How battery runtime is calculated
Start by converting the battery's rated capacity to watt-hours. Then apply the chosen usable depth of discharge and divide the usable energy by the power the battery must actually supply.
For example, 100 Ah at 12 V is 1,200 Wh nominal. At 90% usable depth of discharge that becomes 1,080 Wh. A 120 W AC load through a 90%-efficient inverter draws about 133.3 W from the battery, so the simplified estimate is 1,080 ÷ 133.3 = 8.10 hours.
Why real runtime can differ
The v1 model deliberately keeps the main equation easy to inspect. Temperature, battery age, inverter idle consumption, cycling loads and lead-acid Peukert effects are not yet modelled automatically. They should be treated as planning adjustments rather than hidden corrections.
Typical planning depth of discharge
| Chemistry | Planning DoD | 100 Ah / 12 V usable |
|---|---|---|
| LiFePO4 (LFP) | 90% | 1,080 Wh |
| Li-ion (NMC) | 80% | 960 Wh |
| Gel | 60% | 720 Wh |
| AGM | 50% | 600 Wh |
| Flooded lead-acid | 50% | 600 Wh |
Methodology
Watful's production version should attach named source references and review dates to chemistry assumptions before launch. The calculator code and displayed formula are intentionally based on the same calculation function.