How to Size a Boat Battery Bank: Day Sailers, Fishing Boats, Cruisers and Offshore

Most battery banks we replace were not worn out by age. They were too small for the boat they were on, cycled deeper every weekend until they quit. The fix is not a bigger battery picked off a shelf. It is working out how many amp-hours the boat actually pulls between charges, then sizing the bank so it never has to give up more than its chemistry can afford.

This guide walks through that process, then applies it to five common types of boat: day sailers, fishing boats with outboards running, powerboats with a generator, coastal cruisers and offshore passage makers. The method is the same for all of them. What changes is when the boat uses power and when it gets it back.

Step 1: Do an Electrical System Audit

Everything starts with an energy audit. List every DC load on the boat, its current draw in amps, and how many hours it runs. Multiply amps by hours to get amp-hours (Ah). Anything that cycles on and off, like a refrigerator compressor or an autopilot, gets a duty cycle: the share of the time it is actually drawing current.

Ah = amps x hours x duty cycle

Run the audit for each way you use the boat, because the numbers can differ enormously:

• Underway. On a sailboat this is usually the worst case: autopilot, radar, instruments and nav lights all running with no engine charging.
• At anchor or on the mooring. On a powerboat this is almost always the worst case, because underway the engines carry the load.
• Normal and worst case for each. Normal is a typical day. Worst case is fog, heavy weather, a hot week or resetting the anchor three times.

Use our boat power usage audit template to work through it. Where you can, measure rather than guess. A battery monitor with a shunt will tell you what the boat really used last weekend, which beats any spec sheet.

The loads people get wrong. Victron Energy's Reinout Vader makes the point in his book Energy Unlimited: owners underestimate the small loads that run all day, like refrigeration and electronics, and overestimate the big loads that run for minutes, like a windlass, microwave or bow thruster. A refrigerator drawing 4 A doesn't look like much until you multiply it by 24 hours. A 500 A bow thruster used for two minutes costs about 17 Ah.

Step 2: Find the Amp-Hours Between Charges

The number that sizes the bank is not daily consumption. It is how many amp-hours you take out between recharges. A boat that motors an hour every morning and evening only has to carry half a day on its batteries. A boat that sits on the hook all weekend with no solar has to carry two full days.

So decide honestly how the boat gets its charge back: shore power every night, solar, the main engine, a generator, or some combination. Then add up the audit over the longest stretch you expect to go without one.

Step 3: Divide by What the Chemistry Can Give

No battery gives you its full label capacity in normal use. Lead-acid batteries should not regularly go below about 50 percent, and in the short charging windows most boats have, they rarely get back above 80 percent, because the last 20 percent takes hours of absorption. That leaves roughly 30 percent of the label to work with. Lithium (LiFePO4) can be cycled far deeper and accepts charge right up to full, so much more of the label is usable.

Bank Ah = Ah between charges / usable fraction

We size for both the normal case and the worst case and install the larger result. The worst case is allowed to go deeper, because it is occasional.

Battery type	Usable fraction, normal	Usable fraction, worst case	Bank size vs. normal Ah between charges
Flooded lead-acid	0.30	0.60	about 3.3 x
Quality AGM / TPPL / carbon foam	0.50	0.65	about 2 x
Lithium (LiFePO4)	0.70	0.80	about 1.4 x

These ratios come from the energy systems design method taught by Nigel Calder and BoatHowTo. Victron arrives at almost the same answer from a different direction: start with 50 percent usable lead-acid capacity, keep a 10 percent safety margin, then allow for 20 percent capacity loss as the battery ages, and you land at about 32 percent. Their rule of thumb is a house bank of at least three times what you take out between charges. Two independent methods agreeing on 3 to 1 for conventional lead-acid is a good sign the number is right.

