These guides answer the questions behind the calculator: how backup time is derived, why a 900VA inverter gives you 720W, why half a lead-acid battery is off limits, how to measure your own efficiency in one evening, why inverters trip when a fridge starts, and when lithium is actually worth it.
Each guide opens with a direct answer, then shows the working. No brand recommendations, no sign-up, no gated content.
Backup hours = (Battery Ah x Battery voltage x Inverter efficiency x Depth of discharge) / Load in watts. For a 150Ah 12V battery at 70% efficiency and 50% depth of discharge on a 156W load, that is 630Wh / 156W = 4.0 hours.
Fully charge the battery, run one appliance of known wattage until cut-off, then divide delivered energy by nominal energy: (Watts x Hours) / (Ah x 12V). A 150Ah battery running a 60W fan for 10 hours measures 600/1800 = 0.33.
A refrigerator compressor spikes to roughly three times its running wattage at start-up. Peak demand is total continuous load plus the single largest surge, and an inverter sized only for average load cannot hold that.
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.