Doesn't this ignore completely the efficiency of the batteries? These batteries aren't ideal, you won't get out 100% of the energy that is put in. I don't know the efficiency rating, and I haven't done the math, but I suspect that the losses will cut into the savings significantly.
The page says >92% round trip efficiency. Since they also claim that they're selling them to utilities for exactly this purpose I'd guess it's theoretically possible and probably comes down to the pricing structure offered to consumers in your local area to buy and sell to the grid.