Why 0.70 for lithium and not 0.80 or 1.00? A typical LiFePO4 battery is rated for 80 percent depth of discharge and up to 4,000 cycles, and some makers advertise 100 percent. Rated is not the same as planned. Sizing normal use to 70 percent leaves a reserve for the day the forecast is wrong, keeps the BMS away from its low voltage cutoff, and is still less than half the lead-acid bank it replaces.

Five Boats, Five Different Banks

The examples below use typical loads for each type of boat. Your boat will be different, so treat these as a worked method, not a shopping list. All figures are at 12 V.

1. The Day Sailer

A 25 to 32 foot sailboat out for the afternoon, back on the mooring or the dock at night. Small loads, short days, and a charge waiting at the end of most of them.

Load	Draw	Hours	Ah
Chartplotter and instruments	1.5 A	6	9
Stereo	1.5 A	4	6
Tiller pilot (25% duty)	3 A	6 x 0.25	5
VHF, standby and calls	0.5 A	6	3
Phone charging	varies	varies	3
Bilge pump, standby drains	0.1 A	24	2
Total per day			28 Ah

Recharged every night, that is 28 / 0.30 = about 95 Ah of flooded battery. The worst case is a weekend away from the charger, 56 Ah, which a 95 Ah bank still covers at 0.60. One quality 100 Ah battery does it, flooded, AGM or lithium.

For day sailers the bigger question is usually not capacity. It is keeping that battery charged while the boat sits for a week. A small solar panel on the mooring, a separate start battery for an inboard, and tracking down the standby drains (radios, stereo memory, a bilge alarm) will do more for battery life than buying a bigger one.

2. The Fishing Boat With Engines Running

A center console or bay boat running outboards most of the day. Underway, the engines carry the electronics, so the audit that matters is the time spent drifting, anchored on a spot or idling with the livewells running. Call it four hours off the engines, six on a slow day.

Load (engines off)	Draw	Hours	Ah
Two MFDs with sonar	6 A	4	24
Livewell and baitwell pumps	4 A	4	16
Stereo	3 A	4	12
Fridge drawer or cooler (50% duty)	4 A	4 x 0.5	8
VHF, lights, misc.	1 A	4	4
Total between runs			64 Ah

That gives about 210 Ah of flooded battery, 130 Ah of AGM or 90 Ah of lithium for the normal case. The six hour worst case, 96 Ah, pushes AGM to about 150 Ah and lithium to about 120 Ah, so those become the answer.

Three things matter more on fishing boats than the house bank size:

• Keep every engine's start battery separate from the house loads, with a battery switch or automatic charging relay between them. Drifting with the stereo on should never cost you a start.
• Check what the outboards really put back. Rated charging output is usually quoted at cruise rpm, and some of it is already spoken for by the engine and electronics. At idle or trolling speed many outboards make only a fraction of their rating. If the run home is short, the bank may never refill.
• Size a trolling motor bank on its own. Use the motor's average amp draw times the hours you fish it, divided by 0.8 for lithium. A 24 V motor averaging 30 A for five hours needs about 190 Ah at 24 V.

3. The Powerboat With a Generator

A 35 to 45 foot express or flybridge cruiser with an inverter, an AC refrigerator and a generator. Underway the engines handle everything. At anchor the generator runs in the evening and morning, and the batteries carry the boat through the night. That quiet stretch is what Victron calls the generator free period, and it is what sizes the bank.

Load (10 hour night, generator off)	Draw	Hours	Ah
Refrigerator on the inverter, incl. inverter losses	9.5 A avg.	10	95
Morning coffee maker, 900 W on the inverter	88 A	0.2	18
Inverter idle	1 A	10	10
Cabin fans	1 A	10	10
Electronics standby, AIS, CO detectors	1 A	10	10
LED cabin lights	3 A	2	6
Water pump and heads	varies	varies	5
LED anchor light	0.2 A	10	2
Total per night			156 Ah

For the normal case that is about 520 Ah of flooded battery, 315 Ah of AGM or 225 Ah of lithium. A hot night with the generator off for 12 hours and the refrigerator cycling harder pushes worst case to about 200 Ah, which takes lithium up to about 255 Ah. AGM and flooded stay where the normal case put them.

