Battery Runtime Calculator — How Long Will Your Battery Last?

A runtime number needs a real system.

Plenty of calculators spit out a number from a 50%-rule guess. Wattonomy uses your real loads and usable capacity (LiFePO4 depth-of-discharge, inverter losses), then goes further — sizing the battery, solar, wiring and fuses so the runtime you see is one you can actually build to — built to recognized standards.

From a number to a system

The runtime is the start. The value is everything the answer implies — and we build all of it.

Real usable energy

We use true usable Wh — LiFePO4 depth-of-discharge and inverter efficiency — not an optimistic nameplate.

Per-appliance runtime

See hours of fridge, lights, Starlink or your own loads on a full charge.

Right-sized battery

Turn the runtime you want into the bank you actually need, at the right voltage.

The whole build

Wiring diagram, fuse & cable sizing and a parts list — so the number becomes a system.

What you walk away with — free

Real per-appliance runtime from usable Wh
A full validated system sized to the runtime you want
A wiring diagram with cable and fuse sizing
A shoppable, standards-referenced parts list

Building it for real? Unlock the build binder.

Free gets you a safe, correctly-sized design. The build binder gets you the documents to build it right — and prove it's right.

Printable wiring diagram, yours to keep & tape inside the build

Full parts list, every line sourced

The volt-drop & fusing calculation trail — the "why" behind every spec

Save your build & compare options side by side

Start your design — free

The build binder is a one-time unlock — no subscription, no auto-renew.

Questions

How is runtime calculated?

From your real loads and the battery’s usable energy — accounting for LiFePO4 depth-of-discharge and inverter efficiency — not a flat 50% rule.

Can I size a battery from a target runtime?

Yes — the tool turns the runtime you want into the bank, solar and wiring you need.

Does it cover lead-acid and lithium?

It sizes around modern LiFePO4 usable depth-of-discharge by default, the standard for new builds.

Do I need an account?

No — free, no email wall. Accounts only for saving builds or the build binder.

The standards we build to

Plain version: these are the recognized rulebooks your design is sized against, so the numbers hold up to a surveyor, an inspector or an insurer.

ABYC E-11The US standard for AC & DC wiring on boats — what a surveyor or insurer actually checks.
ISO 13297The international standard for small-craft electrical systems.
Victron datasheetsReal component specs — fuse classes, charge limits, MPPT ratings — not generic assumptions.

Wattonomy applies these standards in its calculations. It is not certified, sponsored or endorsed by ABYC, ISO, NFPA or Victron — it sizes your design to meet what they require, and shows the working.

Calculate & design my system

It takes about a minute. No account, no email.

How long will your battery actually last?