Pumped storage hydropower is one of the oldest and most reliable forms of energy storage, dating back to the early 20th century. Initially developed to store excess baseload electricity generated during periods of low demand, PSH has evolved into a critical component of modern energy grids. The concept involves pumping water from a lower reservoir to an upper reservoir during off-peak times when excess power is available, then reversing the flow during periods of high demand to generate electricity using turbines. This simple yet effective "water battery" has been instrumental in balancing electricity supply and demand for nearly a century.

The first PSH facility in the United States, the Rocky River Plant in Connecticut, began operation in 1929. Since then, plant size and technology have advanced significantly, with projects like the Bath County Pumped Storage Station in Virginia, which remains one of the largest in the world, providing 3,003 MW of capacity. According to the IHA 2024 World Hydropower Outlook (link), there are currently 43 operating PSH projects in the U.S., offering a combined generation capacity of 22,878 MW and an energy storage capacity of over 550,000 MWh, with global installations reaching approximately 270 plants and 161,000 MW.

PSH currently accounts for 96% of all utility-scale energy storage in the United States.