Inverter calculator
Estimate AC runtime from a battery without treating inverter efficiency or its own idle consumption as free. The result shows the complete battery-side power budget.
Calculation breakdown
| Nominal battery energy | 1,280 Wh |
|---|---|
| Usable battery-side energy80% SOC window at 90% health | 922 Wh |
| Battery power requiredAC load after conversion loss, plus inverter idle draw | 456.4 W |
| Estimated battery current | 35.7 A |
Runtime = (V × Ah × SOC window × health) ÷ (AC load ÷ efficiency + idle W) = (12.8 × 100 × 0.8 × 0.9) ÷ (400 ÷ 0.9 + 12) = 2.02 h = 2 h 1 min
Before you rely on this
Confirm inverter low-voltage cutoff, battery/BMS current capability, conductor size and protection. Runtime capacity alone does not prove the battery can supply the load.
Method
The battery's nominal watt-hours are reduced to the entered SOC window and remaining health. The AC load is divided by inverter efficiency, then zero-load consumption is added on the DC side.
Maximum efficiency is often measured near a favorable load point. Manufacturer specifications also publish non-zero idle power, and both values can change with model, voltage, load and temperature.
Usable Wh = V × Ah × (start SOC − minimum SOC) × health Battery W = AC load W ÷ inverter efficiency + inverter idle W Runtime h = usable Wh ÷ battery W
Symbols
Worked examples
Example
The usable SOC window is 100% to 20%, battery health is 90%, inverter efficiency is 90%, and idle consumption is 12 W.
Usable energy = 12.8 × 100 × 80% × 90% = 921.6 Wh Battery power = 400 ÷ 90% + 12 = 456.4 W Runtime = 921.6 ÷ 456.4 = 2.02 h
Result: About 2 h 1 min.
Detail
At large loads, conversion loss usually dominates. At small continuous loads, the inverter's own consumption can become a meaningful share of the battery drain.
| Efficiency | Idle draw | Battery power |
|---|---|---|
| 95% | 8 W | 534 W |
| 90% | 12 W | 568 W |
| 85% | 20 W | 608 W |
Battery power = 500 W ÷ efficiency + idle draw. These are examples, not inverter ratings.
Limits of the model
Assumptions
The AC load is steady and the entered efficiency represents that operating point.
Not covered
Does not model hour-by-hour load cycling, voltage sag, temperature or a chemistry-specific discharge curve.
Sources
Next steps
Convert an inverter's AC load into the DC watts and amps the battery must actually supply. Use voltage under load—not only the nominal label—to avoid understating current.
Size from the loads that can run together, then check the largest startup event. A single watt total is not enough when motors, compressors or poor power factor are involved.
Enter your battery and your load to estimate runtime. The calculator applies depth of discharge, battery health, conversion losses and — for lead-acid only — the Peukert effect, and shows each step of the energy budget.
The label capacity is not the energy available to your load. Set the starting and minimum state of charge, battery health and conversion efficiency to expose the usable window.
Last meaningful update: 2026-08-19. This date changes only when the model, the sources or the guidance change.