Usable capacity
We size every bank on usable capacity, not nameplate. LiFePO4 is taken at 80% depth of discharge, which is a routine working figure rather than a limit. AGM and other lead-acid chemistries are taken at 50%, which is what the chemistry tolerates without a shortened life.
This single difference is why a 400 Ah lithium bank replaces a 600 Ah AGM bank, and why every comparison on this site is drawn in usable amp-hours.
Solar charging calculation assumptions
Panel output is calculated as rated watts times 4.25 peak sun-hours, a national annual working average, then derated by 25% for heat, dirt, shading, angle and controller losses. That gives an average day, not a best day.
When sizing a bank we then credit only half of that figure, and we never size a bank below half the gross daily draw regardless of how much solar is fitted. A bank sized on a perfect solar day fails on the third grey one, and we would rather be wrong in the direction that leaves your fridge running.
DC-DC and alternator sizing
Two ceilings, and we take the smaller. A quarter of the bank’s capacity is the current the cells should be asked to accept from an engine. Half the alternator’s rating is what the alternator can sustain continuously without overheating. The result is rounded down to a charger size you can actually buy.
Above roughly 120 A of alternator output, an externally regulated alternator with a temperature sensor becomes the better answer and the calculator’s recommendation should be read as a floor.
Weights and prices
Pack weights use 102 Wh per kilogram for LiFePO4 and 43 Wh per kilogram for AGM, at the selected system voltage. These are working averages across the brands we list, not any one product’s figure.
Build prices on the hull pages are installed prices on the Australian east coast, including labour and certification, current at the review date on each page. They will be higher in remote areas and lower if you supply your own components.
Claims and test evidence
Product capacities, cycle counts, weights and warranty terms are treated as manufacturer claims unless the page links to a complete independent evidence file. Boat Battery has not yet published an independent product bench result, so comparison tables do not present any product as tested by us.
A future evidence file must identify the exact product, equipment, conditions, method, raw readings and uncertainty. The publication-pending logbook explains that threshold. If we get something wrong, we publish a dated correction rather than quietly changing the record.
Assumptions by calculator
House bank calculator
- 80% usable depth of discharge on LiFePO4, 50% on AGM
- Solar: panel watts × 4.25 sun-hours, derated 25%, then credited at half for sizing
- The bank is never sized below half the gross daily draw
- Bank rounded up to the next 50 Ah; pack weights at the selected system voltage
Trolling motor runtime
- Draw scales with the cube of throttle setting, from the motor's rated maximum draw
- 80% usable capacity on LiFePO4
- Continuous trolling; intermittent use extends runtime proportionally
DC-DC and alternator sizing
- Charge current ceiling: the lesser of 25% of bank capacity and 50% of alternator rating
- Rounded down to a commercially available charger size
- Refill time assumes the charger runs at its rated output throughout
Solar top-up
- 4.25 peak sun-hours as an Australian annual average
- 25% derating for heat, dirt, shading, angle and controller losses
- Average day, not best day: size the bank for the grey ones
AGM to lithium swap
- AGM usable capacity: 50% of nameplate
- LiFePO4 usable capacity: 80% of nameplate
- Pack weights: roughly 8 kg per 100 Ah at 12 V for LiFePO4, 28 kg for AGM
Not electrical advice
Calculator output is a planning starting point. Standards, licensing and certification requirements depend on vessel category, jurisdiction, voltage and the work performed. Confirm the current rules with the relevant maritime and electrical authorities, insurer and a suitably qualified professional. Nothing here replaces that assessment.