Can You Run Air Conditioning From Batteries on a Boat? DC Aircon, Honestly
Every cruiser in Thailand knows the choice: run the generator all night for air conditioning and listen to it hum until dawn, or shut it down and sweat. For years those were the only options, because conventional marine air conditioning needs 230 V AC power. That has changed. DC air conditioning units now run directly from a boat’s 12, 24 or 48 volt battery bank — and with an honest look at the numbers, a quiet, cool night at anchor is achievable on a surprising range of boats. This guide works through the real arithmetic: what the units draw, what battery bank you need, and where the limits are.
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Why DC-direct beats an inverter
You could run a normal 230 V marine air conditioner through a big inverter, and plenty of boats do. But every watt passes through the inverter’s conversion first, and you lose roughly 10–15% of your precious battery capacity as heat in the process — plus the inverter’s own idle draw, all night. A DC unit skips the middleman entirely: the compressor and fans run natively on battery voltage. The same battery bank simply lasts longer, and there is one less big, expensive box in the system to buy, wire and troubleshoot.
The honest arithmetic: what DC aircon really draws
Using real figures from our DC marine air conditioning range: a 7,000 BTU unit — right for a typical sleeping cabin — draws about 450 W cooling, which is roughly 35 A at 12 V. A 12,000 BTU unit for a saloon draws about 600–720 W: 50 A at 12 V, but only 30 A at 24 V or 15 A at 48 V. Those continuous figures are the worst case: once the cabin is down to temperature the compressor cycles, and real overnight consumption typically settles to somewhere between half and three-quarters of the continuous number, depending on insulation, sun-heated decks cooling off, and how cold you like it.
Work an eight-hour night on the 7,000 BTU unit at 12 V: 35 A at a 60% duty cycle is around 170–200 Ah from a 12 V bank. That is a serious but realistic load: a 400–500 Ah AGM bank (cycled to 50%) or a 250–300 Ah lithium bank (cycled to 80%) carries it with sensible margin. Put your own numbers into our battery bank calculator — it does this arithmetic properly, including your other overnight loads, and tells you honestly what bank you need rather than what you hope you need.
Which voltage: 12, 24 or 48?
The physics is simple: the same watts at higher voltage means fewer amps, thinner cables, and less loss. If you have an existing 12 V house bank and want one cabin cool, a 12 V unit drops straight in — just budget for proper heavy cable over a short run. If you are designing a system from scratch, or want 12,000 BTU or more, 24 V halves the current and 48 V quarters it: a 16,000 BTU unit at 48 V draws just 20 A, which is ordinary cable territory and pairs naturally with the 48 V lithium banks becoming standard on modern cruising boats. As a rule: one small unit on an existing bank — stay at 12 V; a whole-boat DC aircon plan — think 24 or 48 V from the start.
Sizing the unit for a tropical cabin
Undersized aircon runs flat out and never quite gets there; oversized wastes battery. In the tropics the old temperate-climate rules of thumb undersize badly — Thai sun through a deckhead is a different proposition to a marina in Europe. Our free marine A/C BTU calculator sizes it from your cabin dimensions, insulation and sun exposure in two minutes. As a sanity check: a typical double sleeping cabin lands around 7,000 BTU, a saloon 12,000–16,000 BTU.
AGM or lithium behind it?
Nightly air conditioning is the textbook case of daily deep cycling, and that is honestly where lithium earns its premium: deep discharge without complaint, fast recharge from a big alternator or charger the next morning, and steady voltage all night. A well-sized AGM bank absolutely can run DC aircon — sealed AGM is robust, simple and much cheaper per bank — but if you plan to run aircon most nights, size the AGM bank generously and recharge it fully and promptly, or the nightly deep cycling will shorten its life. Our guide to AGM versus lithium for boats works through the decision properly. Either way, the recharge plan matters as much as the bank: 200 Ah out overnight has to go back in somehow, and for most boats that is a combination of alternator, shore or generator charging and solar — not solar alone.
Installation realities worth knowing up front
Three practical points save most of the surprises. First, cable: a 12 V unit drawing 35–50 A needs genuinely heavy cable over even a short run, sized for voltage drop, with a proper fuse or breaker at the battery end — this is the hidden cost of choosing 12 V for bigger units, and exactly why the higher voltages exist. Our marine cable size calculator gives you the correct gauge in seconds. Second, the seawater circuit: these are water-cooled units, so plan a dedicated through-hull with a strainer, a pump matched to the unit, and a clear overboard discharge — in warm Thai seawater, keep the strainer cleaning on the monthly list because growth here is quick. Third, condensate: a cooling unit wrings litres of water out of humid tropical air every night, and that drain has to run somewhere sensible — a sump or overboard, never quietly into the bilge foam.
And a word on solar: people often hope the panels will carry the aircon. Run the honest numbers: 200 Ah out of a 12 V bank overnight is roughly 2.5 kWh, which is a full day’s output from around 600 W of well-mounted panels doing nothing else. On most cruising boats solar is the valuable contributor that shortens generator or engine charging time — not a replacement for it. Plan the recharge as a system and the aircon becomes a nightly routine instead of a battery-flattening gamble.
Two situations flip the equation in your favour. Underway, a healthy alternator covers the aircon draw directly — motoring between anchorages with the cabin pre-cooling is free comfort, and you drop the hook with a full bank. At the dock, shore power through a decent charger carries the load continuously, so the same DC unit serves as your marina air conditioning too — one system for every situation, which is precisely the appeal: no generator to service, no changeover, just the bank doing its job.
Where we come in
Drake Marine supplies self-contained, seawater-cooled DC marine air conditioning units from 7,000 to 16,000 BTU in 12, 24 and 48 V — with reverse-cycle heating, full specification sheets and installation drawings with every quotation, and honest sizing advice before you commit. We supply the battery banks too, so the whole system gets sized as one. Start with the BTU calculator, put the unit and your hours into the run-time and battery calculator for the amp-hours, bank, solar and cable, read the installation planning guide, then talk to us at hello@drakemarine.com — we would rather size it right than sell it big.
