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 (after inverter losses ~10–15% if you're running AC).

Use it in 10 minutes. Pick daily Wh and days of backup. Divide by DoD. Divide by system voltage. That's the Ah to buy — then round up to real battery sizes.

Nameplate vs usable (one 100 Ah battery)

Chemistry / voltage	Nameplate Wh	Usable Wh	Hours at 100 W DC
LiFePO4 12 V (80% DoD)	1,200	960	~9.6 h
AGM 12 V (50% DoD)	1,200	600	~6 h
LiFePO4 24 V (80%)	2,400	1,920	~19 h
LiFePO4 48 V (80%)	4,800	3,840	~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

3 kWh/day × 2 days autonomy ÷ 0.80 DoD = 7,500 Wh of usable need → about 625 Ah at 12 V or 156 Ah at 48 V LiFePO4. Parallel strings of cheap 12 V drop-ins look cheaper until one cell takes the pack down. Voltage up, current down, wire smaller.

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