If you have full usage of your charger, then batteries are pointless anyway because you have steady usage no matter what.

But it's not a realistic assumption, at the very least the driver has to park, get out of their car, plug the car, spend some time on the payment interface, then unplug the car and leave.

So even in the maximum theoretical scenario where drivers are lining up at the charging station, your charger isn't going above 80% utilization. Using a single car battery, you can save 20% in terms of connection to the grid (you “just” need a 800kW connection instead of a 1MW one), and you aren't nearly as much of a nuisance to the grid as if you were having constant ups and down of 1MW.

In practice there will a be a trade off between how much you save in connection infrastructure to the grid and how much you spend on batteries, and this calculation will depends a lot on the usage pattern.