Your starting battery size is the lower of average daily solar exports and evening/night use divided by efficiency. Size from usable kWh, not nominal kWh.
The simple sizing rule
Useful battery size ≈ min(daily solar export, night use ÷ 0.90)The 0.90 allows for roughly 10% round-trip energy loss.If you export 12 kWh on an average day but only use 7 kWh after sunset, a 13.5 kWh battery will often sit partly full. If you use 16 kWh at night but export only 8 kWh, the solar system may not fill a large battery without grid charging.
Three common Australian household patterns
Efficient home, gas hot water, little overnight load.
Typical family load with cooking, cooling and evening appliances.
Pool, EV, electric hot water, large HVAC or all-electric home.
Those ranges are orientation only. Seasonal generation, export limits, tariff windows, backup reserve and grid-charging strategy can move the answer materially.
Why oversized batteries can destroy payback
The federal incentive tapers above 14 kWh and again above 28 kWh. More importantly, unused capacity earns nothing. A battery that regularly finishes the night half full has capital sitting idle, while a battery that rarely fills may be constrained by the solar system.
- Ask for usable capacity and maximum continuous output—not just the product name.
- Check whether backup reserve permanently withholds part of the battery.
- Compare at least a winter and summer day, not one annual average.
- Make the installer show the import/export interval data behind the recommendation.