There are 40 million people, and 13.5 million households in California. About 409 pumped hydro installations 3.5 km deep on the seafloor, each with 38 steel bottles, each displacing 365 tonnes of water, would serve the entire population.

Total number of steel bottles = 15,542

If they are stood upright, then each bottle has a 10x10-foot footprint. The whole installation would fit on a square patch of seafloor 380-meters wide. For comparison, this is about the same size as the Hoover Dam, which is 1244-feet (379 meters) long.

No breakthrough technology would be required. It could have been built 100 years ago, had there been a need.