There are a few possibilities for grid-scale storage. Lightsail energy are planning compressed air storage at grid-scale and say they can get 750kWh of storage

There are a few possibilities for grid-scale storage. Lightsail energy are planning compressed air storage at grid-scale and say they can get 750kWh of storage into a shipping container format, or much more underground by using caverns emptied by the oil and gas industry http://www.lightsail.com/ http://www.lightsail.com/ There is also Gravity-power who are planning underground mass storage in the 160MWh range http://www.gravitypower.net/ http://www.gravitypower.net/ And Isentropic, who are using hot gravel storage to build a 6MWh plant in the UK Midlands http://www.isentropic.co.uk/ http://www.isentropic.co.uk/ Also, there are developments in chemical storage like the Ryden battery, which could bring battery costs low enough to just put storage where the power is being used - http://powerjapanplus.com/ http://powerjapanplus.com/