At larger scales, these are called pumped-storage hydroelectric plants. There are currently more than 100 GW of pumped-storage capacity in the world, with efficiency ranging from 70% to 87% [1]. They are, in fact, some of the largest batteries we've ever built.<p>They store electricity by pumping thousands of tons of water uphill when demand is low, and letting it fall back down past a bunch of turbines when demand is high. Water is much easier to handle than a solid block of steel, and it's much more scalable as well. You just need a hill and some water, possibly an already existing reservoir. Pumps can be turned on and off almost instantly to meet fluctuating demand. There's one about 10 minutes' drive from where I live. It's marvelous, and the two artificial lakes (one at the top, one at the bottom) also make nice parks for the public to enjoy.<p>Since pumped-storage plants seem to work so well, I wonder if there will be any need to install smaller versions in each home. It's probably going to be difficult to match the efficiency of much larger units. Maybe these will be more useful as backup batteries.<p>[1] <a href="https://en.wikipedia.org/wiki/Pumped-storage_hydroelectricity" rel="nofollow">https://en.wikipedia.org/wiki/Pumped-storage_hydroelectricit...</a>