Backup time = (Battery Ah x 12V x efficiency x depth of discharge) / load in watts. A 150Ah battery at 70% efficiency and 50% depth of discharge holds 630Wh usable, which runs two ceiling fans and three LED bulbs for about 3 hours 46 minutes. Pick your battery and load below for the full working.
Each page below runs the same engine the live calculator runs, on a specific battery size and a specific appliance mix — with the load table, the usable-energy maths, the inverter size and the start-up surge check all shown.
It depends entirely on capacity and load. A 150Ah 12V battery holds about 630Wh usable, so it runs a 140W two-fan load for roughly 4 hours 30 minutes and a 400W load for about 1 hour 35 minutes. Every combination below is worked out in full.
Start from the outage you actually get and the load you actually run, not from a dealer's round number. Find your load profile below, read across the battery sizes, and pick the first one that clears your typical outage with margin for battery ageing.
Lead-acid tubular, the dominant Indian home inverter chemistry — 12V nominal, 50% depth of discharge, 70% assumed system efficiency. A lithium chemistry toggle is on the roadmap.
Because that is what real Indian tubular-plus-inverter stacks deliver once battery age, cable losses and terminal resistance are counted. Optimistic 85-90% assumptions are the main reason calculators overstate backup.
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.