How to Size a House Battery Bank

Every dead-battery call I get in July starts the same way. The owner swears the batteries are fine, they charged fine last season, and yet the fridge quit at 2am and the anchor light barely glows. Nine times out of ten the batteries are healthy. The bank was just too small for how the boat actually gets used.

Getting the right amount of battery capacity is not guesswork, and it is not about buying the biggest box that fits the locker. It comes down to a short list of numbers you can gather in an afternoon at the slip.

Below is the exact process I walk owners through before I quote a single dollar of copper.

What you need before you start

You do not need a lab. You need a notepad, your boat’s equipment list, and a way to read amperage. A cheap clamp meter that reads DC amps runs about $40 and pays for itself the first time you catch a phantom draw.

A Watt-hour is a unit of energy: volts times amps times hours. On a 12V boat we usually talk in amp-hours (Ah) instead, which is just amps drawn multiplied by how many hours you draw them. A device pulling 5 amps for 3 hours uses 15 amp-hours. That is the whole vocabulary you need to size a house battery bank correctly.

Step 1: List every load and how long it runs

Write down each 12V device on the boat and the amps it pulls. Then estimate honestly how many hours per day it actually runs at anchor, not while the engine is charging.

Refrigeration is almost always the biggest single load. A typical 12V marine fridge pulls around 5 amps but only cycles maybe 40 to 50 percent of the time, so figure 5 amps for 10 to 12 hours across a full day.

Lights, the water pressure pump, phone charging, instruments, and a stereo add up faster than people expect. The mistake I see most weekends is owners forgetting the small stuff, then wondering where 30 amp-hours went overnight.

Step 2: Total your daily amp-hours

Multiply each load’s amps by its daily hours, then add the column. Here is a real tally from a 34-foot cruiser I worked on last spring, a weekend boat that anchors out two nights at a time.

So this boat burns roughly 94 amp-hours in a day at anchor. That single number drives everything that follows.

Step 3: Decide how many days of autonomy you want

Autonomy is how long the bank must run everything with no charging at all. A daysailor who returns to a shore-power slip every night needs one day. Someone gunkholing for a weekend needs two.

For our example boat, the owner wanted two days between meaningful charges, because he anchors Friday and Saturday and does not want to run the engine just to feed the fridge. Two days times 94 amp-hours is 188 amp-hours of actual energy needed.

Step 4: Apply depth of discharge

Depth of discharge (DoD) is how far down you drain a battery before recharging. You almost never use a battery’s full nameplate capacity, because draining it flat wrecks it fast.

Flooded lead-acid batteries live longest if you stop around 50 percent. AGM handles 50 to 60 percent comfortably. Lithium (LiFePO4) can safely use 80 to 90 percent, which is why the same usable energy needs far less nameplate capacity. This tradeoff is real money over the life of the bank, and our guide on What Depth of Discharge Really Costs You shows the cycle-life numbers.

Divide your usable need by the DoD fraction. For flooded at 50 percent: 188 divided by 0.5 equals 376 amp-hours of rated capacity. Same energy on lithium at 85 percent needs only about 221 rated amp-hours.

Step 5: Round up to real battery sizes

Batteries come in standard cases, so you will round to what actually exists. For the flooded example, two 6V golf-cart batteries wired in series make a 12V string of roughly 210 to 225 amp-hours. Two of those strings gives about 420 to 450 amp-hours, comfortably above our 376 target.

How you connect them matters as much as how many you buy. Series wiring adds voltage, parallel adds capacity, and mixing them wrong is a fast way to cook a cell. Our walkthrough on Wiring Batteries in Series and Parallel covers the layouts, and the fuller Marine Battery Bank Setup Guide ties the whole install together.

Chemistry choice affects weight, cost, and where it can live aboard. If you have not settled that yet, read AGM vs Flooded vs Gel Batteries before you buy, because a flooded bank this size weighs well over 200 pounds and needs a vented, secured location.

Step 6: Check that your charging can keep up

A big bank is useless if you can never refill it. As a rule, lead-acid banks accept a charge current of roughly 10 to 20 percent of their capacity, so a 400 Ah flooded bank wants a charger putting out 40 to 80 amps to recharge in a reasonable window.

Undersized charging is why boats limp home at half capacity all season. The American Boat and Yacht Council publishes the wiring and overcurrent standards worth following here, and the ABYC website is the reference I hand every owner. For plain-language background on battery types, BoatUS expert advice is a solid free resource.

Putting the numbers to work

Run the five figures in order: daily amp-hours, days of autonomy, DoD limit, rated capacity, then charging to match. For the 34-foot cruiser that meant a 420 Ah flooded bank, an 60-amp charger, and a second season with the fridge still cold at Sunday breakfast.

Do this once, on paper, before you spend anything. A right-sized bank costs the same to install as a wrong-sized one, and it is the difference between trusting your boat and babysitting it. Measure your loads this weekend, and you will know exactly what to buy.