Interior view of Powerhouse and generatorsThe improvements made to its facilities and the upgrading of its transformers expanded the capacity of the powerhouse sufficiently making it a productive operation for the needs of the twentieth century. While the continued growth in hydroelectric power up to World War II stimulated innovations and improvements in the technology of the electrical power industry, the operation at the Folsom plant continued through the late 1920s and the Great Depression of the 1930s using the same basic machinery of an earlier age. Why the Folsom Powerhouse was retained in the P.G.&E. system is a subject of speculation at this point. Wood bearings and other early technology made the facility increasingly outdated and difficult to operate. The fact that the four original generators and turbines were still running after forty years of operation was evidence of their quality engineering and design. Officials at P.G.&E. no doubt recognized this, and kept the plant running because it could produce needed power that could serve a local market, or be readily integrated into a larger power grid.

The lower powerhouse had problems from its beginnings, and even after a new generator was installed in 1923, replacing the 1897 dynamo, that facility only operated for a short time after being restarted. So despite the investment, this plant was finally shut down in 1924, and was never reactivated.

The historic importance of the main powerhouse, however, was recognized by P.G.&E.'s managers fairly early in this century. A promotional publication of 1911 referred to the power plant as the "Old powerhouse in Folsom," projecting the notion that the company had been a pioneer in the hydroelectric power industry with years of experience behind it. There was also an advantage to be gained from the impressive brick structure that housed the power plant. It presented an image of importance and stability that reflected well on this relatively new company that was expanding in a young industry. As the years passed, P.G.&E. developed the public relations theme of "progress," promoting its contribution to the expansion of electrical power in California at a lower cost to the consumer. Just before the Folsom Powerhouse ceased operation in 1952, the company produced a film recounting its own history and the history of electrical power development in California. The narrator of the film was an "old timer" who knew the industry.

The Department of Parks and Recreation acquired the Folsom Powerhouse project area, along with about half of a mile of the canal, as a donation from P.G.&E. in 1958. The property was in good condition when it was brought into the State Park System, and restoration projects for the buildings were not programmed. Minor site development was done to accommodate park visitors. In the late 1960s or early 1970s, interpretive panels and displays depicting the history of the Folsom Powerhouse were installed in the garage at the north end of the office/shop building. Not long after this, the wood-frame trash screen at the mouth of the forebay under the east side of the concrete bridge was reconstructed. Starting around 1980, district and unit staff undertook more vigorous maintenance and restoration activities, repairing facilities damaged by repeated acts of vandalism and years without proper maintenance. Between 1987 and 1990, three major preservation projects were completed on the lower powerhouse, the main powerhouse, and the office/shop building. The state park volunteers on the Board of the Friends of the Folsom Powerhouse Association were instrumental in acquiring funds for the office/shop project, and the Association funded and planned the rehabilitation of the small visitors' center in the garage.