How to Size a Generator for Marine Air Conditioning

On/ Off AC Marine Air Conditioners
(Powered by Shore Power or Generator)

Pros

• Best fit for real world boat usage
  Most recreational boats spend the overwhelming majority of their life docked and connected to shore power. Engine hour data commonly shows only a few hundred operating hours over 10 to 20 years. AC powered air conditioners operate in their intended environment for well over 95 percent of total ownership time.

• Lower initial cost
  On off AC marine air conditioners typically cost significantly less than DC systems. In many cases the purchase price is roughly half that of a comparable DC variable speed unit. Installation costs are also lower because they do not require large battery banks or major electrical upgrades.

• Lower total cost of ownership
  No need for oversized battery banks, heavy DC cabling, upgraded alternators, battery management systems, or complex charging infrastructure. Shore power
  provides cooling at a much lower cost than storing and cycling energy through batteries.

• System simplicity and proven reliability
  AC on off systems are mature and well understood with decades of real world use. They rely on simple electrical and mechanical components with fewer failure modes and fewer proprietary electronics.

• Lower electrical and fire risk
  AC systems operate at much lower current levels than 12V DC systems for the same cooling output. This reduces sensitivity to connection resistance corrosion and termination quality. Protection devices and wiring practices are
  standardized and widely understood.

• Unlimited practical runtime
  Shore power allows continuous operation without concern for depletion.
  Generator operation supports long stays away from the dock without redesigning
  the vessel electrical system.

• Ease of service and long term support
  Replacement parts are widely available. Most marine HVAC technicians are
  familiar with AC on off systems. Troubleshooting is faster more predictable and
  less expensive.

Cons

• Starting current
  Fixed speed compressors have higher inrush current and may require soft start devices on small generators.

• Generator use away from shore
  Cooling at anchor typically requires generator operation which introduces noise and maintenance. This aligns with how most boats are already configured and used.

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12V ~48V DC Variable Speed Marine Air Conditioners
(Powered by a Battery Bank)

Pros

• Quiet operation for short at anchor periods
  Allows limited duration cooling without running a generator. This benefit applies mainly to overnight anchoring.

• Low starting surge
  Variable speed compressors reduce inrush current compared to fixed speed AC compressors.

Cons

• Much higher initial cost
  DC variable speed air conditioners typically cost close to double the price of comparable AC on off units. When combined with the required battery bank charging upgrades cabling and protection hardware the initial system cost can be substantially higher.

• Limited relevance to actual usage
  Battery powered air conditioning is typically useful for only about 0.5 to 2 percent of total ownership time over a 10 year period. For the remaining 98 percent of use the DC system provides no advantage.

• Large battery bank requirement
  A typical 16,000 BTU DC unit drawing approximately 120 amps at 12V requires about 1,500 amp hours of battery capacity to run for 12 hours. This equates to roughly 350 to 465 pounds of lithium batteries or close to 1,000 pounds of AGM batteries. This weight is carried at all times regardless of whether battery air conditioning is used.

• Permanent fuel efficiency penalty
  The added battery weight increases displacement and fuel burn. Even a 1 to 2 percent fuel penalty applies every time the boat is underway while the battery air conditioning benefit applies only occasionally.

• High current low voltage electrical system
  12V systems require 10 to 20 times more current than AC systems for the same cooling output. This demands very large conductors short cable runs flawless crimps precise torque and strict corrosion control. Small increases in resistance can result in significant heat buildup.

• Increased fire sensitivity
  High current DC systems are more vulnerable to connection related heating. DC arcs are harder to extinguish than AC arcs and require strictly DC rated protection devices. Battery banks can deliver extremely high fault current if protection is inadequate.

• Battery life and replacement cost
  Batteries are consumable assets. Even lithium batteries degrade with time temperature and cycling while AGM batteries often require replacement within a decade. Over a 10 year ownership cycle battery replacement cost can become a major operating expense and frequently erases any marginal efficiency gains.

• Electronics complexity and poor service economics
  DC variable speed systems rely heavily on complex proprietary electronics including inverter drives control boards sensors and communication modules. Qualified technicians capable of diagnosing these systems are scarce and expensive. Troubleshooting often takes many labor hours and may require trial board replacement rather than definitive diagnosis. The marine environment heat humidity salt air vibration and condensation shortens electronic lifespan. Replacement controllers are often non repairable and can cost a large fraction sometimes close to half of the original system price before labor. Post warranty repairs are unpredictable costly and often difficult to justify economically.
  
  
• Fixed speed seawater pump limits efficiency gains
  Most installations use constant speed seawater pumps. Pump power consumption is largely independent of compressor speed and reduces real world efficiency advantages often attributed to DC systems.

Final takeaway

For the vast majority of recreational boats on off AC marine air conditioners powered by shore power or a generator offer lower initial cost lower long term ownership cost greaterreliability easier service and better alignment with real world usage. While12V DC variable speed air conditioning can provide a niche benefit for quietovernight anchoring that benefit applies to a very small portion of totalownership time and comes at the expense of significantly higher cost greater complexity added weight permanent fuel penalties higher service risk and battery replacement expense. In practical marine applications Group A deliversfar more advantages with far fewer compromises making it the preferred solution for most boats and most owners.