The Boat Mill

Battery Bank Runtime Calculator

Estimate how long a house bank lasts under a continuous amp load. Enter nameplate Ah, usable percent (depth of discharge), and load amps.

Deck ticket

Nameplate amp-hours at your system voltage.
~50% lead-acid/AGM; ~80–90% LiFePO4.
House loads averaged to a steady amp draw.
Preset fills usable %; pick custom to type your own.
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How this is calculated

Runtime is usable amp-hours divided by continuous amps. Usable Ah is nameplate Ah times your chosen depth-of-discharge fraction.

usable_Ah = bank_Ah × (usable% ÷ 100)
runtime_h = usable_Ah ÷ load_A

This is a DC continuous-load estimate. It ignores Peukert (high-rate discharge on lead-acid), inverter losses, temperature, and parasitic drains. For watt loads, amps ≈ watts ÷ system volts. Not an ABYC wiring or BMS design tool. Volume/KD: unknown.

FAQ

Why not use 100% of the label?

Lead-acid banks are usually planned around 50% DoD for cycle life. Many LiFePO4 packs allow 80–90% usable before the BMS cuts off. Check your battery maker.

What about Peukert?

High discharge rates shrink effective Ah on lead-acid. This page does not apply a Peukert exponent — if you pull heavy amps, expect less runtime than shown.

Can I enter fridge + lights as one load?

Yes if you average them to a continuous amp figure (or sum duty-cycled averages). For daily Ah planning, multiply each device’s amps × hours/day instead — a different worksheet.

12V vs 24V?

Ah is at the bank’s nominal voltage. Keep load amps on the same bus. Do not mix a 12V Ah label with 24V load amps without converting.

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