Calculate exactly how long your inverter battery will last. Add each appliance with its wattage, quantity and minutes run per hour, set your battery Ah and measured efficiency, and get an honest runtime with the full working.
Backup hours = (Battery Ah x 12V x efficiency x 50% depth of discharge) / average hourly load. Average hourly load counts each appliance at wattage x quantity x (minutes run per hour / 60).
It is the duty cycle. A fridge that cycles 40 minutes in an hour is a 100W load, not a 150W load. Counting duty cycle is what separates this calculator from tools that assume every appliance runs flat out.
Start at 0.70 if you have not measured. Run the efficiency estimator once and use your own factor thereafter — battery age, electrolyte level, cabling and terminals all fold into that single number.
High Load means estimated backup under 2 hours; Low Load means over 5 hours; anything between is Moderate. The bands are a prompt to shed load or size up before the next long outage.
Battery Buddy is free, needs no sign-up, sells no batteries and shows every formula it uses. It is built by Flocci Technologies, an India-first AI-native software studio behind 30+ apps that share one Flocci identity.