Adding energy storage (a battery) to a solar power system increases functionality by keeping power on even when there is a grid power outage, as well as storing extra solar power for use at night (reducing power exporting to the grid and increasing self-sufficiency).
Backup systems are not the same as an off-grid system but are similar in principle. The main difference is that backup systems have an existing grid connection or use the grid as the primary power supply. Unlike off-grid systems, backup systems don't have to provide 100% of power use and are therefore more variable in their capacity.
Most backup power systems have the capacity to run a home autonomously for at least one day. In some cases, backup power can last continuously, depending on the capacity of the system, season, weather, building efficiency, and the users.
Back-up power systems are generally configured in one of two ways:
To backup essential loads like refrigeration, lighting, and most 120V appliances and outlets, a partial-house backup configuration separates these loads from larger loads. Non-backup loads include large loads like electric heat, clothes dryer, pool pumps, or other non-essential items. Two breaker panels are required for this configuration, so installation costs may be higher than a whole-home backup configuration.
To back up a whole house, including larger 240V appliances like water heaters, air-conditioning, and well pumps, the main electric panel can be supported with additional battery capacity.
In either configuration, the duration of backup power depends on the users, load management, the capacity of the solar array, and the availability of sunlight.