Mabru 12V DC Marine Air Conditioning: Running AC on Your Batteries
A quiet anchorage in August, no generator, and a cabin that is still 85 degrees at midnight. For years the only answers were a generator, a shore power cord, or a fan and a wet towel. Mabru Power Systems built a different answer: self-contained marine air conditioners that run directly on 12 volt DC, straight from your house battery bank.
They work. But they only work well on a boat whose electrical system is designed for them. This post covers the Mabru 12V lineup, how many BTUs your boat actually needs, the real amp draws, how much battery and solar you need, and what a correct installation looks like.
Who Is Mabru?
Mabru Power Systems is a Florida manufacturer that has been building marine air conditioners for more than 35 years. Their DC line is self-contained and reverse cycle. One compact unit holds the compressor, evaporator coil, blower, seawater condenser and reversing valve, so it both cools and heats. Every 12V unit ships with its own 12V raw water pump (350 GPH), so nothing in the system needs AC power.
The Mabru 12V DC Lineup
| Model | Cooling (BTU/h) | Heating (BTU/h) | Cooling draw (A @ 12V) | Heating draw (A @ 12V) | Typical space |
|---|---|---|---|---|---|
| SC5DC | 5,000 | 5,500 | 17 to 27.9 | 20 to 33.8 | V-berth or aft cabin |
| SC7DC | 7,000 | 7,200 | 18 to 29.5 | 21.2 to 38.3 | Larger cabin, or two small cabins with a Y duct |
| SC12DC | 12,000 | 12,300 | 22 to 44 | 28 to 48 | Salon or main cabin |
| RadAir 12V dome | 12,000 | 12,300 | 22 to 44 | 28 to 48 | Hardtop or cabin roof mount, no interior space used |
Figures from Mabru's published specifications. Mabru also builds 48V versions of the 5,000, 7,000 and 12,000 BTU units (more on that below).
The RadAir is the unusual one: the whole 12,000 BTU unit lives in a radar dome style housing mounted on a hardtop or cabin roof. On a boat where every cubic foot below is spoken for, that matters.
How Many BTUs Do You Need?
Boat length is a poor guide to air conditioning. Two 40 foot boats can need very different capacity: a sailboat with small portlights and most of its living space below the waterline, and a trawler with a deckhouse salon wrapped in glass. What matters is the floor area of each space you want to cool, how high in the boat it sits, and how much sun comes through the windows.
Step 1: Measure each space
Measure the length and width of the cabin sole (the floor) in each space you want to cool, and multiply to get square feet. Size each cabin on its own. One unit ducted to several spaces only works when the spaces are small and next to each other.
Step 2: Pick a load factor
The standard marine method, used in Dometic's air conditioning selection guide, multiplies floor area by a load factor based on where the space sits and the climate you boat in:
| Space | Temperate (New England) | Tropical (Florida, Caribbean) |
|---|---|---|
| Below deck: hull slopes in, small portlights | 60 BTU/h per sq ft | 90 BTU/h per sq ft |
| Mid deck: main deck level, small or shaded windows | 90 BTU/h per sq ft | 120 BTU/h per sq ft |
| Above deck: large windows, direct sun | 120 BTU/h per sq ft | 150 BTU/h per sq ft |
A common cross-check by volume is 14 to 19 BTU/h per cubic foot of cabin.
Step 3: Do the math
New England examples:
| Space | Floor area | Category | Load | Mabru fit |
|---|---|---|---|---|
| V-berth, 35 ft sailboat | 7 x 6 ft = 42 sq ft | Below deck | about 2,500 BTU/h | SC5DC |
| Aft cabin, 40 ft sailboat | 8 x 7 ft = 56 sq ft | Below deck | about 3,400 BTU/h | SC5DC |
| Main salon, 40 ft sailboat | 11 x 9 ft = 99 sq ft | Below deck | about 5,900 BTU/h | SC7DC |
| Deckhouse salon, 40 ft trawler | 12 x 10 ft = 120 sq ft | Above deck | about 14,400 BTU/h | More than one SC12DC delivers |
Take that same trawler salon to Florida and the load climbs to 18,000 BTU/h. A system sized for a Rhode Island summer can be undersized in the tropics.
