25-Hz units.

The Horse Mesa dam has three conventional hydroelectric generating units rated at a total of 32,000 kW, and a pumped storage unit rated at 97,000 kW added in 1972.

Open-loop Pumped Hydro Storage