I'm thinking it would be more useful to use as a storage device to store off peak energy (cheap) at night and use it during the day. The solar panels are already doing their job and that gets sold to the grid, so I don't see much use in storing that: You're just creating an inneficiency. Removing usage from peak times on the other hand is already an environmental friendly way to reduce solution when all those wind turbines are spinning and peple are asleep - in fact one doesn't even need to have solar panels to do so, just the battery.