Rough guide by boat size
Use this for a first look only. Your own measurements always win.
| Boat | Spaces usually cooled | Typical 12V setup (temperate climate) |
|---|---|---|
| Under 30 ft cuddy or pocket cruiser | One cabin | One 5,000 BTU unit |
| 30 to 36 ft sailboat | V-berth and salon | One 7,000 BTU unit with a Y duct, or a 5,000 plus a 7,000 |
| 36 to 45 ft sailboat | Salon plus one or two staterooms | 7,000 to 12,000 BTU in the salon plus a 5,000 per stateroom |
| 36 to 45 ft trawler or express cruiser | Glass salon plus staterooms | 12,000 BTU or more in the salon plus the staterooms. Total load often points to 48V. |
Add up every unit you expect to run at once. Two SC12DC units cooling together can pull close to 90 amps at 12V. At that point the battery bank, the wiring and the charging system all get expensive, and a 48V system starts to make more sense.
Do not oversize
Bigger is not better on a boat. An oversized unit cools the air quickly and shuts off before it pulls the moisture out, which leaves the cabin cold and clammy. Mabru's variable speed compressors soften this by slowing down, but on a 12V system oversizing also has a direct battery cost. Size to the load, then shrink the load with sun shades, window covers and hatch covers. Shading your glass is the cheapest cooling you will ever buy.
Why Run AC on DC Instead of Through an Inverter?
You can run a conventional 115V marine air conditioner from batteries through an inverter. The problems are efficiency and surge.
- No conversion loss. An inverter typically wastes 8 to 15 percent of the energy turning DC into AC. A DC unit uses battery power directly.
- No startup surge on the inverter. A fixed speed AC compressor pulls a large inrush current every time it starts, which forces you into a bigger inverter than the running load requires. Mabru's DC compressors ramp their speed up and down, which is why the amp draw is listed as a range.
- Variable speed means continuous drying. Instead of slamming on and off, the unit slows down as the cabin approaches set point. It runs longer at low power and keeps pulling moisture out of the air. On a boat, dry air is half of comfort.
- The raw water pump is DC too. No hidden AC load anywhere in the system.
The Honest Part: Battery Math
This is where 12V air conditioning projects succeed or fail. Air conditioning is the largest load most boats will ever ask a battery bank to carry.
Amps times volts equals watts, and amps times hours equals amp hours. At 12.8 volts nominal:
- SC5DC cooling: 17 to 28 amps, about 220 to 360 watts
- SC7DC cooling: 18 to 30 amps, about 230 to 380 watts
- SC12DC cooling: 22 to 44 amps, about 280 to 560 watts
Now run it overnight. These are rough 8 hour estimates assuming the unit averages near the middle of its range:
| Unit | Average draw (assumed) | 8 hours | Lithium bank to stay above 20% |
|---|---|---|---|
| SC5DC | 22 A | about 175 Ah | 300 Ah or more |
| SC7DC | 24 A | about 190 Ah | 300 Ah or more |
| SC12DC | 33 A | about 265 Ah | 400 Ah or more |
Hot, humid nights with the hatches cracked push draw toward the top of the range. A cool night in a well sealed cabin pulls it toward the bottom. Your other loads (refrigeration, electronics, lights, pumps) still need their share of the bank on top of this.
Two conclusions follow:
- Lithium is effectively required. Lead acid and AGM banks should not be regularly discharged past about 50 percent, and their voltage sags under a 30 to 45 amp continuous load. A lithium (LiFePO4) bank delivers most of its rated capacity and holds voltage under load. Our lithium retrofit guide covers what a proper conversion involves.
- You need a way to put it back. Pulling 200+ amp hours out every night means putting 200+ amp hours back every day. That comes from solar, a high-output alternator with an external regulator or a DC to DC charger, or shore power.
Before you buy anything, measure what your boat actually uses. Our free Boat Power Usage Audit sheet lets you list every load and see where an air conditioner fits in your daily energy budget.
Can Solar Run It During the Day?
Partly, and that is the best part of the story. Air conditioning load peaks when the sun is strongest. A well placed 400 watt array at midday can deliver roughly 20 to 25 amps into a 12V bank in good sun. That covers a large share of a 5,000 or 7,000 BTU unit while it runs, so the batteries are not draining all afternoon and are fuller when the sun goes down.
Mabru's own installation manual makes the same point: owners increasingly run air conditioning on solar during the day and on lithium batteries at night. If you are planning solar alongside air conditioning, start with our Marine Solar Design Guide and How Much Solar Do I Need on My Boat?
What About Heat?
All Mabru DC units are reverse cycle, so they heat by pulling warmth from the seawater. Heating draw is higher than cooling draw (up to 48 amps on the SC12DC). Reverse cycle heat works well in New England spring and fall, and loses effectiveness as water temperature drops toward the low 40s Fahrenheit. It is a great shoulder season comfort tool, not a winter liveaboard heater.
