As the sun sets on California’s solar farms, a backup energy source deep in the Sierra Nevada Mountains springs to life. The huge system of reservoirs and turbines can store energy during the day and then crank out electricity for 900,000 homes, using just water and gravity.

As the state tries to make wind and solar work around the clock, officials want to build more like it. It won’t be easy: such projects take years to develop, are expensive and face stiff opposition.he push by California and other states to revive the century-old technology — called “pumped-hydro storage” — underscores the limitations of modern batteries.