Every cooling project on a small boat runs into the same wall. The comfortable answer is a real air conditioner. The affordable answer is moving air around. And the gap between them, once you price it honestly, is not a few hundred dollars — it's the cost of a different boat.
Step one: fans
Before anything else, we're putting 12-volt fans in — that's the next video.
This isn't the consolation prize. A fan doesn't cool the cabin, it cools you, by moving air across your skin so sweat can actually evaporate. On a humid night that's worth several degrees of perceived temperature, and it costs almost nothing to run. A decent 12-volt cabin fan draws a fraction of an amp. You can run several of them all night and barely dent the house bank — which means that once the solar array is up, fans are effectively free.
Good ventilation is the other half of it: hatches, a wind scoop, a dorade or two, and moving the hot air out rather than just stirring it. On a boat this size, fans plus airflow handle most of the season. It's only the genuinely brutal afternoons that need refrigeration.
Why we passed on 12-volt and solar air conditioning
This is the part we want to be honest about, because DC air conditioning is having a moment and the marketing makes it sound solved. It works. It's also very expensive, in a way that isn't obvious until you add up the whole system rather than the unit.
Start with the box itself. A 12,000 BTU 12 volt DC marine unit runs about $5,800. A comparable 115-volt self-contained unit is roughly $2,400. Same cooling, less than half the price — the DC premium is real and it's steep.
Then look at what has to sit behind a DC unit to make it useful away from the dock. Practical Sailor's write-up on running marine air conditioning off batteries is blunt about the infrastructure: for a 16,000 BTU system they recommend at least 600 Ah of lithium — 800 Ah preferred, and note you'd want 2,000 watts or more of solar and a 5,000 watt inverter. With lead-acid you'd double the battery figure. And all that buys you two or three hours of cooling on a sunny day.
| 12V DC + solar | 115V + generator & shore power | |
|---|---|---|
| The unit | ~$5,800 (12,000 BTU DC) | ~$2,400 (11,000 BTU, 115V) |
| Battery bank | 600–800 Ah lithium | Whatever you already have |
| Solar | 2,000 W+ | Not required |
| Inverter | 5,000 W | Not required |
| Runs at anchor | Yes — silently, for a few hours | Yes — noisily, as long as there's fuel |
| Runs at the dock | Yes | Yes |
Prices checked August 2026. The left-hand column isn't wrong — it's genuinely the nicer system. It's just that the electrical infrastructure alone costs more than we paid for a lot of this boat, and the solar half of it is five times the array we're currently building.
The honest version
On a 42-foot boat with a big lithium bank and a hard top covered in panels, DC air conditioning makes a lot of sense. On a 29-foot picnic cruiser, chasing it means rebuilding the entire electrical system in service of one appliance. That's not a refit, that's a different boat.
So: if we've gotta have it, we run the generator
The rule we settled on is simple. Get an air conditioner we can run on shore power or with a generator — and don't tax the 12-volt system with it at all.
That decision does a lot of work. It lets us buy the cheaper, more common 115-volt unit. It leaves the house bank entirely alone, which means the solar array stays sized for the things solar is actually good at — lights, instruments, the Raspberry Pi stack, refrigeration, fans. And it keeps the cooling problem from swallowing every other project on the boat.
The splitter and the converter
To make it work in both places, we picked up a splitter and a converter so the same air conditioner can be fed from the dock pedestal or from the generator, depending on where we are and what's available. Dock when there's a dock, generator when there isn't.
The specific gear and how it's wired goes in the video — that's the part worth watching rather than reading.
If you're copying this, do the shore power part properly
Anything that feeds your boat's AC system from two possible sources needs to make it impossible for both to be connected at once — that's what a proper transfer switch or selector is for, and it's not a place to improvise. Backfeeding a shore cord from a generator is genuinely dangerous, both to you and to anyone working on the pedestal. Fusing, grounding, and an ELCI on the shore inlet all matter here. If you're not certain of your work, this is the system to have a marine electrician look over.
What we gave up
Being straight about the downsides, since a generator is not a free lunch:
- Noise. A generator running at anchor is exactly as antisocial as it sounds, for us and for everyone anchored near us. This is the real cost.
- Fuel and storage. Gas cans on a small boat are their own problem — stowage, fumes, and a jug you have to keep refilling.
- Not an all-night solution. We're not running it while we sleep at anchor. It's for taking the edge off a brutal afternoon, or for the dock.
- One more thing to maintain. Small engines that sit unused are small engines that don't start.
Which is exactly why the fans come first, and why we expect to use them ten times as often as the air conditioner.
Skipper's verdict
Cool the people, not the boat. Fans and airflow do that for almost nothing, and they're the upgrade you'll actually use. Keep the air conditioner for the days that earn it, run it off something that isn't your house bank, and spend the money you saved on the systems that make the boat go.
What's next
- The 12-volt fan install — placement, wiring, and what actually helps at anchor. Next video.
- The AC unit going in: placement, venting, drainage, and the shore power side of it.
- Running it in anger and reporting back on whether the generator plan holds up in practice.
The write-up that started this project is on Substack: Sailboat Air Conditioning. The install videos go up on YouTube as we work through it.
Sources
- Skipper Scott on Substack — Sailboat Air Conditioning (the post that kicked this off)
- Mabru Power Systems — SC12DC 12,000 BTU 12V DC marine air conditioner ($5,800)
- MarinAire — 11,000 BTU 115V self-contained marine air conditioner (~$2,398; 1,250 W, 7.9 A running, 14 A startup)
- Practical Sailor — Can You Run a Marine Air-Conditioner on Battery Power? (600–800 Ah LFP, 2,000 W+ solar, 5,000 W inverter)
All prices checked August 2026 and will drift. Treat them as a shape, not a quote.