How to Size a Battery Bank for Your Boat
One of the most common questions we get from boaters upgrading their electrical systems is some version of: "How big of a battery bank do I actually need?"
It's a fair question — and the answer depends on more than just picking a number that sounds big enough. Oversize your bank and you've spent money on capacity you'll never use. Undersize it and you're running the generator every night or waking up to dead instruments.
This guide walks you through the exact process our ABYC-certified technicians use to size battery banks for marine, RV, and off-grid systems. We'll do the math together, step by step.
What Is a Battery Bank?
A battery bank is simply one or more batteries wired together to store electrical energy for use on your boat. Most boats have at least two banks — a house bank that powers everything onboard (lights, refrigeration, instruments, pumps) and a starting bank dedicated to cranking the engine.
This guide focuses on sizing your house bank, which is where all the real decision-making happens.
Step 1: Calculate Your Daily Power Consumption
Before you can size a battery bank, you need to know how much power your boat actually uses in a day. This is your daily amp-hour (Ah) load.
Go through every electrical device on your boat and estimate how many hours per day you run it. Then multiply by its amp draw to get daily amp-hours.
Here's an example for a typical 35-foot cruising sailboat:
| Device | Amp Draw | Hours/Day | Daily Ah |
|---|---|---|---|
| Refrigerator/freezer | 5A | 24 (cycles ~50%) | 60 Ah |
| LED cabin lights (4x) | 1.5A total | 4 hrs | 6 Ah |
| VHF radio (standby) | 0.5A | 24 hrs | 12 Ah |
| Chartplotter / instruments | 3A | 8 hrs | 24 Ah |
| Autopilot | 4A | 6 hrs | 24 Ah |
| Water pressure pump | 3A | 1 hr | 3 Ah |
| Phone/device charging | 2A | 3 hrs | 6 Ah |
| Total | ~135 Ah/day |
Your numbers will be different. If you're not sure of the amp draw for a device, check the manual or look for the watt rating — divide watts by your system voltage (12 or 24V) to get amps.
Pro tip: Add 15–20% to your total as a buffer for devices you forgot, higher-than-expected usage days, and system inefficiencies. So 135 Ah becomes roughly 160 Ah/day.
Step 2: Decide on Your Autonomy
Autonomy is how many days you want to run your house loads without charging — whether that's from shore power, solar, your alternator, or a generator.
For most cruisers, 2–3 days of autonomy is a practical target. That gives you a comfortable buffer if you're anchored somewhere without sun, wind is light so the alternator isn't charging much, or you're waiting out weather.
If you have solar or a good charging setup, you may need less built-in autonomy because you're topping off every day. If you're doing extended passages or anchoring in overcast regions, you might want more.
Autonomy calculation:
Required capacity = Daily Ah × Autonomy days
160 Ah/day × 3 days = 480 Ah
Step 3: Account for Depth of Discharge
Here's where battery chemistry matters — a lot.
You cannot use 100% of a battery's rated capacity without damaging it. Every battery type has a safe depth of discharge (DoD) limit — the maximum percentage of capacity you should regularly use.
| Battery Type | Safe DoD | Usable % of Rated Capacity |
|---|---|---|
| Flooded lead acid | 50% | 50% |
| AGM | 50% | 50% |
| Lithium (LiFePO4) | 80–90% | 80–90% |
This is one of the biggest reasons lithium batteries have become so popular in marine applications — you get far more usable energy from the same rated capacity.
Adjusted capacity formula:
Required rated capacity = Required capacity ÷ DoD
For lithium (80% DoD): 480 Ah ÷ 0.80 = 600 Ah rated
For AGM (50% DoD): 480 Ah ÷ 0.50 = 960 Ah rated
To store the same usable energy, you'd need 960 Ah of AGM versus 600 Ah of lithium. That's a significant difference in cost, weight, and space — especially on a boat where all three matter.
Step 4: Choose Your System Voltage
Most boats run on 12V or 24V. Some larger vessels and high-power systems run 48V. Here's how to think about it:
12V systems are by far the most common on recreational boats. Most marine equipment — lights, pumps, chartplotters, radios — runs natively on 12V. Simpler to wire, easier to find components, and compatible with almost everything.
24V systems make sense when you have high-power loads (electric winches, electric propulsion, large inverters) or long wire runs. Higher voltage means lower current for the same power, which means smaller wire, less voltage drop, and less heat. Many serious cruising boats run 24V house systems with a DC-DC converter to power 12V devices.
