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Marine Battery Calculator

Size your battery bank from electrical loads

DeviceWatts · Hrs/Day

Battery Bank Required

310

amp-hours @ 50% max DOD

Batteries

4 total

Est. Cost

$1,200

Bank Configuration

Daily Load

1,860 Wh

155 Ah/day

Charge Time

7.8 hrs

4P1S config

Actual Bank Capacity400 Ah

Daily Energy by Device (Wh)

Navigation Lights200 Wh
Refrigerator1440 Wh
VHF Radio120 Wh
LED Cabin Lights100 Wh

Marine Battery Types Compared

TypeMax DODCycle LifeCost/100Ah
Flooded Lead-Acid50%300–500$150
AGM50%500–800$300
Gel50%500–800$280
LiFePO480%3,000–5,000$800

What You'll Need

Star Brite Ultimate Aluminum Cleaner & Restorer 64oz

$18-$254.5
View on Amazon

3M Marine Adhesive Sealant 5200 Fast Cure White 3oz

$14-$204.5
View on Amazon

Shoreline Marine Bilge Pump 600 GPH 12V

$15-$254.3
View on Amazon

Star Brite Ultimate Aluminum Cleaner & Restorer 64oz

$18-$254.5
View on Amazon

3M Marine Adhesive Sealant 5200 Fast Cure White 3oz

$14-$204.5
View on Amazon

Shoreline Marine Bilge Pump 600 GPH 12V

$15-$254.3
View on Amazon

As an Amazon Associate, we earn from qualifying purchases.

Frequently Asked Questions

Q

How do I calculate the battery bank size for my boat?

Sum each device’s watt-hours per day (watts × hours), divide total by system voltage to get daily amp-hours, then divide by the battery’s maximum depth of discharge. For lead-acid batteries use 50% DOD; for lithium LiFePO4 use 80% DOD.

  • Step 1: List every device with wattage and daily run time
  • Step 2: Total daily Wh = sum of (watts × hours) for each device
  • Step 3: Daily Ah = Total Wh / System Voltage (12V, 24V, or 48V)
  • Step 4: Required Ah = Daily Ah / DOD (0.50 for lead-acid, 0.80 for lithium)
  • Example: 1,860 Wh/day ÷ 12V = 155 Ah ÷ 0.50 DOD = 310 Ah bank needed
Battery TypeMax DODNeeded for 155 Ah/dayCost (100Ah ea.)
Flooded50%310 Ah (4 batteries)$600
AGM50%310 Ah (4 batteries)$1,200
Gel50%310 Ah (4 batteries)$1,120
LiFePO480%194 Ah (2 batteries)$1,600
Q

What is depth of discharge and why does it matter?

Depth of discharge (DOD) is the percentage of battery capacity you can safely use before recharging. Lead-acid batteries should only be discharged to 50% to preserve cycle life. Lithium LiFePO4 batteries can safely discharge to 80%.

  • Lead-acid at 50% DOD: 300–800 cycles depending on quality
  • LiFePO4 at 80% DOD: 3,000–5,000 cycles typical
  • Discharging lead-acid below 50% dramatically shortens lifespan
  • LiFePO4 delivers consistent voltage throughout the discharge curve
  • A 100 Ah lead-acid battery gives 50 usable Ah; a 100 Ah lithium gives 80 Ah
Q

How long does it take to charge a marine battery bank?

Charge time depends on the depleted amp-hours and charger amperage. A 30A charger recharging 155 Ah at 85% efficiency takes about 6.1 hours. Larger chargers or alternators reduce time, but bulk charging should not exceed the battery manufacturer’s recommended rate.

  • Charge time = Depleted Ah / (Charger Amps × Efficiency)
  • 85% efficiency factor accounts for heat and conversion losses
  • Lead-acid max charge rate: typically 20–25% of capacity (C/5 to C/4)
  • LiFePO4 max charge rate: up to 50% of capacity (C/2)
  • 30A charger on 310 Ah bank (155 Ah depleted): 155 / (30 × 0.85) = 6.1 hours
Charger SizeBank SizeDepleted AhCharge Time
20A200 Ah100 Ah5.9 hrs
30A310 Ah155 Ah6.1 hrs
40A400 Ah200 Ah5.9 hrs
60A400 Ah200 Ah3.9 hrs
Q

Should I use 12V, 24V, or 48V on my boat?

Most recreational boats under 40 feet use 12V systems. Larger vessels and boats with high electrical loads benefit from 24V or 48V, which reduces wire size and energy losses. The higher voltage means lower current for the same power.

