PowerCalcLab

Battery calculator

Battery Usable Capacity Calculator

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.

Advanced assumptions

Results update as you type. The calculator runs entirely in your browser — nothing you enter is sent anywhere.

Usable delivered energy1,036.8 Wh
STORED ENERGY90%
90% usable10% reserve
The highlighted window is usable after the entered SOC floor, health and conversion loss.

Calculation breakdown

Usable battery capacity90 Ah
Usable depth of discharge90%
Nominal energy1,280 Wh
Energy before conversion loss1,152 Wh
Delivered Wh = V × Ah × health × (start SOC − minimum SOC) × efficiency
= 12.8 × 100 × 1 × 0.9 × 0.9
= 1,036.8 Wh

Method

How the number is reached

Usable depth of discharge is the SOC span you permit, not necessarily the chemistry's maximum. Health reduces the capacity of the entire battery before that window is applied.

Conversion efficiency is shown separately so battery-side usable energy is not confused with energy delivered through an inverter.

Usable DoD = starting SOC − minimum SOC
Usable Ah = rated Ah × health × usable DoD
Delivered Wh = voltage × usable Ah × efficiency

Symbols

SOC
State of charge
DoD
Depth of discharge

Worked examples

The same maths, applied

Example

12.8 V 100 Ah LiFePO4 with a 10% floor

A healthy battery starts full, stops at 10% SOC and feeds a 90% efficient inverter.

Usable Ah = 100 × 90% = 90 Ah
Battery energy = 12.8 × 90 = 1,152 Wh
Delivered energy = 1,152 × 90% = 1,036.8 Wh

Result: 1,036.8 Wh delivered.

Limits of the model

What it assumes, and where it stops

Assumptions

Nominal voltage represents the pack across the energy calculation.

Health is remaining capacity relative to the label rating.

Not covered

Actual delivered energy varies with current, temperature, voltage cutoff and cell balance.

Chemistry defaults are planning values; the BMS and warranty define the real SOC floor.

Sources

Where these figures come from

  • How to Size a Solar System for Your RVVictron EnergyManufacturer sizing walkthrough connecting daily energy, days of autonomy, system voltage and chemistry-dependent usable capacity, with an explicit reminder to include inverter losses.
  • VE.Bus BMS NG — Battery current and discharge-floor settingsVictron EnergyManufacturer guidance for maximum and recommended C-rates, BMS current limits and minimum state-of-charge floors.
  • SI UnitsNIST Office of Weights and MeasuresDefinitions of the volt, ampere, watt and joule used for the unit conventions on this site.

Next steps

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Last meaningful update: 2026-08-19. This date changes only when the model, the sources or the guidance change.