The air conditioning question. Nearly every powerboat owner eventually asks whether the batteries can run the air conditioning overnight. As a rule of thumb, an older 16,000 BTU unit draws about 1.5 kW. Running half the time for eight hours, that is 6 kWh, or about 500 Ah at 12 V for one unit, before anything else on the boat. That is not a bigger house bank. It is a different system, usually 48 V lithium with serious charging behind it, and it has to be designed as one.

When the generator runs all day. On sportfish boats and larger yachts where the generator runs continuously offshore, the bank is a buffer, not a fuel tank. Size it for the overnight generator free period using the same 3 to 1 lead-acid rule, and for the peak loads. With a Victron MultiPlus running PowerAssist, the batteries can carry short peaks above the generator's rating, which often lets a smaller generator do the job and run at a healthier load.

4. The Coastal Cruiser

A 34 to 42 foot sailboat spending weekends and a couple of summer weeks at anchor. This is the same boat we used in How Much Solar Do I Need on My Boat?: refrigerator, electronics, fans, lights and a water pump add up to about 76 Ah per day at anchor. A hot week with the refrigerator running harder and fans on most of the day puts worst case at about 100 Ah.

How the boat recharges	Flooded	AGM	Lithium
Engine or solar every day (76 Ah between charges)	about 255 Ah	about 155 Ah	about 125 Ah
Two nights at anchor, no charging (152 Ah between charges)	about 510 Ah	about 310 Ah	about 250 Ah

That table is the whole argument for solar on a cruising sailboat. Solar that keeps up with the daily load halves the bank you need. Without it, carrying two days means either 500 Ah of flooded lead in the bilge or around 250 Ah of lithium. Where the worst case decides the size, as it does for lithium here, we have used it.

5. The Offshore Passage Maker

A 40 to 50 foot sailboat on passage, sailing day and night. Underway is the worst case, the loads never stop, and every amp-hour has to come back from an engine, a generator or renewables.

Load (24 hours under sail)	Draw	Hours	Ah
Chartplotter, instruments, VHF	2.5 A	24	60
Satellite internet left on	2.5 A	24	60
Autopilot, below decks (30% duty)	6 A	24 x 0.3	43
Refrigerator (45% duty)	4 A	24 x 0.45	43
Radar, night watches	3 A	12	36
Watermaker	10 A	3	30
AIS transponder	0.5 A	24	12
Cabin lights, fans, devices	varies	varies	10
LED tricolor	0.3 A	12	4
Total per day			298 Ah

In heavy weather the autopilot works harder and in fog the radar stays on, which puts worst case at about 363 Ah.

Recharge schedule	Flooded	AGM	Lithium
Once a day	about 1,000 Ah	about 600 Ah	about 455 Ah
Twice a day	about 500 Ah	about 300 Ah	about 230 Ah

Two things stand out. First, charging twice a day cuts the bank in half, which is why so many offshore boats run the engine or generator morning and evening. Second, a 1,000 Ah flooded bank weighs well over 600 pounds. This is where lithium stops being a luxury and becomes the practical choice.

The bank is only half of an offshore system. Putting 300 Ah or more back every day takes real charging. Victron puts the practical limit of a main engine alternator at about 300 Ah per day into a 12 V bank, and a lead-acid bank needs more than it used back, because charging losses run around 20 percent. Beyond that, the answer is a high-output alternator with an external regulator, solar, a hydrogenerator, or moving the house bank to 24 or 48 V.

Step 4: Check the Bank Against the Rest of the System

A bank sized only for energy can still fail the boat. Before buying anything, check it four more ways.

