12,000 BTU marine air conditioner sizing
The most-sold size in the country. A 32–38 ft boat in the subtropics, or a bigger boat in Hobart. Also the size the DC units are built around.
Boat Size and Climate for 12,000 BTU Air Conditioning
12,000 BTU is 3.5 kW of cooling. Run the sizer backwards and that is the right answer for a boat of roughly this length — which moves a long way depending on the water the boat sits in.
| Where it lives | Boat length | What that is |
|---|---|---|
| Tropics — Cairns, Darwin | about 32 ft | 9.8 m. Thirty-degree seawater takes 10–15 % off the label before the unit has cooled anything, so the same box does a smaller boat. |
| Subtropics — Brisbane, Perth | about 34.5 ft | The reference case for most figures on this site. |
| Temperate — Sydney, Hobart | about 38 ft | 11.6 m. Cooler water means the unit makes close to its rated output, so it stretches further. |
These are the sizing calculator solved for length instead of capacity, at a typical cabin count for each size. A dark hull or a lot of glass adds about 10 % each, which is a real difference at this capacity.
12,000 BTU Marine Air Conditioning Power and Cost
About 5 A at 240 V with the compressor running. That leaves 5.0 A of a 10 A marina lead, or 10.0 A of a 15 A one, for everything else aboard.
Around $2.28 for a night, at the typical Australian tariff and the duty cycle a real boat sees rather than the compressor running flat out.
A Self-contained job on the boat this capacity suits — supplied and fitted, ex GST, including the seawater side and the electrical work.
Off the shore lead this capacity is a different question entirely. A unit this size on batteries wants a serious bank and, in most cases, a DC unit rather than an inverter — the running costs page works through both.
12,000 BTU Marine Air Conditioner Options
Mabru SC12DC, Webasto BlueCool S13. The closest on the site is the Mabru SC12DC at 12,000 BTU — near enough that the sizing decision is really about the boat, not the box.
Choosing a Higher or Lower Cooling Capacity
Oversizing is the more common mistake and the more expensive one. A unit too big for the space cools the air fast, satisfies the thermostat, and shuts down before it has removed much water — so the cabin ends up cold and clammy, and the compressor short-cycles itself to an early death. Undersizing simply runs constantly and never quite gets there, which is uncomfortable but not destructive. When the arithmetic lands between two sizes, take the smaller one.