Power plants must take into account peak and off-peak periods, a power outage is unthinkable during these times. Batteries can be used to store energy during off-peak periods, when there is less demand for energy. When there is more demand for electricity during a peak period than that can be produced by the power station, the stored energy in the batteries can help.
We also refer to this as a back-up battery, for which maintenance-free & stationary batteries are chosen.
