Maybe good for an LED Lighting circuit?
My first thought about the Powerwall, was it'd be great if it could be fed from solar or wind devices and to then do a simple re-wire of the house lighting circuits, and replace existing bulbs with LED's.
The re-wire wouldn't be expensive, as it just needs disconnecting from the output of the distribution unit and re-wired into the Powerwall output. Likewise, no DC->AC->DC conversion required if the bulbs are all DC to begin with - so no invertors needs.
Likewise, extra DC outlets could be provided to run/recharge DC devices (routers, iPads, mobiles, etc)
OK, so lots of "low energy" bulbs consume little power, but LED's can consume much less, so although you might still be dependant on the grid for high powered AC-mains items, you could possibly save quite a bit of energy cost.
I guess in the end it comes down to the power provided by the Powerwall, vs upfront cost (and some re-wiring, plus new LED bulbs) vs lifetime. Oh, and the cost of the solar panels/wind turbines too. Ah....I can see there might be an issue now :( Oh well.