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80%

Usable capacity is the only number that matters.

A 100 Ah battery is not 100 Ah you can spend. LiFePO4 gives you about 80%. AGM gives you about 50%. The brochure Ah is a trap.

Bank Wh = Ah × volts. Usable Wh = bank Wh × DoD. Runtime hours = usable Wh ÷ load watts.

Print it. Pick daily watt-hours and days of backup. Divide by depth of discharge. Divide by system voltage. That's the amp-hours to buy — then round up to real battery sizes.

Nameplate vs usable (one 100 Ah battery)

Chemistry / voltageNameplate WhUsable WhHours at 100 W DC
LiFePO4 12 V (80% DoD)1,200960~9.6 h
AGM 12 V (50% DoD)1,200600~6 h
LiFePO4 24 V (80%)2,4001,920~19 h
LiFePO4 48 V (80%)4,8003,840~38 h

DoD windows match the live calculator: LiFePO4 80% usable, AGM/lead-acid 50% usable. Not a datasheet substitute — check the exact battery.

Cabin shorthand

The calculator is Ah = (Wh/day × days × 1.25) ÷ voltage ÷ DoD. For 3 kWh/day, 2 days, and 0.80 DoD, the watt-hours to store are (3,000 × 2 × 1.25) ÷ 0.80 = 9,375 Wh. That is 781.25 Ah at 12 V (9,375 ÷ 12) and 195.3125 Ah at 48 V (9,375 ÷ 48). The directory table prints those as 781 Ah and 195 Ah. Dividing 3,000 × 2 by 0.80 and skipping the 1.25 produces 7,500 Wh, 625 Ah, and 156 Ah, and that shorthand is short of this formula.

Amp-hours and watt-hours

Section added October 3, 2026.

Watt-hours are amp-hours times volts. A 100 Ah battery at 12 V is 1,200 Wh nameplate, which is the first row of the table above. Usable watt-hours are that nameplate times depth of discharge: 1,200 × 0.80 = 960 Wh on LiFePO4, and 1,200 × 0.50 = 600 Wh on AGM. The other direction is the sizing formula. Daily watt-hours go in, amp-hours come out.

Weekend cabin, worked: 1,000 Wh/day, 2 days, 12 V, 0.80. Ah = (1,000 × 2 × 1.25) ÷ 12 ÷ 0.80 = 260.4167 Ah, about 260 Ah. Nameplate watt-hours of that result are 260.4167 × 12 = 3,125 Wh, which is also (1,000 × 2 × 1.25) ÷ 0.80. Usable watt-hours at 0.80 are 3,125 × 0.80 = 2,500 Wh, the same 1,000 × 2 × 1.25 the formula started from. At 48 V the amp-hours shrink and the watt-hours do not: 65.104 Ah × 48 V is the same 3,125 Wh.

A common mistake is to stop at “Wh = Ah × V” and buy 2,500 ÷ 12 = 208 Ah, which drops both the 1.25 and the depth of discharge. Another is the 625 Ah figure this section replaces. Shelf loss is not inside either number: the Battle Born spec table’s 2–3% per month, applied once to 260 Ah, is 5.2–7.8 Ah, on how long a battery holds a charge. The AGM case of this same cabin, 416.7 Ah and four 105 Ah batteries, is the AGM guide.

Compiled with AI assistance from the sources below. Spot an error? Email jpanasuk@gmail.com.

Sources & Standards

References for depth of discharge and cycle life:

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Frequently asked questions

Is this calculator free?

Yes — free to use, no signup, no email required.

How accurate is the estimate?

Estimates follow industry-standard sizing rules and common safety margins. Always confirm with a licensed professional for final sizing.

Can I use this on mobile?

Yes — the page is fully responsive and works on phones, tablets, and desktop.

Where does the math come from?

Formulas are shown inline on the result panel so you can verify each step yourself.