Batteries, which are charged during non-peak times, can feed the grid during peak times. This levels out the demand and reduces the number of power plants needed to be kept on standby to support that peak usage. Batteries won’t replace the powerplants supplying base load, but they can replace the ones that only exist to supply peak load.

When there is no feed to the battery, it’s typically because of storms or other temporary interruptions which cut you off from the powerplants. In which case, more batteries will help, but more powerplants will not.