There are several key principles to consider:
1、Based on load — how many kilowatt-hours (kWh) of electricity are needed during the night?
2、Based on PV generation capacity — can the excess electricity generated by the PV system during the day fully charge the battery?
3、Based on backup power requirements — how long do you want to maintain power supply during an outage?
For a simplified estimation, the following formula can be used:
Capacity (kWh) = (Daily electricity consumption × Backup days) / Depth of Discharge (typically 80–90%)
Then account for efficiency losses (approx. 10%) and a margin.
