Marine Battery Bank Calculator
Size your marine battery bank for days of autonomy without shore power or engine charging. Enter your daily power consumption and battery bank capacity. The calculator applies ABYC's 50% depth of discharge guideline for lead-acid batteries to find usable capacity and days of autonomy.
Battery bank sizing formula
Usable capacity (Ah) = Bank capacity × (DOD% / 100)
Days of autonomy = Usable capacity / Daily consumption
Bank needed (Ah) = Daily consumption × Target days / (DOD% / 100)
ABYC E-11 recommends 50% DOD for lead-acid (flooded or AGM) house banks. Lithium iron phosphate (LFP) can use 80 to 90% DOD, allowing a smaller bank for the same usable capacity. Always verify with your battery manufacturer's specifications.
Typical marine loads (reference)
| Device | Typical draw (A) | 8 hrs/day (Ah) |
|---|---|---|
| LED anchor light | 0.3 | 2.4 |
| VHF radio (receive) | 0.5 | 4.0 |
| Chartplotter 9 in | 0.8 | 6.4 |
| Refrigerator 12V | 4.0 | 32.0 |
| Bilge pump (cycling) | 5.0 | 10.0 |
| Interior LED lighting | 2.0 | 16.0 |
Marine battery bank calculator: frequently asked questions
How do I calculate how long my marine battery bank will last?
Days of autonomy = (Battery bank Ah * Usable depth of discharge) / Daily consumption (Ah/day). For example, a 400 Ah lead-acid battery bank at 50% DOD gives 200 Ah usable. If your boat uses 50 Ah per day, you have 4 days before recharging is needed. ABYC recommends not discharging lead-acid batteries below 50% state of charge.
What is depth of discharge (DOD) for marine batteries?
Depth of discharge (DOD) is the percentage of battery capacity that is used before recharging. For flooded and AGM lead-acid batteries, ABYC E-11 standards recommend not exceeding 50% DOD regularly, as deeper discharge significantly reduces battery life. Lithium (LFP) batteries can be discharged to 80 to 90% DOD without similar lifespan impact.
How large a battery bank do I need for my boat?
A common rule of thumb is to size the battery bank to provide 2 to 3 days of autonomy without charging. Add up all 12V loads in amps, multiply by the hours each device runs per day to get daily Ah consumption, then multiply by 2 to 3 days and divide by the usable DOD fraction. For example: 60 Ah/day * 3 days / 0.5 DOD = 360 Ah total bank.
What is the difference between Ah capacity and reserve capacity (RC)?
Amp-hour (Ah) capacity is the total charge a battery can deliver (e.g., 100 Ah = delivering 5 amps for 20 hours). Reserve capacity (RC) is the number of minutes a fully charged 12V battery can deliver 25 amps before dropping to 10.5 volts. To approximate Ah from RC: Ah = RC * 25 / 60. Use the 20-hour Ah rating for battery bank sizing.
Can I mix battery types in a marine battery bank?
No. Mixing battery types (flooded lead-acid with AGM, or lead-acid with lithium) in a parallel bank is not recommended. Different internal resistances cause uneven charging, which damages batteries prematurely. ABYC E-11 and battery manufacturers advise using identical batteries (same chemistry, brand, and age) in a parallel bank.
Official sources
- American Boat and Yacht Council (ABYC): ABYC E-11 AC and DC Electrical Systems on Boats (DOD and battery sizing standards).
- US Coast Guard: USCG Marine Electrical Safety.
Reviewed by the CalculatorHub team, edited by James Graham, 14 June 2026. See our methodology.