This plan would cost alot of money, because the batteries are not currently connected to the grid in a way that they can provide the power to the grid.

Batteries wear during charge/discharge, so putting this additional load on the batteries will require them to be replaced more often.

Batteries perform substantially worse when cold, so they will have to be heated somehow during the winter -- the power for which has to come from somewhere (e.g. the battery itself, which would put even more wear on it).