How Much Usable Capacity Does a 100Ah LiFePO4 Battery Pack Really Have

FIELD ESTIMATE / TYPICAL 12.8V PACK / 25°C

For a typical 12.8V pack, the quick calculation is 12.8V x 100Ah = 1,280Wh, or 1.28kWh of nominal energy. In normal operation, expect roughly 1.0 to 1.15kWh of usable DC energy, depending on the battery management system and the selected depth of discharge. This article shows how to calculate the number for a real installation.

Start With Nominal Energy

Use the battery’s nominal voltage rather than its maximum charging voltage:

Nominal energy = nominal voltage x rated capacity

For a 12.8V LiFePO4 pack:

12.8V x 100Ah = 1,280Wh

The same 100Ah capacity produces different nominal energy at different voltage levels. A 25.6V pack stores about 2.56kWh, while a 51.2V pack stores about 5.12kWh. The amp-hour figure stays at 100Ah, but the voltage changes the energy available to the load.

The pack’s BMS also keeps a small reserve at the top and bottom of the cell voltage window. That reserve protects the cells from overcharge and over-discharge. A pack rated for 100Ah at 0.2C and 25°C may not provide the same result at 1C, -10°C, or after years of cycling.

What You Can Actually Use

Depth of discharge, or DoD, gives the clearest first adjustment. If the system allows 90% DoD, the theoretical DC energy becomes:

1,280Wh x 90% = 1,152Wh

At 80% DoD, it becomes 1,024Wh. If an inverter fuori rete operates at 92% efficiency, an 80% DoD calculation gives approximately 942Wh of AC energy before cable losses and low-voltage cut-off effects. This is why a buyer should not size a load from the 1.28kWh nameplate alone.

The following figures use a 12.8V 100Ah pack at 25°C. They are planning values, not a substitute for the manufacturer’s discharge curve.

A Practical Capacity Comparison

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