Texas utility Oncor commissioned a study (pdf The Value of Distributed Electricity Storage in Texas) of whether it would be cost-effective to deploy storage throughout the Texas grid (called ERCOT), placing the energy storage at the ‘edge’ of the grid, close to consumers.
The conclusion was an overwhelming yes. The study authors concluded that, at a capital cost of $350 / kwh for lithium-ion batteries (which they expected by 2020, but which Tesla has already beaten), it made sense across the ERCOT region to deploy at least 15,000 MWh of battery storage. (That would be 15 million KWh, or the equivalent battery capacity of nearly 160,000 Tesla model 85Ds.)
You can also see that at a slightly lower price of storage than the $350/kwh assumed here, the economic case for 8,000 MW (or 24,000 MWh) of storage becomes clear. And we are very likely about to see such prices.
8,000 MW or 8 GW is a very substantial amount of energy storage. For context, average US electrical draw (over day/night, 365 days a year) is roughly 400 GW. So this study is claiming that in Texas alone, the economic case for energy storage is strong enough to motivate storage capacity equivalent to 2% of the US’s average power draw.