Understanding your marine house bank
The direct answer for a typical cruising boat
A boat using 1,200Wh a day for 2 nights at anchor needs about 400Ah with a 50% depth of discharge AGM bank, or about 250Ah with an 80% depth of discharge LFP bank, both at 12V. The chemistry choice alone changes the required Ah rating by well over 100Ah for the same usable energy, which is why picking chemistry before sizing matters. The US Coast Guard's electrical compliance guidelines has the current version, so check there before you rely on a summary.
Why the house bank and starter battery are always separate
The engine starter battery is built for a short, high-current burst to crank the engine and otherwise sits mostly idle between starts. The house bank is built for sustained, repeated discharge over hours or days running cabin systems, a fundamentally different duty cycle. Combining them into a single bank risks draining the battery you need to start the engine, which is why nearly all marine electrical systems keep them isolated through a battery switch, isolator, or combiner. Panel wattage means little if the inverter cannot carry your loads. The solar inverter size calculator in our Inverters & Equipment section can help you size it for the biggest appliance you run.
AGM versus LFP for a house bank
AGM batteries remain a common choice on boats for their lower upfront cost, broad compatibility with older charging equipment, and proven tolerance of the constant vibration and motion boats experience underway. LFP batteries offer a much higher usable depth of discharge and significantly lighter weight for the same capacity, along with a longer cycle life, but typically cost more upfront and may need a charging system upgrade to work correctly with LFP-specific voltage profiles. The lithium vs lead-acid comparison tool in our battery chemistry storage tools goes deeper into this trade-off outside the marine-specific context. If you want to check this estimate against a second method, the quick 5-minute system size estimator in our System Sizing section works from your daily load and peak sun hours.
Heat and storage location affect real capacity
Batteries stored in a warm bilge or near the engine compartment run hotter than batteries in a ventilated, temperature-controlled space, and sustained heat gradually reduces both usable capacity and battery lifespan. Boats cruising in consistently hot climates or storing batteries near heat sources should plan for some capacity derate rather than assuming the manufacturer's rated Ah holds indefinitely. The battery lifespan estimator in our maintenance and lifespan tools factors heat exposure into its projections if you want a longer-range estimate.
What to check once you have a house bank size
With the amp-hour target set, confirm the panels can keep the bank charged using the boat marine solar sizing calculator, and check the inverter sizing calculator if you're running AC appliances off the bank. Battery economics referenced throughout trace back to the National Laboratory of the Rockies Annual Technology Baseline, and the US Coast Guard's electrical compliance guidelines cover the wiring and isolation standards this kind of setup needs to meet.