  • 12V: standard for boats under 40 ft, widest equipment compatibility
  • 24V: common on larger sailboats and trawlers with 3,000+ Wh daily loads
  • 48V: used on large yachts and electric propulsion boats
  • Higher voltage = thinner wires, less voltage drop over long runs
  • Mixing voltages requires DC-DC converters for 12V accessories
Q

What are the most common electrical loads on a boat?

The biggest power consumers on most boats are refrigeration (60W, 24 hrs = 1,440 Wh/day), navigation electronics (50–100W), lighting (20–60W), and VHF radio (25–30W on receive). Air conditioning and electric cooking are the largest loads if equipped.

  • Refrigerator: 60W × 24h = 1,440 Wh/day (often the single largest load)
  • Navigation lights: 25W × 8h = 200 Wh/day
  • VHF radio (receive): 30W × 4h = 120 Wh/day
  • LED cabin lights: 20W × 5h = 100 Wh/day
  • Chartplotter/radar: 50–100W while underway

Example Calculations

1Cruising Sailboat with AGM Batteries

Inputs

Nav Lights25W × 8h = 200 Wh
Refrigerator60W × 24h = 1,440 Wh
VHF Radio30W × 4h = 120 Wh
LED Lights20W × 5h = 100 Wh
Battery TypeAGM (50% DOD)
System12V, 100 Ah batteries, 30A charger

Result

Required Bank310 Ah (4 batteries)
Daily Load1,860 Wh (155 Ah)
Actual Bank400 Ah
Charge Time7.8 hours

Daily Wh = 200 + 1,440 + 120 + 100 = 1,860. Ah = 1,860 / 12 = 155. Required = 155 / 0.50 = 310 Ah. 4 × 100 Ah batteries. Charge: 400 × 0.50 / (30 × 0.85) = 7.8 hrs.

2Weekend Powerboat with LiFePO4

Inputs

Nav Lights25W × 4h = 100 Wh
Stereo50W × 6h = 300 Wh
Fish Finder40W × 8h = 320 Wh
USB Charging15W × 4h = 60 Wh
Battery TypeLiFePO4 (80% DOD)
System12V, 100 Ah batteries, 30A charger

Result

Required Bank82 Ah (1 battery)
Daily Load780 Wh (65 Ah)
Actual Bank100 Ah
Charge Time3.1 hours

Daily Wh = 100 + 300 + 320 + 60 = 780. Ah = 780 / 12 = 65. Required = 65 / 0.80 = 81.3 Ah. 1 × 100 Ah LiFePO4. Charge: 100 × 0.80 / (30 × 0.85) = 3.1 hrs.

Formulas Used

Daily Amp-Hours

Daily Ah = Σ(Watts × Hours) / Voltage

Converts total daily watt-hours into amp-hours at system voltage.

Where:

Watts= Power draw of each device in watts
Hours= Daily run time of each device in hours
Voltage= System voltage (12V, 24V, or 48V)

Required Battery Capacity

Required Ah = (Daily Ah × Days) / DOD

Calculates minimum battery bank size accounting for depth of discharge and autonomy days.

Where:

Daily Ah= Total amp-hours consumed per day
Days= Days of autonomy without charging (1–3)
DOD= Maximum depth of discharge (0.50 lead-acid, 0.80 lithium)

Charge Time

Time = (Bank Ah × DOD) / (Charger A × 0.85)

Estimates hours to recharge from maximum discharge at 85% charging efficiency.

Where:

Bank Ah= Total battery bank capacity in amp-hours
DOD= Depth of discharge fraction
Charger A= Charger output in amps
0.85= Charging efficiency factor

Sizing Your Marine Battery Bank

A properly sized battery bank is the foundation of a reliable marine electrical system. Undersized banks drain too deeply on each cycle, shortening battery life and risking power loss at sea. Oversized banks waste money and add unnecessary weight. The key is matching your daily energy consumption to the right bank capacity for your battery chemistry.

Start by listing every electrical device on your boat with its wattage and estimated daily run time. Multiply watts by hours to get watt-hours, then sum everything. Divide total watt-hours by your system voltage (usually 12V) for daily amp-hours. Finally, divide by the maximum depth of discharge for your battery type.

Lithium LiFePO4 batteries cost more upfront but provide 80% usable capacity versus 50% for lead-acid, meaning you need fewer batteries for the same usable energy. Over their 3,000–5,000 cycle lifespan, lithium often costs less per cycle than lead-acid batteries that last only 300–800 cycles.

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Last Updated: Mar 25, 2026

This calculator is provided for informational and educational purposes only. Results are estimates and should not be considered professional financial, medical, legal, or other advice. Always consult a qualified professional before making important decisions. UseCalcPro is not responsible for any actions taken based on calculator results.

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