That is effectively a form of arbitrage, I suppose. You're charging the battery when electricity is either cheap (middle of the night), or free (from solar on y

That is effectively a form of arbitrage, I suppose. You're charging the battery when electricity is either cheap (middle of the night), or free (from solar on your roof) and then using it in place of expensive day time electricity. You could argue using battery electricity in stead of bringing in grid electricity is functionally equivalent to selling it to the grid, both in terms of pricing incentives, and the effect on the grid – in the same way that people are discussing being able to switch fridges on and off minute by minute as being equivalent to building grid generators which can be switched on and off on that timescale.