How to Size a Home Emergency Backup Battery Bank
Essential loads, hours of autonomy, depth of discharge, and inverter losses in one sizing path.
Whole-house backup sounds luxurious until you price a mansion-scale battery. Emergency sizing focuses on survival circuits: fridge, lights, router, well pump if needed. List watts, pick hours, add margin.
Define the load list
Write each device and its running watts. Note which are surge loads—pumps and compressors need starting multipliers. Add ten to twenty percent for unknowns.
Convert to watt-hours
Wh needed = total watts × hours of autonomy. Divide by inverter efficiency to get DC energy the bank must deliver. A 800 W essential panel for eight hours is 6,400 Wh before losses.
Depth of discharge and chemistry
Lithium banks often plan at eighty percent usable; lead-acid may use fifty. Divide required Wh by usable fraction to get nameplate Wh, then convert to Ah at system voltage.
Solar coupling
Grid-down events stretch longer when solar recharges the bank daytime. Pair storage sizing with a realistic daily harvest estimate—not summer peak only.
Test before you trust
Run a controlled outage drill: kill the main and log voltage under load. Software models help, but a Saturday test reveals cable drops and phantom loads spreadsheets miss.
Emergency backup is engineering triage. Size for what keeps life safe and communications alive; expand later if budget allows.