Ah = daily Wh × days ÷ (usable depth of discharge × efficiency × voltage). Energy is Wh = V × Ah, so a 12.8 V 100 Ah LiFePO4 battery holds 1,280 Wh. Lead-acid and AGM batteries last much longer if you only use about half their rating, which is why they need roughly twice the amp-hours of lithium for the same job.
Safety: These are general estimates, not a design. Follow your equipment manufacturer's instructions and local electrical code, and consult a qualified electrician for anything connected to shore power, a house, or high-current batteries.
Total your daily watt-hours, divide by battery voltage, then by usable depth of discharge (0.9 for LiFePO4, 0.5 for AGM) and by about 0.9 for losses. 1,200 Wh/day on 12 V lithium is about 116–130 Ah.
Parallel keeps the voltage the same and adds amp-hours; series adds voltage. Use identical batteries and follow the maker's limits on series/parallel connections.
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