There are a lot of grid-related engineering problems that come from increasing decentralization (e.g. wind, solar, hydro) because it increases unpredictability in supply.
I suspect there is quite a bit of personal demand that could be shifted easily.
"Smartening" appliances can lead to surprising security issues  as well. If you have the time (and interest) for an entertaining primer into the subject, watch any of Mikko Hyppönnen's recent Talks explaining "Hyppönnen's Law".
Soneone here recently ran the numbers assuming you could move rocks unencumbered up and down earths gravity well and the mass required to generate enough electricity to power the world even for a brief movement. The numbers were sobering.
The only advantage is that there is a very low switching time on the load from charge to discharge compared to pumped hydro. This could be extra useful for load balancing a distributed grid.
Beacon Power has a complete solution but they went bankrupt in 2012, then got bought by Rockland Capital who intend to rehire the staff and build a 20 MW plant in Pensylvania.
Also, here's an article of such an energy storage project in Ireland by Schwungrad Energie Ltd.
The power:energy ratio is high, but has few applications on the grid. Even lithium ion's typical 4:1 MW:MWh is a bit high.
The really unexplored territory is seasonal storage. Electricity to hydrogen, methanol, etc. have huge round trip efficiency hits, like 30%-50%, but may make a ton of sense.
There are also more traditional 1-2MW batteries that fit in tractor-trailers, etc.
- compressed air cavern
- molten salt
- regenerative crane
- pumped hydro
- inclined rail