Can water pumped uphill and big batteries keep the lights on?

Electricity is hard to keep, so for a century the grid mostly let it flow and matched supply to demand on the spot. Now there is a way to bank it. Grid energy storage is a set of technologies that store energy for later use, storing excess electricity from sources such as solar and nuclear power and releasing it when needed. The biggest form is surprisingly old and wet: pumped-storage hydroelectricity.

Here is how it works. Two reservoirs sit at different heights. When there is low-cost surplus power, pumps push water to the upper one, storing energy as gravitational potential energy. When demand is high, the water runs back down through turbines and makes electricity. The plant gets back about 70 to 80 percent of what it uses, and it earns money by selling power when prices are highest. As of 2020 it made up around 95 percent of active storage installations worldwide. The first pumped hydro was built in the Alps at the end of the 19th century, and it spread fastest in the 1960s to 1980s nuclear boom, because nuclear plants cannot quickly adapt to changes in demand.

Batteries are the newcomer. Lithium-ion is now the most used grid battery, their costs fell by 90 percent from 2010 to 2023, and battery power is by far the fastest-responding of all plants, reacting at millisecond timescales. They suit short storage, under eight hours. In Ventura County, California, 142 Tesla Megapacks providing 100 MW replaced a gas peaker plant.

Neither is magic. Pumped hydro needs hilly country, and the more solar and wind a grid has, the longer the storage it needs.