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How this is calculated#

The tool sizes a home battery in kWh from your daily use and your solar array. It also gives the inverter rating and points at AS/NZS 5139.

1. Evening use. It takes your evening share of the daily use.
2. Energy to deliver. It multiplies that by the days of autonomy.
3. Efficiency. It divides by the square root of the round-trip efficiency. That is the discharge efficiency.
4. Usable capacity. The result is the usable capacity in kWh.
5. Nominal capacity. It divides the usable capacity by the depth of discharge.
6. Suggested bank. It rounds up to whole modules of the size you enter.
7. PV surplus. It finds the daily PV energy, then subtracts the daytime use. The tool checks that the surplus fills the evening share.

Worked example#

• 20 kWh a day, 60 % in the evening, 6.6 kW of PV. The usable capacity is 12.65 kWh. The nominal capacity is 14.05 kWh. The bank is 3 × 5 kWh. The inverter is 5 kW continuous. The PV surplus is 15.1 kWh a day.

Limits#

The location rules apply to a pre-assembled integrated BESS. They are in AS/NZS 5139:2019 Cl 4.2.2.2 and Cl 4.2.5. The tool does not hold Amendment 1 (2025). It does not size the DC or AC cable. It does not model tariffs.

Check the usable capacity against the battery data sheet. The nominal capacity is larger, because the battery cannot use all of its charge. The suggested bank is a number of equal modules. Round to the modules the maker sells. Check the inverter against the maximum demand of the home.

See also: Home batteries and AS/NZS 5139, Solar battery size for a family home and PV string sizing.