Solar-power development over canals is an emerging response to the energy-water-food nexus that can result in multiple benefits for water and energy infrastructure. Case studies of over-canal solar photovoltaic (PV) arrays have demonstrated enhanced PV performance due to the cooler microclimate next to the canal. Further, shade from the PV panels has been shown to mitigate evaporation and could mitigate aquatic weed growth. However, the evaporation savings and financial co-benefits have not been quantified across major canal systems. Here we use regional hydrologic and techno-economic simulations of solar PV panels covering California’s 6350 km canal network, which is the world’s largest conveyance system and covers a wide range of climates, insolation rates, and water costs. We find that over-canal solar could reduce annual evaporation by an average of 39 ± 12 thousand m3 per kilometer of canals. Furthermore, the financial benefits from shading the canals outweigh the added costs of cable-support structures required to span canals. The net present value (NPV) of over-canal solar exceeds conventional over-ground solar by 20% to 50%, challenging the convention of leaving canals uncovered and calling into question our understanding of the most economic locations to locate solar power.