There are two distinct uses for a grid-connected battery bank. The first, and the most common is a power backup system. The purpose is to provide temporary power in the instance of grid loss. This is similar to a UPS system but is typically on a much larger scale with higher storage capacity.
The second is a grid-tied system with battery backup. This system setup is used to generate and sell excess power produced by a renewable source to your utility provider when you are connected to the grid. In the event of a grid failure, the battery bank then provides storage power for the critical loads during the outage. Systems are configured according to how much power is to be sold to the utility vs. how much will be stored. If a higher percentage of the power generated is pushed to the utility, this will decrease the remaining amount available for backup and vice versa. This may be determined by personal preference or may be limited or regulated by the utility.
For grid-tied battery backup systems, the battery bank should only be sized to handle the loads supported for the duration of a temporary outage. These systems are commonly used to run household necessities such as limited lighting and refrigeration, etc. It is ideal to limit the size of the supported load to essential services only as higher capacity battery banks require more maintenance as well as power usage to hold at full charge, reducing the amount of generated power that can be sold to the utility.
For grid-connected backup systems, Installers will typically size battery banks to operate to a greater depth of discharge to lower initial installation costs. Cycling of grid-connected systems is significantly less frequent than off-grid applications where this typically occurs on a daily basis. With infrequent cycling, a maximum 80% DOD is acceptable as long as the customer understands that the overall cycle life is affected when the battery bank is discharged beyond 50% SOC. These systems should not be designed to fully discharge the battery bank.
In regions where the grid may not be reliable and extended or intermittent outages occur frequently, it is necessary to size the battery bank and charging source(s) to support a larger load over a longer period of time. This prevents the battery bank from over-discharging and/or deficit cycling.