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Marine Battery Bank Sizing Calculator

Amp-Hour Capacity, Battery Count, and Charging Requirements for House, Starting, and Bow Thruster Banks per ABYC E-10

Free marine battery bank sizing calculator for marine electricians, boat builders, and cruising sailors who need to determine the correct battery capacity for onboard DC loads. Enter your daily electrical loads (amps and hours of use for each device), select the battery chemistry (flooded lead-acid, AGM, gel, or lithium LFP), set the desired depth of discharge and days of autonomy, and the calculator returns the required amp-hour capacity, the number of batteries in parallel, and the minimum charging source needed to replenish the bank.

Battery bank sizing on boats is more critical than in land-based systems because there is no utility grid backup. An undersized house bank means running the generator all night or losing refrigeration, navigation electronics, and cabin lighting. An oversized bank adds unnecessary weight (lead-acid batteries weigh 60-70 lbs per 100 Ah), takes up limited space, and may never get fully charged by the available charging sources. The correct sizing methodology starts with an accurate daily load audit, then applies derating factors for the battery chemistry's recommended depth of discharge — typically 50% for lead-acid and 80% for lithium LFP — and multiplies by the desired days of autonomy without charging.

The calculator also sizes the charging system: alternator output, shore power charger capacity, and solar panel wattage needed to fully recharge the bank in a reasonable time (typically 4-6 hours of engine run time or 6-8 hours of shore power). ABYC E-10 and E-13 provide the framework for battery installation, ventilation, and charging system requirements that the calculator references.

Pro Tip: When sizing a lead-acid house bank, the rule of thumb is that your daily consumption should be no more than 25-30% of the total bank capacity. This keeps you above 50% state of charge at the end of each discharge cycle, which dramatically extends battery life. A bank that is regularly discharged to 50% SOC will last 2-3 times longer than one regularly discharged to 20% SOC. With lithium LFP batteries, you can safely use 80% of the capacity, so the bank can be much smaller for the same daily load.

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Marine Battery Bank Sizing Calculator
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How It Works

  1. Build Your Load Audit

    List every DC load on the vessel: navigation electronics, autopilot, refrigeration, cabin lighting, water pumps, anchor windlass, entertainment systems, and instrument panels. Enter each load's current draw in amps and the estimated daily hours of use.

  2. Select Battery Chemistry

    Choose flooded lead-acid, AGM, gel, or lithium LFP. Each chemistry has a different recommended maximum depth of discharge, cycle life, weight, and charging profile. The calculator applies the correct derating factor for your selection.

  3. Set Autonomy and Depth of Discharge

    Enter the number of days of autonomy you want without charging (typically 1-3 days for coastal cruising, 3-5 days for offshore passages). Select or accept the default maximum depth of discharge for your battery chemistry.

  4. Review Bank Size and Charging Requirements

    Check the required total amp-hours, the number of batteries in parallel, the total weight, and the minimum charging source capacity (alternator amps, shore charger amps, and solar watts) needed to replenish the bank in a reasonable time frame.

Built For

  • Cruising sailors sizing a house bank for multi-day passages without engine run time or shore power
  • Marine electricians specifying battery capacity for a repower that includes upgrading from flooded to AGM or lithium batteries
  • Boat builders determining the battery capacity and charging system for a new production boat's standard electrical package
  • Liveaboards optimizing their house bank size to balance adequate capacity with weight and the ability to fully recharge from solar alone

Features & Capabilities

Daily Load Audit Worksheet

Built-in load entry table with common marine loads pre-populated (navigation, lighting, refrigeration, pumps, electronics). Enter amps and hours for each load, and the calculator totals the daily amp-hour consumption.

Four Battery Chemistries

Supports flooded lead-acid (50% DOD, 500 cycles), AGM (50% DOD, 800 cycles), gel (50% DOD, 1000 cycles), and lithium LFP (80% DOD, 3000+ cycles). Each chemistry applies the correct derating and shows the weight and volume implications.

Charging System Sizing

Calculates the minimum alternator output, shore power charger capacity, and solar panel wattage needed to fully recharge the bank. Shows the estimated engine run time and shore power charge time for each scenario.

Weight and Space Comparison

Displays the total bank weight and approximate volume for each battery chemistry, making it easy to compare the weight savings of lithium (typically 50-60% lighter than lead-acid for the same usable capacity) against the higher upfront cost.

Frequently Asked Questions

Multiply each device's current draw (amps) by the number of hours it runs per day, then sum all the results. For example, a refrigerator drawing 5 amps with a 33% duty cycle (running 8 out of 24 hours) consumes 40 Ah/day. LED cabin lights drawing 2 amps for 5 hours add 10 Ah/day. A chartplotter drawing 1.5 amps for 12 hours adds 18 Ah/day. Total these up for every load on the vessel. Most 30-40 foot cruising sailboats consume 100-200 Ah/day; powerboats with air conditioning can consume 400+ Ah/day.
For flooded lead-acid and AGM batteries, do not exceed 50% depth of discharge (DOD) regularly. Discharging below 50% SOC dramatically reduces cycle life. For gel batteries, 50-60% DOD is acceptable. For lithium LFP, 80% DOD is the standard recommendation, and many lithium batteries can handle 90-100% DOD without significant life reduction. The calculator automatically applies the correct DOD limit for each chemistry.
For coastal cruising with regular access to marinas, 1-2 days of autonomy is usually sufficient. For coastal passages of 2-3 days, plan for 2-3 days. For offshore passages, plan for 3-5 days and ensure your engine alternator and/or solar panels can charge the bank sufficiently during the passage. Liveaboards often size for 2-3 days to cover cloudy weather periods when solar output is reduced.
Lithium LFP batteries cost 3-5 times more than equivalent lead-acid per amp-hour, but the usable capacity is 60% higher (80% DOD vs 50%), the cycle life is 4-6 times longer (3000+ vs 500 cycles), and the weight is 50-60% less. For a cruising boat that cycles the house bank daily, lithium typically pays for itself in 3-5 years through longer life and reduced generator run time. The main considerations are the need for a battery management system (BMS), compatible charging sources, and proper low-temperature cutoff protection.

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