That's the neat thing, it doesn't have to. Every kWh that goes into these batteries is a kWh not purchased from the grid. The payback for the solar panels would very quickly add up.
In fact, trying to 100% charge the batteries via solar would not be the most economical way to operate as then you are just wasting money sending power back to the grid.
In fact, trying to 100% charge the batteries via solar would not be the most economical way to operate as then you are just wasting money sending power back to the grid.