12V battery runtime examples
The table uses a 100Ah LiFePO4 battery at 12.8V, 100% state of charge, 90% depth of discharge, 92% efficiency and a 10% reserve. Loads that cycle or surge will produce different results.
| Example load | Planning runtime | Battery current |
|---|---|---|
| 20W router or small electronics | About 47.7 hours | About 1.7A |
| 60W appliance | About 15.9 hours | About 5.1A |
| 100W load | About 9.5 hours | About 8.5A |
| 300W inverter load | About 3.2 hours | About 25.5A |
How to calculate 12V battery runtime
First convert amp-hours to watt-hours: nominal watt-hours equal voltage multiplied by amp-hours. A 12.8V 100Ah battery therefore stores 1,280Wh nominally. Reduce that figure for the allowed depth of discharge, current state of charge and the reserve you want to keep, then divide by the battery-side load.
For an AC appliance, battery-side watts equal appliance watts divided by inverter efficiency. A 100W appliance through a 92% efficient inverter asks the battery for about 109W, before any separate inverter idle consumption.
Why LiFePO4 and lead-acid runtime differ
LiFePO4 batteries commonly support a deeper planned discharge and hold voltage more steadily under load. AGM, gel and flooded lead-acid batteries are often planned around a shallower discharge to protect cycle life. Selecting a chemistry applies a practical starting assumption—90% for LiFePO4, 50% for AGM/Gel and 50% for flooded lead-acid—but the battery manufacturer's limit should take priority.
High current draw can also reduce the effective capacity of lead-acid batteries because of the Peukert effect. This calculator does not claim a universal Peukert correction because the exponent varies by battery; treat its lead-acid result as an estimate and consult the maker's discharge-rate table for high loads.
Theoretical versus real-world runtime
Theoretical runtime assumes full labeled capacity and constant ideal conditions. Real runtime changes with battery age, temperature, wiring voltage drop, inverter idle draw, startup surges, BMS or low-voltage cutoffs and loads that cycle. A planning reserve helps prevent an estimate from using every calculated watt-hour and leaves room for those uncertainties.