The concept of storing electricity generated in a utility grid has been tried since the beginning of the power industry. In the U.S., large-scale storage projects flourished in the 1960s, '70s, and '80s as utilities added 18 GWs of pumped hydro facilities to support the rapid build out of the fleet of nuclear power plants across the nation. Nuclear plants run best at higher power ratings, so pumping water in these hydro plants presented ideal off-peak loads during nights and weekends when customer demands are lowest. This method of grid storage has been improved during the past two decades, and today these plants provide more than 2 percent of the total capacity of the national grid.