List the loads, with each one's current in amps and hours a day, and the calculator gives the daily use and the smallest house bank to cover it. The starting list is Sailboat Cruising's example for a day on passage, without the fridge.
The method
Sailboat Cruising's method is to add up the daily amp hours and then double them for lead-acid batteries:
- Daily use. Each load's amps × hours a day, added up.
- Lead-acid, AGM or gel. Never regularly below 50%, so the bank is twice the daily use.
- LiFePO4. Often up to 80% or 90%. The calculator takes 80%.
- Size it for a passage. The guide works on a 24 hour passage, when the engine may not run.
Why to round up
The amp hour rating is at a 20 hour discharge rate and 25°C. Pull current faster, or let the bank get cold, and you get less: Sailboat Cruising gives the example of a 100 Ah battery delivering 80 Ah in the cold. Many alternators also stop charging at 70 or 80%, so a lead-acid bank often starts the day short. Their worked example needs at least 470 Ah, and they'd fit 600.
On the survey
Compare the bank on board with the loads the boat carries. An autopilot, fridge and inverter on a small lead-acid bank means deep cycling and short battery life. Note the battery type and date codes and how they're secured. For lithium, check there's a battery management system.
Citing it in a report
"House bank 2 × 105 Ah AGM, 210 Ah. Owner's stated daily use on passage about 160 Ah, which would take the bank below 50%. Recommend the bank is enlarged or the loads reduced." Separate what you measured from what the owner told you.