This is just a pun, but of course his proposal for energy storage already exists, and (surprise surprise) its lithium ion batteries. Significant parts of the projected gigafactory capacity are reserved producing cells for SolarCity home energy storage appliances and other storage systems.

In fact, right now the Tesla factory has 4GWh of lithium ion batteries installed for smoothing energy demands and they are actively building 400kWh storage units:

http://www.greentechmedia.com/content/images/articles/straub...

I'm sure that the end game idea in his mind is using the 85kWh+ batteries of Teslas everywhere for distributed energy storage.

Eclectic energy