Suppose essential loads are four LED lights at 10 W each, two fans at 60 W, a 100 W refrigerator average draw and a 20 W router. Running load is roughly 280 W, but the refrigerator startup surge may be much higher. For four hours, simple energy need is about 1.12 kWh before losses.
If the inverter is 90% efficient and only 80% of nominal battery capacity is intended to be used, nominal storage must be greater than 1.12 kWh. Add ageing margin. Then separately ensure the inverter can handle refrigerator surge plus other simultaneous loads.
Why air conditioning changes everything
Adding a 1.5-ton AC can multiply both power and energy requirements. Instead of automatically sizing a huge battery, decide whether comfort loads can be excluded during outages. Essential-load subcircuits often produce a much cheaper system.
Check recharge time
A battery that lasts through an outage but cannot recharge before the next one may still fail your use case. Consider grid charging power and realistic solar generation.
Warranty metrics
Compare usable energy, cycle warranty, throughput where stated and end-of-warranty capacity terms. Do not compare batteries only by nominal kWh.
A good worksheet turns every appliance into watts, hours and surge, then tests both the worst outage and recharge period.