Peak Load

The bank has to deliver the biggest current the boat can draw at once without a fuse blowing or a BMS disconnecting. A 2,000 W inverter pulls more than 160 A at 12 V. A bow thruster at 500 A plus 30 A of house loads is a 530 A peak. For lead-acid, the battery's CCA rating should at least equal the peak, and preferably double it if the load runs more than a few seconds. For lithium, check the BMS continuous and peak discharge limits. Exceeding them shuts off every DC load on the boat, which is a safety event, not an inconvenience. Thrusters and windlasses are often better served by a dedicated battery near the load.

Charge Acceptance

Every battery has a limit on how fast it can be charged. Flooded batteries accept about 20 to 30 percent of their capacity in amps, AGM 30 to 40 percent, and TPPL AGM wants at least 40 percent. A 500 Ah AGM bank tops out around 150 to 200 A. Victron's guidance is that a lead-acid bank should be at least three times the alternator output, or it will overheat and refuse the current. Lithium is set by its C rating, commonly 0.3C to 1C: a 400 Ah bank takes roughly 120 A at 0.3C. Add up every source that can charge at once, alternator, solar and shore, and make sure the bank can take it.

Recharge Time

Divide the amp-hours you need to replace by what your charging sources actually deliver, not what they are rated for. If the answer is four hours of engine running a day, the bank is fine on paper and miserable in practice. Either add charging, cut loads, or accept a different recharge schedule and size again.

Temperature and Age

Lead-acid loses capacity in the cold and loses more every year it is cycled. Lithium cannot be charged below freezing and wants to live between 32 and 100°F, so keep it out of the engine room. If you are sizing lead-acid with less than the 3 to 1 ratio, you are spending the margin that was meant for the battery's old age.

Ways to Shrink the Bank You Need

Every amp-hour you stop using saves three amp-hours of lead-acid battery. Before buying more capacity, look at:

• Recharging more often. Twice a day instead of once halves the bank.
• Refrigeration. Usually the largest load on a boat without air conditioning. Better insulation and a water-cooled condenser can cut it dramatically. An inefficient 50 W compressor running nonstop can take 100 Ah a day on its own.
• Always-on electronics. Satellite internet, MFDs and inverters left idling add up overnight. Switch them off when you do not need them.
• LED lighting. If there are still incandescent or halogen fixtures aboard, they are the cheapest amp-hours you will ever save.
• Renewables. Solar that covers the daily load changes the whole calculation. Start with the Marine Solar Design Guide.
• Higher charge acceptance. TPPL AGM and lithium refill faster in the same engine hour, which lets a smaller bank do the same job.

Measure It Once It Is In

An audit is an estimate. A battery monitor with a shunt is the proof. Voltage alone cannot tell you how much is left in a lithium bank, and by the time a lead-acid bank shows low voltage it has already been discharged too deeply. A monitor that counts amp-hours in and out, such as a Victron SmartShunt or BMV, shows whether your bank is sized right within a few weekends. Browse our meters and monitoring.

The Short Version

1. Audit the boat underway and at anchor, normal and worst case.
2. Total the amp-hours between charges, not per day.
3. Divide by 0.30 for flooded, 0.50 for quality AGM or 0.70 for lithium. Check the worst case at 0.60, 0.65 or 0.80, and install the larger result.
4. Check peak load, charge acceptance, recharge time and temperature.
5. Keep the start battery separate, and fit a battery monitor.

When in doubt, err large. An oversized bank costs weight and money. An undersized one costs batteries, every couple of seasons, for as long as you own the boat. For chemistry choices, see selecting batteries for your boat, and if you are switching, read Going Lithium Without Going Wrong before you buy.

Want Us to Run the Numbers?

Sea & Land Yacht Works designs and installs marine electrical systems to ABYC standards from our shop in Wakefield, Rhode Island. We can do the electrical system audit on your boat and size the bank, charging and monitoring together. Call 401-753-7479 or request a consultation. Browse the lithium batteries, batteries and battery chargers we carry.

Mike Garretson is an ABYC Master Technician and the owner of Sea & Land Yacht Works. He writes about marine electrical systems for Soundings magazine.