Sizing
How Long Will a 200Ah Lithium Battery Last on a Boat?
About 160 Ah of usable capacity, which on a typical 38 ft cruising yacht drawing 149 Ah a day is a little over one night at anchor.
How Long Will a 200Ah Lithium Battery Last on a Boat?: explanation
The honest answer depends entirely on your loads. A fridge at 45% duty on a Queensland summer day is 54 Ah on its own; a trailer boat with a sounder and a livewell might use 14 Ah in a full day.
That is why every capacity page on this site works run time from a real load profile rather than quoting a single number.
More on sizing
How many amp-hours do I need on my boat?Add up every load in amps multiplied by the hours it actually runs, divide by 0.8 for lithium, and multiply by the nights you spend away from shore power.Why can I only use 80% of a lithium battery?We use 80% of nameplate capacity as a planning allowance, leaving a reserve. It is not a universal LiFePO4 limit; permitted discharge depends on the exact battery and its instructions.