Battery runtime calculator

Runtime is usable watt-hours divided by the load in watts. A 100 Ah 12 V lithium battery at 80% depth of discharge and 90% efficiency has 864 usable Wh, which runs a 120 W load for 7.2 hours.

Two losses stack between the battery and the load. The first is depth of discharge, which is a decision about battery life rather than a physical limit. The second is conversion: an inverter turning 12 V DC into 230 V AC is typically 85 to 92% efficient, and it also draws idle current whether or not anything is switched on. For small loads over long periods that idle draw can dominate — a 20 W inverter overhead running a 10 W load wastes twice what it delivers, which is why DC-native appliances are so common in vans and boats.