AirCon on Board Without a Generator – Pluses & Perils of the New DC Boat

Increasingly sophisticated power generation, storage, and management options make onboard electrical systems more efficient and versatile than ever. BoatHowTo expert Nigel Calder explains the (almost) limitless possibilities of today's battery and alternator technology.

A previous version of this article was initially published in Professional BoatBuilder Magazine.

If you want to learn how to install and maintain a safe and reliable DC system on your boat that covers your comfort needs, check out our Boat Electrics 101 online course.

So what if you want to enjoy the same energy-intensive lifestyle afloat as at home, without running a generator 24/7? It can be done. A constantly improving array of equipment and technologies make this possible for just about any boat. Assembling the available pieces to create an efficient and trouble-free energy system is not rocket science, but it requires attention to detail, a methodical approach, and balancing numerous variables against one another. In this article we give an overview of the process. If you want to learn more about the topic, we recommend you check out our modules on Alternators, Generators and Optimized Energy Systems which are part of our Advanced Marine Electrics program.

The Energy Calculation

The starting point is a calculation of the energy needs. If you have signed up to our Boat Electrics 101 classes, you can use our BOATHOWTO Boat Electrics planner, which we developed specifically to match the factors described in this article. But you can also  find various free energy calculators online, such as the one from Ocean Planet Energy. Primarily we are looking for two key numbers: the cumulative energy load over time, which is expressed in kilowatt-hours (kWh); and the maximum peak (short-term) load the system may see, expressed in amps.

The cumulative energy number is commonly calculated for a worst-case 24 hours (i.e., those days when the energy needs are at their highest). The 24-hour calculation is then adjusted to reflect the maximum cumulative energy requirement (kWh) between engine runtimes (either the propulsion engine/s or generator runtimes), or between dockside charging opportunities. With sailboats, especially those loaded with navigational electronics, an autopilot, and radar, the worst case is typically when under sail. With powerboats, which have a continuous source of energy from the alternator when under way, it is at anchor. The worst-case number will drive the rest of the design process.

The bulk of the daily energy consumption is likely to come from low- to medium-draw devices running for extended periods, notably refrigeration, lights, navigational electronics, and above all, air-conditioning.

Air Conditioning on a Boat: A major consumer of energy

For older air-conditioning units, in the absence of better data, assume every 16,000 Btu of rated output creates a 1 1⁄2 kW load, and then estimate how many hours a day, or overnight, the air-conditioning runs.

Like other increasingly complex battery options, this substantial bank of modern Lithionics lithium-ion batteries is fitted with an external battery management system (the gray box above the batteries).

Multiply the kW load by the hours to derive the total energy consumption in kWh. For example, over a 24-hour period a 16,000-Btu unit running 50% of the time (the rest of the time it has cycled to “off ”) equates to an energy consumption of (1 1⁄2 kW x 12 hours) = 18 kWh. If the unit is rated at less than 16,000 Btu, reduce the load proportionately; e.g., an 8,000-Btu load is (1 1⁄2 kW x 0.5) = 0.75 kW. If more than 16,000 Btu, increase the load proportionately; e.g., a 40,000- Btu load is (1 1⁄2 kW x 2.5) = 3.75 kW. Once again, multiply the load by the hours the unit is running. Note that the resulting load and energy calculations will be conservative.

The author tests battery volts and amps on a large (14-kWh) bank of thin plate pure lead (TPPL) batteries operating under a 1C load. (Charge acceptance rate, CAR or C-rate, is explained here)

Newer air-conditioning units frequently have variable-speed compressors, which are significantly more efficient than the fixed-speed compressors on units more than 10 years old. These newer compressors often run more hours but at much lower and variable loads, so a fixed load number cannot be applied. You’ll need to estimate the average load and runtime. If this is a retrofit (as opposed to a new build) and the worst-case air-conditioning loads can be simulated, a really useful tool is a portable watt-hour meter, which will accurately measure the cumulative energy use.

Short-term high-amp loads

On most boats there are some short-term, high-load, devices such as electric windlasses and bow and stern thrusters. Briefly running these devices consumes comparatively little energy (kWh), and as such has little impact on the overall energy budget. But it’s important to note their peak potential short-term power demands (in amps) on the system. This might be, for example, when a bow and stern thruster or a bow thruster and a windlass are used simultaneously. This peak-amp number has design implications that are sometimes overlooked, with negative consequences, especially if lithium-ion batteries are used in the system.

Do I Need a Generator?

If daily energy needs are above 3 kWh–4 kWh, conventional wisdom has long held that a generator will be needed, either running a few hours a day or, in the case of higher energy needs, especially air-conditioning, 24/7. With the advent of ever more powerful and efficient alternator- based charging devices this is no longer the case. With today’s technology, energy needs on a monohull of up to 20 kWh, and on a catamaran of up to 40 kWh (because there are two engines and the potential for two powerful alternator devices), can be met without a generator.

Properly designed, an alternator- based system will deliver this energy more efficiently, with reduced engine runtime, and with significantly improved onboard lifestyles as compared to a generator-based system. The generator, with its space, weight, fuel, and maintenance requirements, can be removed from the boat.

Energy needs higher than 20 kWh or 40 kWh are frequently best met with a hybrid system that combines an alternator-based solution with an intermittently used, downsized generator.

Battery Capacity to Meet Demands

The design process begins based on the worst-case energy requirement calculated above. This energy requirement for DC systems, as well as AC systems run from DC-to-AC inverters, must be met from stored energy in batteries (with the possible exception of solar, wind, and water energy). This cumulative energy requirement defines the necessary size of a battery bank. If the battery bank is undersized, the system will perform poorly and the batteries will likely fail prematurely. If the battery bank is oversized, it adds unnecessary weight, volume, and expense.

