House bank calculator

Boat Battery Size Calculator

Set your loads, your nights away from shore power, and what you already have on the arch. The maths runs live and every assumption is stated: 80% usable on LiFePO4, 50% on AGM, solar derated a quarter for Australian reality then credited at half for sizing, and a floor of half your gross daily draw under the bank no matter how good your panels are.

System voltage

LoadDrawHrs/dayAh/day
5.0 A54.0
4.0 A36.0
2.0 A12.0
2.2 A11.0
0.9 A10.8
1.2 A9.6
7.0 A3.5
3.5 A10.5
4.5 A—
85 A—
25 A—

Nights away from shore power

Solar fitted

Alternator output

Your numbers

147Ah

Per day, your loads

400Ah

Lithium recommended

State of charge, night by night

N1 · 76%

N2 · 51%

N3 · 27%

Solar return, average day100 Ah/day
Credited for sizing (half)50 Ah/day
Net draw the bank must cover98 Ah/day
Practical build4 × 100 Ah in parallel
Same job in AGM600 Ah
Weight off the boat128 kg
DC-DC charger to fit40 A
Refill from flat, solar only3.2 days

Read this before you buy

Charge through a 40 A DC-DC charger rather than straight off the alternator. It sets the current your engine can sustain all day and keeps the BMS off your cranking circuit.

This is a house bank figure. Keep a separate cranking battery unless your engine builder has approved lithium starting in writing — that is a warranty question before it is an electrical one.

LiFePO4 should not be charge-cycled below 0 °C without a heated or low-temperature-protected BMS. Rarely relevant in Queensland, entirely relevant in a Tasmanian winter.

Turn this into a refit checklistHow the maths works

How to do it by hand

Boat battery capacity calculations and assumptions

How to Plan a Boat Lithium Battery ConversionWork out your daily amp-hour draw first, size the bank from that, then budget the charging and protection around it — on most boats the hardware around the battery costs between a third and a half of what the battery costs.12V vs 24V Boat Electrical Systems: How to ChooseStay at 12 V unless your inverter or windlass currents are forcing cable sizes you cannot run. Twenty-four volts halves every current in the boat, but it means new chargers, a converter for 12 V accessories, and no going back cheaply.How to Calculate Boat Electrical Loads in Amp-HoursMultiply each load's current in amps by the hours it genuinely runs in a day, then add them up. A 5 A fridge cycling at 45% duty is 5 × 10.8 = 54 Ah, not 5 × 24.How to Size a Boat Lithium House Battery BankTake your daily amp-hour draw, subtract half of what your solar returns on an average day, multiply by the nights you spend away from shore power, then divide by 0.8 because you cycle lithium to 80%. Round up to a bank you can buy.

The other calculators

Boat battery charging and runtime calculators

Trolling motor runtimeThrust rating, the speed you actually troll at and your bank size, returning hours on the water and the bank you need for a full day.DC-DC and alternator sizingAlternator rating and bank capacity, returning the charger to fit and the time to refill from your engine.Solar top-upPanel watts against your daily draw, returning the average daily return and whether the array covers your consumption.AGM to lithium swapYour existing AGM capacity, returning the lithium bank that delivers the same usable energy and the weight you take off the boat.Start from your hull insteadNine boat types with the load profile already filled in, one click into this calculator.

Turn the estimate intoan installation plan.

Take the load estimate, charging sources and cable runs to a qualified marine electrician. Our installation guide helps you ask about isolation, over-current protection, ventilation and compliance before work begins.

Read the installation guideCheck the bank size