<b>Substantial Global Pumped Hydroelectric Storage Resource Persists Under Environmental Constraints</b>

Long-duration energy storage is essential for stabilizing electricity systems dominated by wind and solar power¹, yet the global potential of pumped hydroelectric storage (PHS) under realistic hydrological, environmental, and social constraints is unresolved. Here we present a global, bottom-up assessment of PHS by integrating river discharge, long-term precipitation–evaporation balance, topography, protected landscapes, land-use exclusions, population displacement thresholds, and infrastructure accessibility. Screening 2.89 million river and 50 million mountain valleys, we identify a total of 19.03 ± 2.67 PWh of economically and environmentally viable PHS resources. This resource is strongly concentrated in high-elevation terrain across Asia and the Americas, while low-relief and water-limited regions exhibit significant intrinsic constraints. Under projected electricity demand conditions (2050), the identified PHS can provide more than 100 hours of storage duration for 58.4% of global electricity consumption. Although PHS remains the most cost-effective option for long-duration energy storage beyond three hours, its large-scale deployment is fundamentally constrained by geography, hydrology, environmental suitability, and transmission accessibility. These constraints define a hard physical boundary on the role of PHS in future power systems and underscore the necessity of integrating complementary storage technologies to achieve resilient, low-carbon electricity systems.