This planner does not guess a perfect battery size. It shows the two quantities that bound ordinary daily solar shifting: energy available to store and energy you actually need after solar.
Australian Government guidance says batteries store excess solar for later use and that larger morning/evening consumption can justify greater capacity. This calculator therefore exposes the energy balance instead of applying an opaque industry multiplier.
A 10 kWh battery does not automatically supply a 10 kW load. If you enter a desired simultaneous backup load, BecSpec only reports that as a minimum continuous-output requirement to verify; motor/compressor startup and site design remain separate.
CTs, smart meters, controlled-load circuits and export controls can change what the battery sees and how it behaves.