48V systems are increasingly common on boats with substantial electric propulsion (electric inboard motors, sail drives) where the efficiency gains and wiring savings are significant at that power level.
For most cruising sailboats and powerboats upgrading their house bank: stick with 12V unless you have a compelling reason to go higher. If you're adding an electric outboard, we'd recommend having a dedicated propulsion bank at the voltage that motor requires.
Step 5: Pick Your Battery Chemistry
For a new or upgraded house bank, the choice today is essentially LiFePO4 lithium vs AGM.
LiFePO4 (lithium iron phosphate) is what we install in most new builds and upgrades. Key advantages:
- Deeper, safer discharge (80–90% usable)
- Significantly lighter — roughly half the weight of equivalent AGM
- Longer lifespan: 2,000–5,000+ charge cycles vs 300–500 for AGM
- Faster charging — accepts a higher charge current, so your alternator and solar top it off more quickly
- Flat discharge curve — voltage stays consistent until nearly depleted, so your instruments and appliances behave predictably
AGM still makes sense in some situations: when you need a drop-in replacement without any system changes, when budget is the primary constraint, or when your charging system isn't lithium-compatible and you're not ready to upgrade it. Just remember the 50% DoD limit.
We carry lithium batteries from Victron Energy, Epoch, and MG Energy — each with different strengths depending on your system size and budget.
Putting It All Together: A Real Sizing Example
Let's say you have that 35-foot cruising sailboat, and you want to:
- Run 160 Ah/day of loads
- Have 3 days of autonomy without charging
- Use LiFePO4 lithium batteries
- Run a 12V system
Here's the math:
- Daily load: 160 Ah
- 3 days autonomy: 160 × 3 = 480 Ah needed
- LiFePO4 at 80% DoD: 480 ÷ 0.80 = 600 Ah rated capacity
So you're targeting a 600 Ah, 12V lithium bank. A practical way to build that is two Victron Lithium NG 12.8V 300Ah batteries wired in parallel, giving you 600 Ah at 12V.
Don't Forget the BMS
Any lithium battery system needs a Battery Management System (BMS) — either integrated into the battery (most modern lithium batteries include one) or external. The BMS protects the cells from overcharge, over-discharge, and excessive current.
Most quality lithium batteries like Victron's Lithium NG series have an integrated BMS, which simplifies installation. For larger or more complex systems, an external BMS gives you more control and monitoring capability.
Browse our Battery Management Systems collection if you're building a custom bank.
A Note on Battery Monitors
Once you have a properly sized battery bank, you need a way to know how much energy you have left. A battery monitor is to your battery bank what a fuel gauge is to your gas tank — essential for actually managing your power.
The Victron SmartShunt is what we recommend for most installs. It integrates with the Victron ecosystem and gives you real-time state of charge, time remaining, and historical data — all accessible via the VictronConnect app on your phone.
Common Sizing Mistakes to Avoid
Underestimating your loads. People consistently forget about phantom draws — devices on standby, the occasional watermaker run, an anchor light left on all night. Build in that 15–20% buffer.
Ignoring DoD. Especially if you're coming from a world of vague "marine battery" sizing, the DoD calculation is the step most people skip. If you size to raw Ah without accounting for usable Ah, you'll end up with half the autonomy you planned for.
Assuming more is always better. A massively oversized battery bank that you never fully charge is bad for battery longevity. LiFePO4 batteries like being cycled regularly. Size for realistic autonomy, then let your solar and alternator top it off.
Mixing battery chemistries. Don't combine lithium and AGM in the same bank. Different charge profiles mean they'll fight each other, and the AGM will be damaged in the process.
Ready to Size Your System?
Battery bank sizing is straightforward once you run the numbers — but the right configuration for your boat depends on your charging sources (solar, alternator, shore power), your loads, your physical space constraints, and your budget.
If you want to talk through your specific setup, our ABYC-certified team is here. We do this every day.
Schedule a free system consultation →
Or browse our full selection of lithium batteries and battery accessories to start building your bank.
Related reading:
Lead Acid or Lithium: What's the Right Battery for Electric Propulsion?
Marine Battery Specifications & Ratings: What Do They Mean
The Next Generation Lithium Battery: Introducing Victron's Lithium NG