99% of all grid storage is Hydro Power because it&#x27;s cheap, efficient, and relativity high density. <a href="https:&#x2F;&#x2F;en.wikipedia.org&#x2F;wiki&#x2F;Pumped-storage_hydroelectricity" rel="nofollow">https:&#x2F;&#x2F;en.wikipedia.org&#x2F;wiki&#x2F;Pumped-storage_hydroelectricit...</a><p>Running the numbers we could build several days worth of 100% electric storage for fairly minimal costs. (10TWh = 1 day&#x27;s storage.)  The problem is having a back to your backup costs money and someone needs to cover the costs for a facility that effectively never gets used.<p>Another approach is to simply over build, renewables are rapidly becoming the cheapest option so simply over building say 10% can drastically reduce the need for grid storage.