Backup duration is usable energy divided by the power of the circuits you actually kept. List those circuits. Add running watts for the ones that will overlap. Note start watts for anything with a motor. Take usable kilowatt-hours from the battery sheet and the inverter's cutoff. Divide after that list exists. Hours printed before the list are a slogan.
Only the circuits that transfer
A circuit on the utility side of the transfer is dark in the outage, and it does not belong in the hour count. A range, a well, or a heater counts only if the design moved it to the backup panel. Home battery backup for outages explains the transfer and why a whole house is a different project from a critical-load panel. Walk the breakers with the drawing and write the names. A later owner who moves a space heater onto a backed-up outlet has changed the duration without changing the battery.
Running watts, then the start
Running watts are what the nameplate draws while the load is already running. Start watts are the brief extra a motor can ask for. How to add up inverter loads is the method, including a hypothetical list that is not your house. Replace that list with the plates in your panel. The inverter's continuous rating has to cover the overlap. Its surge rating has to cover the start, for the seconds that inverter actually allows.
A pump that the inverter refuses at startup ends the backup in the first second. Kilowatt-hours still sitting in the battery do not start a motor the inverter has already declined. Write the start beside the running watts so the hour math is not the only test the design has to pass.
A hypothetical four hours
Use a hypothetical case only as arithmetic. The backed-up circuits draw 500 watts continuously, and usable energy is 2 kilowatt-hours. Before losses, 2 kilowatt-hours divided by 0.5 kilowatts is 4 hours. Label that stack hypothetical. Do not call it a typical outage, a national backup time, or a quote. Your nameplates and your usable energy replace both inputs.
What those four hours leave out
The division ignores inverter conversion losses, the inverter's own idle draw, and energy the battery does not return on the way out. Real hours run shorter once those are included. The division also ignores a second load plugged in during the outage. Usable energy is the portion the sheet lets you cycle, which may be less than the sticker. How depth of discharge is defined is in depth of discharge and charge stages.
During the outage the grid cannot refill the battery, because the grid is what failed. Refill comes from solar only if the design allows the array to run while the island is up, or from a generator if one was part of the job. Plan the hours you calculated as hours you must live inside, unless that recharge path is written down. Revisit the list when the house gains a freezer, a pump, or a workshop tool. The battery did not shrink. The list grew.
Write usable kilowatt-hours the way the sheet defines them, at the power you will actually draw. A round number on the sticker is the label, not always the energy you may take. The inverter cutoff can arrive before the sheet's deepest allowed discharge, and then the usable figure is the smaller of the two. The hour count uses that smaller figure. If two packs are paralleled, use the combination both manuals allow, not a doubled sticker written in the margin. The watt sum still comes from the load list. This page only turns an honest watt sum into hours.
