Environment & Energy

It's purpose is not to store energy for days or weeks, rather it's there to stabilize the grid when wind or solar power drop out, allowing sufficient time for the idling gas plants to power up and pick up the load.

California has a great deal of experience with gigawatt scale pumped storage systems. The state water project can source or sink quite a lot of electric power, for example, the Castaic Power Plant which has a nominal capacity of 1.5 gigawatts.

But you can do the math yourself. If you had an off-grid house, how many solar panels and batteries would you need to get yourself a 99% reliable source of electricity? (Mind you, that's three and a half days without power each year...) Now multiply that cost by 8 billion. The problem is the same at any scale.

You can watch California's electric grid in action here:

https://www.caiso.com/TodaysOutlook/Pages/supply.html

If you want to model how a 100% renewable energy grid might work you can examine real world data from California's gigawatt scale solar, wind, and storage infrastructure.

It's clear that storage is the limiting factor.