Marine Battery Sizing Calculator
Size your marine house battery bank based on hotel loads, usage hours, battery voltage, and depth of discharge for reliable cruising power.
Hotel Load Profile
Chartplotter, radar, AIS, VHF, instruments.
Interior lights, anchor light, navigation lights.
Marine fridge and freezer (compressor cycling average).
Bilge pump, freshwater pump, anchor windlass (average).
Entertainment, USB charging, cabin fans, instruments.
Hours the battery bank must supply loads per day.
12V standard for small boats, 24V for larger yachts.
LFP: 80–90%, Lead-Acid: 50% recommended.
Accounts for wiring losses and BMS overhead.
Sizing Results
Bank Recommendation
For a 430W hotel load over 14 hours at 12V, a 12V 700Ah LFP bank (8.4 kWh) provides approximately 14 hours of runtime.
Formulas & Worked Examples
Mathematical Formulas
Marine battery sizing sums all hotel loads, multiplies by usage time, and adjusts for depth of discharge and system losses:
The DoD and efficiency factors ensure the battery bank is sized conservatively for real-world conditions.
Example 1: Day Cruiser — Overnight Anchoring
A 28ft day cruiser is anchored overnight. The owner needs navigation lights, a fish finder, VHF radio, cabin lighting, and a small fridge running for 8 hours before the engine is started at dawn.
Given Values
- Navigation lights: 20W
- Fish finder: 15W
- VHF radio (standby): 5W
- Cabin lights: 30W
- Compact fridge: 45W
- Usage duration: 8 hours
- System voltage: 12V
- DoD limit: 80% (LFP)
- System efficiency: 90%
=
=
=
=
Final Answer
A 12V 120Ah LFP battery bank (1.44 kWh) is recommended.
This provides 8 hours of overnight hotel loads with a 13% safety margin above the calculated minimum. The bank weighs approximately 14 kg—manageable for a small cruiser. Starting the engine at dawn for 1–2 hours will replenish the bank via alternator.
Example 2: Cruising Yacht — 24-Hour Hotel Loads
A 45ft cruising yacht at anchor runs extensive hotel loads: chartplotter, radar overlay, LED lighting throughout, refrigeration, watermaker, and entertainment systems. The owner wants full 24-hour autonomy without engine charging.
Given Values
- Chartplotter + radar: 120W
- AIS transponder: 30W
- LED lighting (all zones): 90W
- Refrigeration + freezer: 180W
- Watermaker: 250W
- Entertainment + USB: 60W
- Pumps + auxiliaries: 50W
- Usage duration: 24 hours
- System voltage: 24V
- DoD limit: 80% (LFP)
- System efficiency: 92%
=
=
=
=
Final Answer
A 24V 1,100Ah LFP battery bank (26.4 kWh) is recommended.
This meets 24-hour autonomy for a full hotel load profile. The bank weighs approximately 195 kg, requiring proper structural mounting below the waterline. Solar panels (300W+) on the bimini could offset 2–3 kWh daily, reducing bank size requirements.
Example 3: Trolling Motor — High Discharge Analysis
A 20ft bass boat uses an 80lb-thrust trolling motor drawing 46A at 12V. The angler wants to fish for 6 hours continuously, running the motor at medium speed (60% throttle) with intermittent fish finder and navigation lights.
Given Values
- Trolling motor (60% throttle): 27.6A × 12V = 331W
- Fish finder: 15W
- Navigation lights: 20W
- Usage duration: 6 hours
- System voltage: 12V
- DoD limit: 80% (LFP)
- System efficiency: 88%
=
=
=
=
=
Final Answer
A 12V 260Ah LFP battery bank (3.12 kWh) is recommended.
At 30.5A continuous draw, a 260Ah bank provides 6 hours with a small safety margin. Note: at high discharge rates, battery voltage sag becomes significant—LFP maintains voltage better than lead-acid under this load. A 300Ah bank provides comfortable margin for longer fishing days.
Marine Electrical System
Dual battery bank setup with alternator charging and load distribution
Marine Battery Standards
| Standard | Scope | Key Requirements | Voltage Drop Limit |
|---|---|---|---|
| ISO 13297 | Marine electrical installation | Cable sizing, fusing, labeling | 3% critical / 10% non-critical |
| ABYC E-11 | AC/DC systems on boats | Wire color coding, overcurrent protection | 3% for helm instruments |
| USCG 33 CFR 183 | Electrical systems on boats | Ignition protection, fuel tank bonding | N/A (safety focused) |
| IEC 60092 | Marine electrical installations | Shipboard wiring, switchboards | Varies by circuit type |
Key marine electrical standards governing battery system design
Frequently Asked Questions
What are typical hotel loads on a cruising yacht?
Hotel loads vary by boat size and equipment. A 40ft cruiser typically draws 150–250W for navigation and electronics, 50–100W for lighting, 80–150W for refrigeration, and 20–60W for pumps and other auxiliaries. Total hotel loads often range from 300–600W.
How many hours of usage should I design for?
For overnight anchoring without engine charging, design for 12–16 hours of usage. For extended cruising without shore power, 24 hours provides comfortable margin. For emergency backup, 48–72 hours ensures resilience in bad weather.
What depth of discharge is safe for marine batteries?
LFP (LiFePO4) batteries can safely discharge to 80–90% DoD. Lead-acid (AGM/Gel) should not exceed 50% DoD for acceptable cycle life. Deep-cycle lead-acid rated at 200 cycles at 50% DoD; LFP can deliver 3000+ cycles at 80% DoD.
Should I use 12V or 24V for my marine battery bank?
12V is standard for most recreational boats under 50ft due to widespread 12V equipment compatibility. 24V or 48V systems reduce wiring current for larger vessels with higher loads, reducing cable costs and voltage drop. Converters are available for either configuration.
