This will depend on various factors including the size of the solar PV array and its’ alignment, the profile of household electricity demand and, therefore, how much surplus electricity will be available for storage.
For domestic properties, the general rule is that the battery should have a capacity sufficient to provide power overnight until the following morning when the PV system will start generating one again and recharge the battery. This is complicated by the seasonal variation in daylight hours and greater demand for electricity during the winter when generation is at its lowest.
When purchasing a battery system, it is important to understand that it is likely that during the summer the battery may be fully charged and never fully discharged due to long day length, whilst in winter the converse may happen.
The ability to charge batteries using off-peak electricity can also influence the decision to add a battery to a solar PV installation