You're mixing up the use of batteries for grid stabilization with their use for storage.
But it might come as a shock to you, cities are taking in order of MW to GW
A typical grid-storage battery will have enough storage to dispatch power at 100% of its rated capacity for 4 hours.
Of course if you draw out the energy at less than full capacity, it will take longer to fully discharge.
The largest battery currently in the US - Moss Landing - for example is rated 750 MW/3,000 MWh - which means it can sustain 750MW of output for 4 hours and it does this daily.
This is not "20 minutes".