How do alternator charging limits affect my bank size?
Most marine alternators produce 50–100A. A 200Ah LFP bank at 50% state of charge needs 100Ah to recharge—which takes 1–2 hours at full alternator output. If your daily hotel load exceeds what your alternator can replenish during engine runs, you need a larger bank or solar support.
Can I mix lead-acid and lithium batteries in a marine system?
<span class="text-primary font-bold">Never mix chemistries in parallel.</span> Lead-acid and lithium batteries have different charge curves, internal resistances, and safe DoD limits. Charging them together causes one chemistry to overcharge or the other to undercharge, creating safety risks and reducing lifespan.
What is the difference between anchor load and cruising load?
Anchor load is the power drawn while stationary—typically navigation lights, instruments, refrigeration, and cabin lighting (300–500W). Cruising load adds autopilot, radar overlay, fishfinder, and wind instrument loads (500–900W). Size your bank for the higher cruising load if you plan underway usage without alternator support.
How does vibration affect marine battery selection?
Marine environments produce constant vibration from engine, waves, and hull flex. LFP batteries with sealed prismatic cells are vibration-resistant. Flooded lead-acid batteries can spill electrolyte under heavy vibration. AGM and Gel are sealed but still subject to internal plate damage. Mount all marine batteries with vibration-dampening brackets.
Should I install a battery monitor for my marine bank?
Yes—highly recommended. A battery monitor with a shunt measures actual current flow, tracks state of charge, estimates remaining runtime, and logs historical usage. Victron BMV or similar units give real-time visibility that simple voltage readings cannot provide, especially under load.
Do I need separate banks for engine starting and house loads?
Best practice is to use a dedicated starting battery and a separate house bank, connected via an isolator or dual-battery switch. This prevents hotel loads from draining your engine start reserve. For sailboats with electric winches, consider a third dedicated thruster/winch bank.
How does cold weather affect marine battery sizing?
Below 0°C (32°F), LFP capacity drops 5–10% and charging must be limited to prevent lithium plating. Lead-acid capacity drops 20–30% at -10°C. If you cruise in cold climates, oversize the bank by 15–25% and consider battery heaters or insulated battery boxes.
What role does solar play in marine battery systems?
Solar panels on a bimini or hardtop can supplement alternator charging, providing 100–400W during daylight hours. This extends anchor time without engine running. Solar is best viewed as a maintenance charger rather than primary source—it offsets hotel loads but rarely fully recharges a large bank in one day.
Need a deeper engineering review?
Use this calculator for preliminary estimates. For vessel-specific analysis, battery sizing review, emissions reduction planning, or hybrid feasibility support, contact the Battery Calculators engineering support team.
What Is Marine Battery Sizing?
Why This Calculation Matters
→ Undersized marine battery banks leave you without navigation lights or autopilot power during overnight passages—a serious safety hazard at sea.
→ Oversized banks add unnecessary weight and cost; on a 35ft sailboat, every 100Ah of LFP capacity adds roughly 12 kg to a weight-sensitive hull.
→ Incorrect DoD assumptions accelerate battery degradation—discharging lead-acid below 50% DoD can halve its cycle life from 500 to under 250 cycles.
→ Ignoring alternator charging limits means your engine cannot replenish the bank fast enough, leaving you progressively drained on multi-day cruises.
→ Failing to account for hotel load profiles during anchoring versus cruising leads to runtime shortages exactly when you need power most.
Practical Applications
Overnight Anchoring
Size your house bank for 12–16 hours of hotel loads while at anchor without engine charging or shore power.
Extended Bluewater Cruising
Design for 24–72 hour autonomy when sailing remote passages with limited alternator runtime each day.
Liveaboard Houseboat Systems
Calculate continuous daily power needs for full-time living aboard with refrigeration, watermakers, and entertainment loads.
Trawler & Motor Yacht Auxiliaries
Separate house bank from starting battery to ensure hotel loads never compromise engine cranking capacity.
Common Mistakes to Avoid
✗ Using automotive batteries for marine house banks — car batteries are designed for cranking, not deep cycling; they will fail rapidly at 50% DoD.
✗ Ignoring salt water corrosion on terminals and connections — marine environments require tinned copper, heat-shrink connections, and dielectric grease.
✗ Mixing lead-acid and lithium batteries in parallel — different charge curves cause one chemistry to overcharge while the other undercharges.
✗ Omitting vibration isolation mounts — constant wave-induced vibration can damage battery plates and internal connections without proper mounting.
✗ Not separating house bank from engine starting battery — hotel loads can drain the start battery, leaving you unable to start the engine.
✗ Ignoring alternator charging limits — a standard 50A alternator cannot replenish a 200Ah bank from 50% DoD in a reasonable engine run time.
✗ Using voltage-only state-of-charge reading for lithium — LFP's flat discharge curve makes voltage-based SOC unreliable; use a shunt-based monitor.
✗ Forgetting that cold weather reduces capacity 10–30% — winter cruising in northern waters requires oversizing the bank or adding battery heating.
Why Trust These Calculations?
This calculator uses transparent formulas aligned with ISO 13297 marine electrical standards and ABYC E-11 guidelines. All sizing factors—DoD limits, efficiency derating, and voltage drop assumptions—are documented so you can verify the math independently.
View our methodology and formula derivations →Runtime Calculator
Estimate how long your battery powers specific loads.
Voltage Drop Calculator
Ensure cable runs meet marine voltage drop requirements.
Battery Sizing Calculator
General-purpose battery bank sizing with temperature derating.
C-Rate Calculator
Verify charging current limits for your marine bank.
Energy Conversion
Convert between Ah and Wh at your system voltage.
Home Backup Calculator
Compare marine sizing to residential backup approaches.
Was this calculator helpful?