How to Size a Boat Lithium House Battery Bank

Equipment and information required

Start from your measured daily draw
Amps times running hours for every load, totalled. On a 38 ft cruising yacht with a fridge, freezer, instruments and lighting this typically lands between 120 and 180 Ah a day at 12 V.
Work out what solar genuinely returns
Panel watts × 4.25 sun-hours × 0.75 for heat, dirt and shading, divided by the nominal bank voltage. On a 12 V bank, 400 W of panels returns roughly 400 × 4.25 × 0.75 ÷ 12.8, about 100 Ah on an average day.
Credit that solar at half
Subtract only half the solar figure from your daily draw. A bank sized on a perfect solar day fails on the third grey one, and grey days are when you need the reserve. Never let the result fall below half your gross daily draw.
Multiply by your nights at anchor
Three nights is the usual planning figure for coastal cruising. This is the number of days the bank has to carry the deficit before a real charge.
Divide by 0.8 for usable depth
You cycle LiFePO4 to about 80% of label capacity. A bank that needs to deliver 240 Ah has to be a 300 Ah bank. This is the step most often skipped, and it is why banks come out a fifth too small.
Round up to a bank that exists
Round up to the next 50 Ah, and never size below 100 Ah. Then check the result against the cases actually sold in Australia rather than a theoretical number.

Sizing on label capacity and assuming you can use all of it.

Divide by 0.8. A 200 Ah case gives you 160 Ah.

Giving solar full credit for its average day.

Credit it at half. The bank exists for the days solar underperforms.

More on sizing