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
The calculator UI, the example table, the chart and the displayed formula intentionally call the same calculation function, so what you see is what is computed. The planning depth-of-discharge defaults come from the sources below and can always be overridden.
- LiFePO4 planning DoD 80–100% (90% used here) — LiFePO4 manufacturer datasheets, e.g. Battle Born Batteries and Victron Energy lithium documentation
- AGM and flooded lead-acid planning DoD ~50% — Trojan Battery user's guide; Victron Energy "Energy Unlimited" handbook
- Gel planning DoD ~60% — Victron Energy gel and AGM battery datasheets
- Inverter efficiency 85–95% (90% default) — Typical pure-sine inverter datasheets (e.g. Victron Phoenix, Renogy)
- Lead-acid capacity falls at high discharge rates (not yet modelled) — Peukert's law, W. Peukert (1897)
Assumptions last reviewed on 26 August 2026. If a source disagrees with your battery or equipment datasheet, trust the datasheet.
Frequently asked questions
Why can real battery runtime be shorter than the estimate?
Temperature, battery age, cycling loads, inverter idle draw and battery chemistry can all reduce real-world runtime.
What does depth of discharge mean?
Depth of discharge is the share of a battery's rated capacity that you plan to use before recharging.
Do I need inverter efficiency for a DC load?
Not for a direct DC load. Set inverter efficiency to 100% unless another converter is involved.