LiFePO4 Bank Sizing for Liveaboards

Most people sizing a battery bank for liveaboard life work backward from a number they saw on a forum: “400Ah is plenty” or “you need 600.” Both answers are useless without knowing what you actually run overnight, how long you go between charges, and how cold it gets where you keep the boat. Sizing is arithmetic, not opinion, and the arithmetic is friendlier than it looks.

I spend my working weeks pulling old lead-acid banks out of sailboats and trawlers and dropping lithium in their place. The single biggest reason people are unhappy after a lithium refit is not the batteries. It is a bank that was guessed at instead of measured.

So let’s do it the measured way. By the end you’ll have a real amp-hour target for your own boat, with numbers you can defend.

What you’ll need before you start

This is a planning job first and a wiring job second. The planning takes an afternoon at the galley table with a notebook.

Gather these: a battery monitor or clamp meter to read actual current (a Victron BMV or a $40 clamp meter both work), the wattage or amp rating off every appliance label, and an honest log of how many hours a day each thing runs. If you already have a shunt-based monitor installed, even better, because it has been counting for you.

LiFePO4 means lithium iron phosphate, the specific lithium chemistry used in marine house banks because it tolerates heat and abuse far better than the lithium in a laptop. When I say lithium in this guide, that’s what I mean. If you want the wider view of the whole refit, the Lithium Boat Battery Upgrade Guide covers the parts list and wiring side that this article deliberately skips.

Step 1: Build a real energy budget

Everything starts here. You are going to list every DC load on the boat and estimate its daily amp-hours (Ah), which is simply amps multiplied by hours of run time.

Work in amp-hours at 12 volts. If an appliance is rated in watts, divide watts by 12 to get amps. A 60-watt fridge compressor draws about 5 amps while running, but a compressor fridge only runs maybe 40 percent of the time, so over 24 hours that’s roughly 5 amps times 9.6 hours, or 48Ah a day.

Here is a budget from a real 38-foot cruising sailboat I refit last spring. The couple aboard cook, work remotely, and anchor out four nights a week.

Round it up. Call this boat a 115Ah-per-day home. Your list will look different, and that’s the point: yours is the only one that matters.

Step 2: Decide how much autonomy you want

Autonomy is how many days the bank can carry you between full charges. For weekend sailors, one day is fine. For liveaboards who anchor out and want to run the generator or the engine less, aim for 1.5 to 2 days.

Two days of autonomy on our example boat is 115Ah times 2, which is 230Ah of usable energy. Notice the word usable. That is the energy you actually get to pull out, not the number printed on the case.

Why two and not one? Because weather happens. A cloudy stretch kills solar, and if your bank only holds a single day you’re running the engine at anchor every evening, which is exactly the noise and diesel cost you bought lithium to avoid.

Step 3: Convert usable Ah to rated Ah

This is where lithium pulls ahead of lead-acid, and where most people oversize by accident. A flooded or AGM battery should only be discharged to about 50 percent to protect its life. A quality LiFePO4 bank can give you 90 to 100 percent of its rating, day after day.

So to get 230Ah of usable energy, you don’t need a 460Ah lithium bank. You need roughly 230 to 255Ah of rated capacity. I’d spec 250Ah here and sleep fine. That difference in mindset is the whole argument in Lithium vs AGM for Boats: Which Wins?, and it’s why a lithium bank that looks small on paper outperforms the lead bank it replaced.

Step 4: Sanity-check against real product sizes

Lithium comes in common building blocks: 100Ah, 200Ah, and 300Ah 12V units. You wire them in parallel to reach your target. For our 250Ah goal, three 100Ah batteries (300Ah) is the clean answer, giving a little headroom for a growing load list.

Here’s how a few liveaboard profiles shake out. These assume 12V and healthy LiFePO4.

Prices move, but in mid-2026 a solid 100Ah marine LiFePO4 runs roughly $250 to $500. That 300Ah example bank lands somewhere around $900 to $1,400 for the batteries alone, before charging gear and wiring.

Step 5: Match the charging to the bank

A bank you can’t refill is a bank that’s always half-empty. Size your charging so it can replace a full day’s use in the daylight or engine hours you actually have.

Our 115Ah-a-day boat needs to put back 115Ah plus a little inefficiency, call it 130Ah. With 400 watts of solar in decent sun that’s easy in summer. In a gray northern winter, that same panel might make 20Ah a day, and you’ll lean on the alternator or shore power instead.

Alternators are the sneaky problem. Lithium accepts charge so greedily that a stock alternator can overheat trying to feed it, which is why a DC-DC charger or an external regulator with a temperature sensor belongs in almost every lithium refit. The wiring, fusing, and bus-bar layout that ties all this together is covered in the Marine Battery Bank Setup Guide.

Step 6: Account for temperature and margin

LiFePO4 hates being charged below freezing. Charging a cold cell plates lithium metal inside it and permanently damages it. If your boat lives anywhere that hits 32°F (0°C), you need batteries with a low-temperature cutoff, an internal heater, or a heated locker.

Capacity also fades slightly in the cold and over years of cycling. I add about 10 percent margin on top of the calculated bank for age, and I round up to whole batteries. That’s how our 250Ah target became a tidy 300Ah of hardware.

The worst call I get is from someone who nailed the summer math and never thought about a January cold snap. Their beautiful new bank refused to charge at the dock, and they assumed it was dead. It was just protecting itself. A little temperature planning up front saves that panic. For general cold-water and cold-weather boating guidance, BoatUS is a reliable place to read up.

Putting your own numbers together

Run the five real steps in order: total your daily amp-hours, pick 1.5 to 2 days of autonomy, multiply, keep the rated size close to the usable target because lithium lets you, then round up to real battery sizes and add a temperature plan. A liveaboard who does this ends up with a bank that fits the boat instead of the forum.

Do the budget first, even a rough one on a napkin. The couple on that 38-footer are on their second full season anchoring out four nights a week, and they’ve run their engine for charging maybe a handful of times. That’s not luck. That’s a bank sized to the way they actually live aboard.