Boat Battery Size Calculator

Estimate the battery bank capacity your boat needs from daily energy use, system voltage, required autonomy, battery type, and a planning reserve.

Daily battery use Daily Wh ÷ System V = Daily Ah
Required bank (Daily Ah × Autonomy days) ÷ Usable DoD
Recommended bank Required bank × (1 + Reserve)
Battery bank inputs
Wh/day
Enter the total energy your boat uses in a typical day.
Use the nominal voltage of the battery bank.
How many days should the bank cover without recharging?
The DoD is a planning assumption. Always follow your battery manufacturer's limits.
Covers planning uncertainty and avoids sizing the bank exactly to the minimum.

How to Size a Boat Battery Bank

1. Start with daily energy use

Add the energy used by your onboard electrical loads over a normal day. If a device uses 60 W for 4 hours, it contributes 240 Wh. Add the contributions from lights, refrigeration, electronics, autopilot, pumps, communication equipment, and other loads.

2. Convert watt-hours to amp-hours

The calculator converts daily energy to battery amp-hours using:

Daily Ah = Daily Wh ÷ System Voltage

3. Allow for the battery's usable depth of discharge

The battery's nominal Ah rating is larger than the amount you should plan to use. The calculator therefore divides the required usable energy by the selected planning DoD.

4. Add an operating reserve

A reserve margin helps prevent the design from landing exactly on the calculated minimum. The default 20% reserve is a planning choice rather than a manufacturer specification.

Example

With 2,400 Wh/day on a 12 V system, the daily battery draw is:

2,400 ÷ 12 = 200 Ah/day

For 2 days of autonomy and an 80% planning DoD:

(200 × 2) ÷ 0.80 = 500 Ah required bank

With a 20% reserve:

500 × 1.20 = 600 Ah recommended bank

Battery type assumptions

The calculator uses 50% planning DoD for flooded lead-acid and AGM, 60% for gel, and 80% for LiFePO4. These are planning assumptions, not universal limits. Battery manufacturers can specify different usable capacity, discharge floors, temperature limits, and cycle-life targets.

What this calculator does not include

Real installations can require additional derating for low temperature, high discharge rates, inverter losses, cable losses, battery age, charging efficiency, and manufacturer-specific limits. For a final marine electrical design, size the bank against the actual battery datasheet and the rest of the boat's electrical system.