Useful for the two questions owners actually ask: can the bank get through the night, and can it get through the passage?
Usable energy, not nominal energy
A bank's nominal energy is its capacity times its voltage. The energy you can actually plan to use is that figure times whatever share of the capacity you are prepared to consume.
That share is a decision, not a specification. Running a bank to empty on every cycle is legal, possible, and harder on the cells than stopping earlier. More importantly, a bank with nothing left in it has no reserve for navigation, communications or bilge pumping when something goes wrong.
Constant loads are rare
The calculation assumes a steady draw. Real loads mostly are not:
- Refrigeration cycles. Its average consumption over 24 hours is much lower than its running draw, and it depends on ambient temperature, how full the box is and how often it is opened.
- Autopilot consumption varies with sea state, trim and steering system. A measurement taken in flat water badly understates a rough passage.
- Inverter loads come and go, and the inverter's own idle draw continues between them.
For anything that cycles, use measured average consumption over a realistic period.
Cold reduces available capacity
Lithium cells deliver less of their rated capacity at low temperature, and charging them below the manufacturer's stated minimum can cause permanent damage — many LiFePO4 products specify a minimum charge temperature around +5°C. For southern Australian winter cruising this is a real design constraint, not a footnote.
Discharging is generally more tolerant than charging, but the available energy still falls.
The BMS may stop you first
The result assumes the bank delivers the energy you asked for. A BMS may disconnect earlier — on low cell voltage, on cell imbalance, on temperature, or on current. When it does, everything downstream stops at once, which is why the boat should be designed so a house-bank shutdown cannot remove navigation, steering support, communications or engine starting.