Installation: What Has to Be Right
A self-contained unit looks simple. The details are what make it quiet, dry and reliable.
Location
- Never in the bilge or engine room. Mabru's manual is explicit about this, because the blower can pull fumes and carbon monoxide into living spaces.
- The compartment must be sealed, with return air coming only from the cabin you want to cool.
- Low and central is best, with room for service access and the condensate drain.
Air side
- Respect the minimum duct size (4 inch for the SC5DC, 5 inch for the SC7DC) and the minimum return air grille area. Undersized ducts and grilles are a leading cause of iced coils and weak performance.
- Keep duct runs short, with as few bends as possible.
- One air filter per unit, in the unit or in the return grille, and keep it clean.
Seawater side
- Dedicated thru-hull with a seacock and a raw water strainer.
- Mount the included 12V pump below the waterline so it stays primed when heeled or underway.
- Put the discharge where you can see it. If seawater flow stops, the unit will fault, and a glance at the discharge stream is the quickest check.
- Lead the condensate pan drain to a sump, not the bilge.
DC wiring
This is a 40 to 50 amp continuous load. Wire it like one.
- Size the conductor for voltage drop, not just ampacity. We size compressor loads for 3 percent drop. Example: an SC12DC at 48 amps (heating), 10 feet from the distribution panel (20 feet round trip), needs 4 AWG tinned marine wire. 6 AWG is not enough.
- Overcurrent protection at the source per ABYC E-11, sized to Mabru's installation manual.
- Use a dedicated circuit, and make sure your battery monitor shunt sees the load so you can track it.
- If your main DC distribution is already marginal, adding a 45 amp continuous load is the time to fix it.
12V or 48V?
Mabru offers 48V versions of the same 5,000, 7,000 and 12,000 BTU units. At 48 volts, the same wattage needs one quarter of the current, so wiring shrinks and voltage drop stops being a design problem. If your boat already has a 48V house bank (common on electric propulsion repowers and newer lithium systems), or you plan to run two or more units, 48V is usually the better design. On a typical 12V boat cooling one or two cabins, 12V keeps things simple.
Is a Mabru 12V Unit Right for Your Boat?
Good fit
- Sailboats and trawlers roughly 30 to 45 feet cooling one or two cabins
- Boats without a generator, or owners who want silent nights at anchor
- Boats with, or planning, a lithium bank of 300 Ah or more plus solar
- Center consoles and express boats with a cuddy, where the RadAir saves interior space
Not the right tool
- Larger yachts needing 30,000+ BTU across multiple zones. That is generator or 48V territory.
- Boats with a small lead acid bank and no plan to upgrade
- Owners expecting it to heat the boat through a New England winter
Mabru 12V Air Conditioning FAQ
How long can a Mabru 12V air conditioner run on batteries?
It depends on bank size and conditions. Mabru advertises the SC07DC running at least eight hours on a battery bank. As a planning number, expect 175 to 265 Ah over an 8 hour night depending on the model, which points to a lithium bank of 300 to 400 Ah or more.
What size air conditioner does my boat need?
Measure the floor area of each space and multiply by a load factor. In New England, use about 60 BTU/h per square foot for cabins below deck, 90 for main deck spaces with small windows, and 120 for spaces with large windows in direct sun. In the tropics, add 30 to each. Size each cabin separately rather than by boat length.
Do I need an inverter?
No. The unit and its raw water pump run directly on 12V DC.
Does it heat as well as cool?
Yes. All Mabru DC units are reverse cycle heat pumps. Heating works best when seawater is above the low 40s Fahrenheit.
Can I install it myself?
A handy owner can handle mounting and ducting. The thru-hull, seawater plumbing and high current DC wiring are where mistakes get expensive or dangerous. At minimum, have a qualified marine electrician check the wiring and overcurrent protection.
What refrigerant does it use?
The 12V DC units use R-134a.
Plan It Before You Buy It
A 12V air conditioner is not just an appliance. It is a system decision that touches your battery bank, your charging sources and your DC distribution. Get those right and you get a cool, dry, silent cabin at anchor. Get them wrong and you get a dead battery bank at 3 a.m.
Sea and Land Yacht Works designs and installs lithium, solar and DC power systems throughout Rhode Island, Connecticut, Massachusetts and Long Island. If you are considering 12V air conditioning, we will size the whole system with you before you spend a dollar on hardware.
Call 401-753-7479 or reach out through our contact page.
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