A battery can store some solar exports for later use, but its cost and performance should be compared with the potential bill reduction. The decision depends on hourly consumption, export-credit terms and the installed system.
Review hourly consumption
Review several months of data and estimate how much energy is exported during the day and needed in the evening or at night. Annual consumption alone does not show which capacity is suitable.
Compare cost and usable capacity
Ask for usable capacity, charge and discharge power, efficiency, inverter compatibility, warranty and installation cost. Compare quotes with and without a battery using the same consumption data.
Backup is different from storage
A battery does not guarantee power during an outage. Backup depends on the inverter, wiring, system configuration and protected circuits. Ask for written details of which loads could remain powered.