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.
How to calculate battery runtime
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.
Questions
At 80% usable and 90% efficiency, about 7.2 hours running a 120 W load. Lead-acid at 50% usable would give about 4.5.
Yes, by 8–15% in conversion, plus a constant idle draw of 5–25 W whether or not the load is on.
Age, cold, and discharge rate. Capacity falls with cycles and drops sharply below freezing, especially in lead-acid.
No. Regular deep discharge shortens life dramatically in lead-acid and moderately in lithium. The depth of discharge setting reflects that.
Watts, because it is voltage-independent. Amps only make sense alongside the voltage they are drawn at.