Solar battery calculator
Size a home battery for the solar evening load.
Reads AS/NZS 5139:2019. Tables checked 1 Oct 2026.
Try a job 6
Each job fills in example values. Replace them with your values.
- Size a solar battery for a family homeThe kWh a 25 kWh a day home needs to run its evenings on an 8 kW array.
- Check where a home battery can goThe AS/NZS 5139 rules for a battery on a garage wall backing onto a bedroom.
- Size a battery for the evening peakThe kWh to carry 60 % of 22 kWh a day into the evening.
- Size a battery for 6.6 kW of solarThe battery for a 6.6 kW array and a home using 18 kWh a day.
- Size a battery for a day of backupThe battery for one day of backup with a 5 kW peak.
- Size a battery in 5 kWh modulesThe number of 5 kWh modules a home using 24 kWh a day needs.
Inputs
Worked example with the default inputs. The live calculator replaces it when the page loads.
- Daily use
- 20 kWh
- Evening share
- 60 %
- Days of autonomy
- 1
- Peak load on the battery
- 5 kW
- PV size
- 6.6 kW
- PV yield per day
- 3.5 kWh/kW
- Usable depth of discharge
- 90 %
- Round-trip efficiency
- 90 %
- Module size
- 5 kWh
- Location
- Garage
- On a wall to a habitable room
- No
Usable capacity
12.65kWh
- Nominal capacity
- 14.05kWh
- Suggested bank
- 3 × 5kWh
- Inverter, continuous
- 5kW
- PV surplus per day
- 15.1kWh
This result is a design aid. Verify it against the current standard.
Calculation steps
- Evening use12kWh
- Energy to deliver12kWh
- Discharge efficiency (square root of round-trip)0.95x
- Usable capacityResult12.65kWh
- Nominal capacity at the depth of discharge14.05kWh
- Suggested bank
3 × 5 kWh
- Inverter continuous rating5kW
- PV energy per day23.1kWh
- PV surplus after daytime use15.1kWh
- Energy to fill the evening share13.33kWh
About this calculator
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.
- Evening use. It takes your evening share of the daily use.
- Energy to deliver. It multiplies that by the days of autonomy.
- Efficiency. It divides by the square root of the round-trip efficiency. That is the discharge efficiency.
- Usable capacity. The result is the usable capacity in kWh.
- Nominal capacity. It divides the usable capacity by the depth of discharge.
- Suggested bank. It rounds up to whole modules of the size you enter.
- 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.
Questions
Each answer describes what this tool calculates. The result is a design aid. Verify against the current standard.
How big a solar battery do I need?
Size it on the energy you use after sunset. The tool takes your daily use and the evening share, then adds the losses. It divides by the usable depth of discharge to give the nominal size on the data sheet.
What is the difference between usable and nominal kWh?
Nominal kWh is the full chemical capacity. Usable kWh is what the battery lets you draw before it stops, set by the depth of discharge. A 10 kWh battery at 90 % gives 9 kWh usable.
Can my solar panels fill the battery?
The tool multiplies the PV size by a daily yield, then takes off your daytime use. If the surplus is less than the evening share plus losses, it warns you. Winter yields are lower than the yearly average.
Where can a home battery go?
AS/NZS 5139:2019 keeps an all-in-one home battery out of habitable rooms, ceiling spaces, wall cavities, under stairs and escape routes. It also sets clearances from exits, windows and other appliances. The tool lists the rules for the location you choose.
How is this different from the battery bank tool?
The battery bank tool sizes an off-grid or backup bank in Ah at a set voltage from a list of loads. This tool sizes a grid-tied home battery in kWh from your bill and your solar.
Clauses and tables this tool reads
- AS/NZS 5139:2019 Cl 4.2.2.2Restricted locations for a home battery
- AS/NZS 5139:2019 Cl 4.2.3.2IP rating of a home battery
- AS/NZS 5139:2019 Cl 4.2.4.2Barrier to a habitable room
- AS/NZS 5139:2019 Cl 4.2.5Working space at a home battery
- AS/NZS 5139:2019 Cl 4.2.2 to 4.2.5Location, clearance and barrier rules for a pre-assembled integrated BESS
Every clause and table, with how each was verified
Tables last checked on 1 Oct 2026.
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