It's simple power vs energy equations. Let's say you need to supply steady 1MW to town for 1hr, 2hr or 10hr. This needs total of 1MWh 2Mwh or 10Mwh respectively. Let's say battery would cost $200/kwh, then this would cost 200k, 400k or 2M resp. Battery storage is generally used for less than 2hr duration currently where they are price competitive. For the rest, either gas peaker, coal or hydro is used.