Battery does not have to beat the grid. It has to assist in balancing the load on the grid.
Practically, to achieve 100% electricity generation from renewables, there is always going to have to be a certain degree of overcapacity in generation simply to compensate for the fact that the wind and sun are inconstant as well as intermittent. But there's also an upper limit how much under-utilisation of generating plant that can be financially tolerated. A small amount of storage can go a long way toward limitting the amount of over capacity needed to meet peak needs.