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Battery Amp Hour Calculator

Already have a battery and want to know how long it'll actually run your load? Enter its Ah rating, voltage, and chemistry to get a realistic runtime — not the optimistic number on the label.

Your battery & load

Runtime estimate

how long this battery will run this load —hours
Load current—
Usable capacity (after DoD)—
Usable energy—
—

How this calculator works

Formula: Runtime (hours) = usable Ah ÷ effective load current, where usable Ah = battery Ah × depth of discharge%, and effective current accounts for system losses. This is the reverse of our Amp Hour Calculator, which starts from your required runtime and tells you what size battery to buy — this one starts from the battery you already have.

Why the number on the label is optimistic

A battery's rated Ah (say, "100Ah") is measured under lab conditions at a specific discharge rate, usually a slow 20-hour rate. Draw more current than that (a bigger load) and the usable capacity drops — plus you should never fully drain lead-acid or AGM batteries anyway. This calculator applies your chemistry's safe depth of discharge and your stated system losses so the runtime figure reflects real use, not the lab number.

Depth of discharge by chemistry

Battery typeSafe DoDNotes
Flooded lead-acid~50%Regular deep discharge below 50% shortens life fast
AGM~50%Similar discharge limits to flooded lead-acid
Lithium (LiFePO4)~80% (some rated to 100%)Tolerates much deeper, more frequent discharge

Common uses

Solar battery banks (how long will stored power last through the night), RV and marine house batteries (how long between charges), portable power stations, and job-site backup batteries for alarm panels or NVRs where you already know the installed battery's rating and just need the runtime.

Note: real-world runtime is usually a little lower than this estimate at high discharge rates (fast draw reduces effective capacity further, known as the Peukert effect) — treat this as a planning figure, not a guarantee, especially for loads that draw more than about C/5 (a fifth of the battery's Ah rating in amps).

Frequently asked questions

Why is my real runtime shorter than the simple Ah divided by amps math?

Because that simple math ignores depth of discharge and system losses. This calculator applies your chemistry's safe discharge limit and your stated losses, so the runtime figure reflects what the battery will actually deliver, not the optimistic label number.

What's the Peukert effect and should I worry about it?

It's the drop in usable capacity that happens at higher discharge rates, a battery delivers less than its rated Ah when drained quickly. It matters most for loads drawing more than about a fifth of the battery's Ah rating in amps. For lighter loads, the effect is small enough to ignore.

Can I use this for a battery bank instead of a single battery?

Yes. Add up the Ah ratings of batteries wired in parallel and enter that total, the voltage and runtime math work the same way. For batteries wired in series, enter the bank's total voltage instead and keep the single battery's Ah rating.

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