Size your marine house battery bank for your electrical needs
Basic electronics and lights
Full systems, refrigeration, etc.
High-power electronics for a day trip
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Download NowThe house battery bank is the boat's power reserve, and it is sized by a simple, unforgiving question: how many amp-hours do you actually draw, and for how many hours before you can recharge? Undersize it and you are running the engine at anchor to keep the fridge alive, or stranded with a dead start battery on a dark night.
Capacity is not what the label says. A lead-acid battery should rarely be discharged past half, while lithium tolerates eighty or ninety, so the usable capacity and therefore the bank size depend heavily on chemistry and the depth of discharge you allow. That single factor usually decides between a comfortable bank and a chronically stressed one.
Enter your average load in amps and the hours you want to run off it, choose the chemistry, and the calculator sizes the bank, applies a safety margin and warns you when the depth of discharge is pushing lead-acid too hard.
Amp-Hours Needed = Load (A) × Hours; Required Capacity = Ah ÷ Depth of Discharge; Bank = Required × 1.20 safety| Chemistry | Safe Depth of Discharge | Relative Size | Note |
|---|---|---|---|
| Flooded lead-acid | 50% | Large | Cheap, needs upkeep |
| AGM | 50% | Large | Sealed, maintenance-free |
| Gel | 50 - 60% | Large | Sensitive charging |
| Lithium (LiFePO4) | 80 - 90% | Small | Light, costly upfront |
Usable capacity by battery chemistry.
Formula verified against standard naval-architecture references · Updated October 2026
Multiply your average amp draw by the hours you want to run, divide by the depth of discharge your chemistry tolerates, and add about twenty percent margin. A 5 amp, 10 hour load on lead-acid needs roughly a 120 Ah bank; the same on lithium needs about 70 Ah. Enter your load and hours here and the calculator sizes it and flags when you are asking lead-acid to work too deep.
Because draining a battery deeply shortens its life, dramatically for lead-acid. A lead battery's cycles collapse if you regularly pull past about half its capacity, so only half the label is usable; lithium keeps most of its capacity over thousands of deeper cycles. The depth-of-discharge division in the calculation is exactly this reality, and it is the main reason lead banks are physically large.
Where weight, space or frequent deep discharge matter, usually yes. Lithium lets you install a smaller, far lighter bank that accepts a deeper discharge and lasts many more cycles, and it holds voltage better under load. It costs more upfront and needs correct charging and protection, so it pays off on cruisers who live off the bank, less on boats that top up daily.
Add their average draw over the period they run, not their peak label current. A compressor fridge cycles, so estimate its duty and average amps; electronics draw steadily. Log or estimate an average anchorage load including lighting and instruments, then feed that into the hours you want off-grid. The average, not the maximum, is what sizes the bank honestly.
Yes. A dedicated, isolated starting battery guarantees you can crank the engine even after the house bank has been worked down, which is a genuine safety feature and not a luxury. Charging Combiners or battery switches let one alternator fill both while keeping the duties apart, so a hard night of house loads never leaves you unable to start.
A bank is only as good as how fast it refills. If your alternator or solar can only return a few tens of amp-hours an hour, a very large lead bank may never fully charge between uses and will sulfur and fade. Match capacity to real charging capability on top of the load calculation, so the bank returns to full within your normal sailing or anchoring routine.
They shrink the usable side of it. A bank loses capacity with age and cycles, so a bank sized exactly to need becomes marginal as it wears. That is why a safety margin matters: build in headroom so the bank still covers your load after a few seasons of normal capacity fade, rather than starting out at the very edge of what it can do.