Pick the appliances you need to keep running (or enter your own wattage) and see exactly how many hours your battery bank will cover.
Runtime assumes a steady load — real appliances cycle on and off (fridge compressors, etc.), so actual backup time in practice is often somewhat longer than this steady-state estimate.
Further reading: Solar Battery Storage 101: Sizing, Cost, and Whether It's Worth It
This is the inverse of battery sizing: instead of solving for capacity given a target runtime, it solves for runtime given a capacity you already have (or are considering). Check the appliances above that you'd actually want running during an outage — most people are surprised how far a modest battery goes once the load is trimmed to real essentials.
During an outage most people only power an "essentials" circuit — fridge, some lights, wifi, phone charging — not the whole house at once. That's why this defaults to a modest load rather than a whole-home total.
Use the Battery Size Calculator — enter your target hours/days there and it solves for the capacity you need, the reverse of this tool.