How to Size a House Battery Bank: Load List, Usable Capacity and Charging

Walk the boat and the breaker panel and write down everything that uses 12-volt power: lights, electronics, VHF, fridge or cooler, pumps, fans, stereo, USB outlets, windlass, trim, livewell, autopilot, and anything that runs through an inverter. Battle Born suggests the same starting point: list every load on the boat, then work out how much each one uses in a day.

Get the amps for each device from its manual or label (if only watts are given, divide by about 12 for a 12-volt device). Better still, measure: with a DC clamp meter around the device's positive wire, or by watching your battery monitor's amps reading while you switch one thing on at a time. Battle Born describes this method: switch everything off, note the baseline ("vampire") draw, then turn each item on in turn and note how much it adds.

Don't forget the small always-on loads: stereo memory, monitors, radio standby, CO and bilge alarms. They are small but run 24 hours a day.

For each load, estimate how many hours it runs in a typical day of the way you actually use the boat. Use average use: a fridge compressor cycles on and off, an anchor light runs all night, the windlass runs a few minutes. Blue Water Sailing advises being conservative. If you anchor out and also cruise, Battle Born suggests working out an "at anchor" day and an "underway" day separately and using the bigger of the two.

Multiply amps by hours for each load and add them up. A simple example (illustrative only; use your own numbers):

AC appliances running through an inverter draw far more from the batteries than people expect. Blue Water Sailing's rule for a 12-volt system: divide the appliance's watts by about 12.5 volts, then add about 10 percent for inverter losses, then multiply by hours of use. A coffee maker that runs briefly can be manageable; a space heater or air conditioner on an inverter usually is not. See inverter safety.

If you have a battery monitor with a shunt, do a real test: with the charger off, use the boat normally for a day and read the amp-hours consumed. Battle Born describes this as the second, easier audit method. Compare it with your paper total; real life is usually higher.

A battery's rated amp-hours are not all usable:

• Flooded, AGM and gel lead-acid: Soundings' rule is to never discharge the house bank below about 50 percent, so the bank must be at least twice the amp-hours you use between charges. Blue Water Sailing gives the same rule of thumb for long battery life.
• Lithium (LiFePO4): BoatUS notes lithium batteries can be discharged much deeper without damage, so more of the rating is usable. Many owners plan on using most, but not all, of the rated capacity (around 80 percent is a common planning figure); your battery maker's datasheet and warranty give the real limit and the BMS cut-off.

How deep you cycle also decides how long the bank lasts. Victron, for example, publishes its LiFePO4 "Smart" batteries' design life at three depths: about 2,500 cycles when discharged 80 percent each time, 3,000 at 70 percent and 5,000 at 50 percent. Other makers rate theirs differently, so compare datasheets at the same depth of discharge, not just the headline cycle count. A slightly bigger bank that you only half-empty can outlive a smaller one you run down every day.

So in the example, 95 Ah a day needs roughly 190 Ah of lead-acid, or roughly 120 Ah of lithium on that planning figure, before any reserve.

Then add margin:

• Charging gaps: Soundings suggests three times the expected load for extended cruising, because recharging is harder and batteries often don't get back to full.
• Partial charging: lead-acid batteries charged by an alternator on short runs often don't reach full charge, so the "top" of the bank isn't always there.
• Aging and cold: capacity drops as batteries age and in cold weather.
• Growth: Battle Born suggests sizing up about 30 percent to leave room for loads you will add later.

A bank is only as good as its charging. List your sources and what each can realistically return per day:

• Engine alternator: depends on alternator size, rpm and how long you run. Small outboards return little; see battery isolator types. Mercury points out that the rated output on a spec sheet is a maximum; what actually reaches the batteries is less, after the engine's own loads, and output climbs with rpm, so hours at idle or trolling speed return much less than the rating suggests.
• DC-to-DC charger for a lithium bank: limited to its rating while the engine runs; see the DC-to-DC charger guide.
• Shore charger: at the dock, sized to the bank. Boating Magazine notes a lithium bank usually needs a lithium-specific shore charger to reach full between trips.
• Solar and wind: Soundings says they can be deducted from the consumption total; see solar panel care for keeping panels producing.
• Generator: for big boats; see generator load management.

If your charging can't replace a day's use in a reasonable time, a bigger bank only delays the problem. Sometimes the right answer is more solar or a bigger charger, not more batteries.

The cheapest amp-hour is the one you don't use. Soundings points to LED lighting and newer, more efficient refrigeration compressors as the biggest wins. Turn off unused electronics at the panel, and run big AC appliances only on shore power or the generator.

With your target capacity:

• Don't mix old and new batteries, or different types, in one bank. Blue Water Sailing calls adding new batteries to old "bad science," because the old ones pull the new ones down.
• Fit the physical space, weight limits and ventilation you have. Lead-acid weighs much more per usable amp-hour than lithium.
• Size every cable, fuse and switch for the new bank, and set every charger to the new battery type. A bank upgrade is a system change, not a battery swap.

Log your load list, daily amp-hours, chosen bank size and charging sources in BoatMend. After the first season, compare with what the battery monitor shows and adjust. Re-run the numbers whenever you add something big, like a fridge, inverter or stereo amplifier (see stereo amplifier battery bank).