Nuclear is a good example. What do you do with the power in the middle of the night? We don't have a battery system to store it, although the system in the state water project that pumps water uphill from one dam to the higher, to be used at a later time, functions sort of like a battery. But you could also use that unused, cheap, excess energy to crack water into hydrogen.
We have two of those water storage "battery" systems near here, one to east one one to the west. San Louis Reservoir takes water from the Calif Aqueduct to fill the reservoir, then generates power when it drops back down to the O'Neill Forebay. The Helms project raises and drops water 1500' between Courtright and Wishon reservoirs.
