Step 1: decide what stays on
List critical loads and estimate average power during an outage. Do not automatically size storage around the home’s normal peak demand if you plan to shed large loads.
Step 2: multiply load by required hours
Average kW × hours gives the energy required by the loads. Then allow for inverter losses and the fraction of battery energy you intend to use.
Step 3: check battery power
A battery bank with enough kWh can still fail the project if its continuous current/power limit is too low. Check sustained and peak output with the inverter.
Step 4: ask how the battery will recharge
If outages are short and the grid normally returns, recharge power may be less important. If the system must operate through long outages, PV array size and weather variability become central.
Step 5: leave room for reality
Loads change. Batteries age. Weather varies. A design with no headroom can be frustrating even if the spreadsheet is technically balanced.
Try the battery runtime calculator and appliance load calculator together.