Existing battery technology works best when you don’t charge to 100% or discharge to 0%. On top of this you want oversized batteries to deal with battery degradation over time. This means any large scale battery system includes a buffer beyond normal use which you would only use if wholesale prices spiked.
On top of this grid operators want generation redundancy in case an individual power plant goes offline for whatever reason. Combining both you get quite a lot of excess capacity compared to the current grid.
