FigureDesk

Battery Capacity Calculator

Calculate the battery capacity (Ah) needed to run a given load for a target number of hours, from voltage, depth of discharge, and inverter efficiency you provide.

Your numbers

h
V
W
%

From your inverter's datasheet — there is no default.

%

From your battery's datasheet — there is no default.

Result

Required battery capacity

92.59 Ah

Required nominal energy
1111.11 Wh
Battery-side power draw
111.11 W
Discharge current
9.26 A
C-rate
0.1C
This is an estimate, not a guarantee of a specific battery's real-world performance. Buying some margin above this figure — commonly 10–20% — accounts for temperature, age, and discharge-characteristic effects this calculation does not model.

How it works

This is the Battery Runtime Calculator worked backwards: instead of asking how long a battery you already have will last, it asks how big a battery you need to buy for a load you already know and a runtime you want. The energy your load actually needs (accounting for inverter loss on AC loads) divided by your usable depth of discharge, divided by voltage, gives the required capacity in amp-hours.

battery-side power (AC load) = load W / (inverter efficiency / 100)
battery-side power (DC load) = load W
required usable Wh = battery-side power × target runtime h
required nominal Wh = usable Wh / (DoD / 100)
required Ah = nominal Wh / voltage

Worked example

A 100 W AC load, run through an inverter rated at 90% efficiency, needed for 5.4 hours, on a 12 V battery with 50% usable depth of discharge:

  • Battery-side power draw: 111.1 W (higher than the 100 W load, due to inverter loss)
  • Required nominal energy: 1200 Wh
  • Required battery capacity: 100 Ah

Assumptions and limitations

  • A simple "load × hours ÷ voltage" calculation undersizes the battery, because it silently assumes you can discharge to 0% and, for an AC load, ignores what the inverter itself consumes. This calculator requires depth of discharge and (for AC loads) inverter efficiency as explicit inputs specifically so the result isn't an optimistic best case.
  • Depth of discharge and inverter efficiency are values you enter, not looked-up defaults — FigureDesk does not guess these from a battery chemistry. Check your battery and inverter datasheets.
  • This does not use Peukert's correction — it assumes a constant discharge rate at the power you enter, not a battery's reduced effective capacity at high discharge currents.
  • This is an estimate. A common practice is buying some margin above the computed figure — 10–20% is typical — to account for temperature, age, and discharge-characteristic effects this calculation doesn't model.
  • Distinct from the Solar Battery Size Calculator, which sizes a battery bank from daily energy consumption and days of autonomy for a solar off-grid system. This calculator instead takes a direct load wattage and a target runtime — the framing that fits sizing a battery or portable power station against one specific device or job, rather than a whole-house daily energy budget.

FAQ

How is this different from the Battery Runtime Calculator?

It's the exact inverse. Battery Runtime takes a battery you already have (capacity, voltage) and a load, and tells you how long it will run. This calculator takes the load and the runtime you want, and tells you what capacity battery you need to buy.

How is this different from the Solar Battery Size Calculator?

Solar Battery Size works from daily energy consumption (Wh/day) and a number of autonomy days — the framing solar system designers use. This calculator works from a specific load's wattage and a target runtime in hours — the framing that fits sizing a battery for one device or job, like a portable power station for a specific appliance.

Should I buy exactly the capacity this calculator gives me?

Most people buy some margin above it — commonly 10–20% — since real batteries lose some capacity to age and temperature over time, and running right at the calculated minimum leaves no headroom. The exact margin is a personal risk tolerance choice, not something this calculator computes for you.

What depth of discharge and inverter efficiency should I use?

Check your battery and inverter's own datasheets — both vary meaningfully by product and chemistry, so there's no single correct number to assume. A lower DoD or lower inverter efficiency both increase the capacity you need for the same target runtime.

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FigureDesk results are advisory and do not replace a licensed electrician, a qualified engineer, or the applicable local electrical code. Verify any electrical work against your local code and with a qualified professional before relying on it.