Battery shopping becomes much easier once you separate two questions: what loads must stay on and for how long?
Start with energy, not brand names
Add the average watts of your critical loads and multiply by the number of hours you want them to run. Divide by 1,000 to convert watt-hours to kilowatt-hours.
Allow for usable capacity and losses
A battery’s nameplate capacity is not always the energy delivered to appliances. Allow for depth-of-discharge limits, inverter losses, reserve capacity, temperature and aging.
Runtime and whole-home backup are different
A refrigerator, lights and internet may need only a modest average load. Air conditioning, water heating and cooking can multiply both battery and inverter requirements.
Estimate runtime → Size from your loads →
Always verify battery/inverter communication, voltage compatibility, maximum current and installation requirements before purchase.