Off-Grid Solar Calculators

Battery storage

Battery Runtime Calculator

Estimate how many hours a 12V, 24V or 48V battery lasts under a constant DC load. The math is Hours = (Ah × V × DoD) ÷ W, which is usable watt-hours divided by watts. Choose LiFePO4 (~95% DoD) or lead-acid (often 50%) so the result is usable energy, not the label.

Inputs

Ah

Rated amp-hour capacity of one battery or the whole parallel bank.

System voltage of the battery or series string.

Depth of discharge you are willing to use. Lower DoD protects lead-acid cycle life.

W

Average power draw of lights, fridge, electronics or other DC loads.

Results

Estimated runtime
11 h 24 min

Time until the chosen depth of discharge is reached at a constant load.

Usable energy
1,140Wh

Watt-hours you can actually draw before hitting the DoD limit.

Recommended gear

Engineering methodology

Runtime hours = (capacity in Ah × nominal voltage in V × depth of discharge) ÷ load in watts. That is the same as usable watt-hours ÷ watts. Usable energy is Ah × V × DoD, from the identity Wh = Ah × V. The tool treats the load as constant DC power. It does not apply Peukert’s equation, inverter efficiency, or temperature derating.

12V vs 24V vs 48V at the same watt-hours

Usable energy is Ah × V × DoD. Doubling voltage at the same Ah doubles watt-hours, so runtime at the same watt load doubles. For a fixed energy (same Wh), 48 V simply uses fewer amp-hours and lower current (I = P ÷ V), which is why cable and fuse sizes shrink on higher-voltage banks.

LiFePO4 depth of discharge vs lead-acid

A 100 Ah 12 V LiFePO4 pack at 95% DoD is about 1,140 Wh usable. The same 100 Ah label on flooded lead-acid at 50% DoD is 600 Wh. That is why the chemistry selector changes hours so much without changing the Ah box.

Frequently asked questions

What depth of discharge should I use for LiFePO4 vs lead-acid?

LiFePO4 house batteries are commonly used to about 95% of rated capacity. Flooded or AGM lead-acid is often limited to about 50% DoD for cycle life, or up to about 80% in deep-cycle / emergency use. Lower DoD means fewer usable watt-hours from the same nameplate Ah.

Does battery runtime include inverter losses?

No. Hours = usable Wh ÷ DC watts. If the load is on the AC side of an inverter, divide the AC watts by inverter efficiency first (for example 100 W AC through 90% efficiency is about 111 W from the battery), then enter that DC wattage.

Why is this different from Peukert’s law for lead-acid?

Peukert’s equation reduces available amp-hours as discharge current rises on lead-acid. This calculator uses a constant DoD factor only. High-current lead-acid loads will run shorter than the number shown. LiFePO4 is much less Peukert-sensitive at typical house-battery C-rates.

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