Battery Runtime Calculator

The simplest possible version of this math — battery capacity times voltage, divided by your load — quietly assumes two things that usually aren't true: that you can discharge the battery all the way to 0%, and, for an AC load, that the inverter converts every watt from the battery with no loss. Neither holds in practice: depth of discharge limits how much of the battery you can safely use, and a real inverter consumes some of the battery's power just doing the DC-to-AC conversion. This calculator asks for both DoD and inverter efficiency directly so its answer reflects that reality instead of the optimistic best case. It still doesn't model temperature, battery age, or the reduced effective capacity batteries show at high discharge currents (Peukert's effect) — all of which can push real-world runtime lower still, especially under heavy load.

Depth of discharge varies by chemistry, but FigureDesk doesn't have a sourced, citable default for each one yet — rather than guess, this calculator asks for your battery's actual usable DoD directly, which you can find on its datasheet. The runtime math itself is identical for every chemistry once you know DoD and voltage.

Check your battery's datasheet or manufacturer specification. Different chemistries and even different products within the same chemistry vary — there is no single correct number to assume.

Inverters aren't perfectly efficient — some power is lost converting DC to AC, usually as heat. A 90%-efficient inverter needs about 11% more power from the battery than the AC load actually draws, and a less efficient inverter needs proportionally more still. This is exactly the gap a runtime estimate that ignores inverter efficiency would miss, and why it matters for AC loads specifically — a DC load skips this loss entirely.