Twelve 100-Ah TPPL batteries were installed on the author’s boat as part of some extensive electric-propulsion experiments. These advanced lead-acid cells offer improved capacity and efficiency compared to those with conventional lead-acid chemistry

Battery capacity is typically rated in amp-hours (Ah). But for energy system calculations, we need it in kilowatt-hours (kWh). To convert Ah to kWh, multiply a battery’s rated Ah by its voltage. This gets us to watt-hours (Wh). Divide Wh by 1,000 to get to kWh. For example, a 12V battery rated at 180 Ah has a capacity of (180 Ah x 12V) = 2,160 Wh/1,000 = 2.16 kWh. The designed battery capacity in kWh must be equal to the maximum cumulative energy requirement between engine runtimes, which were previously calculated, multiplied by a “battery factor,” which varies with battery type and chemistry.

Let’s say this results in a target battery capacity of 10 kWh. If it is necessary to convert this back to Ah to determine how many batteries are needed, multiply the kWh by 1,000 to derive Wh, and then divide by the system voltage. For example, if this is a 12V system: (10 kWh x 1,000) = 10,000 Wh/12V = 833 Ah.

Battery Factors

Various forms of lead-acid (PbA) and lithium-ion (li-ion) batteries are standard in marine applications:

Wet-cell PbA 

This type of battery needs periodic topping up with distilled water. For several reasons these are the least-suitable batteries for powerful alternator-based energy systems. The battery factor is 3 or 4 (i.e., if the daily load is calculated as 4 kWh, the battery capacity should be between 12 kWh and 16 kWh). Properly sized battery banks are invariably large, heavy, and bulky, and in general perform poorly.

Sealed PbA

These come in gel-cell and absorbed glass mat (AGM) versions. The batteries most suited for alternator-based energy systems are two variants of AGM batteries known as thin plate pure lead (TPPL) and carbon-foam (the Firefly brand of batteries). For TPPL, the battery factor should be at least 2 and preferably 3; for carbon-foam it can be as low as 2. For a number of reasons, the carbon-foam batteries have the best set of characteristics of any PbA battery for alternator-based energy systems.

Various TPPL and carbon-foam batteries are capacity tested on the author’s boat after aggressive high-capacity DC-systems testing.


When compared to any PbA battery, lithium-ion batteries have dramatically improved performance in alternator-based energy systems. For a given battery capacity, the batteries are typically less than half the weight and often half the volume. The battery factor can be as low as 1.4, which further reduces the size, weight, and volume of the necessary battery pack as compared to any PbA-based pack. Li-ion then has a substantially greater life expectancy. Despite the extremely high cost of lithium-ion, if its capabilities can be fully exploited, over time it can be more cost-effective than any PbA solution.

There are three li-ion chemistries used in marine applications: lithium-ion iron phosphate (LFP), nickel manganese cobalt (NMC), and nickel cobalt aluminum (NCA). If sufficiently abused, all three have the potential to catch fire, with LFP being the least likely to do so. Li-ion fires are rare, but when they occur they are generally catastrophic. Aside from ensuring the necessary performance characteristics, my own (personal) criteria for putting a li-ion battery on my boat is that it either has to have passed rigorous third-party abuse testing based on an appropriate standard (I prefer UL 1973), or it must come from a recognized marine vendor with a significant li-ion track record established over a number of years, backed up by an excellent warranty and a substantial liability insurance policy.

We cover design criteria for standards-compliant lithium-ion battery installations in the corresponding module of our Advanced Marine Electrics program.

A 10-kWh bank of Torqeedo lithium-ion batteries were installed as part of the author’s latest iteration of a high-capacity DC-energy system.

Alternator Calculations

Whatever energy is taken out of a battery between charge cycles must be put back, plus a margin to compensate for inefficiencies within the battery during the charge and discharge. With wet-cell PbA, these inefficiencies can be as high as 40%, with TPPL and carbon-foam around 15%, and with li-ion 5%–10%. The byproduct of inefficiency is heat — the higher the inefficiency, the more the heat, and the greater the likelihood that this will become a limiting factor in system design. (For more on battery temperature issues, see below).

The conventional OEM-installed 80-amp alternator that comes on many small diesels like this one doesn’t come close to meeting the generating needs of a powerful DC-based energy system.

Ignoring heat issues for the moment, the crudest form of alternator-based supply-side calculation takes the amount of engine runtime that will be available from normal boat operations and divides this into the amount of energy needed to run the boat between engine runtimes (or between plugging into shore power at dockside), modified to account for battery inefficiencies.

 To take a fairly extreme example, for a sailboat that will use its propulsion engine for 40 minutes to maneuver in or out of a harbor, and if the boat’s energy needs are 5 kWh between engine runtimes, and the batteries are 85% efficient carbon-foam, the necessary battery charging energy is (5 kWh/0.85) = 5.88 kWh. Given an engine runtime of 0.66 hours, this gives us a target alternator output of (5.88 kWh/0.66 h) = 8.9 kW. To put this in perspective, that would be 636 amps at an alternator output voltage of 14V—not impossible but pushing the boundaries with existing technology.

Let’s assume we could find an alternator, or combination of alternators, rated to deliver the desired output. Now, a number of practical design and operational issues come into play:

  • The speed (rpm) at which the alternator reaches its rated output can vary significantly for two different alternators with the same nominal output. The best fit is the alternator that achieves the target output at the normal operating speed of the engine driving it.
  • The efficiency with which alternators generate electricity varies enormously and in turn varies with alternator rpm and levels of output. The peak efficiency of many alternators is not much above 50% (the other 50% of the input energy from the drive belt is converted to heat inside the alternator), while others deliver above 80%. Powerful alternators operating at low efficiency generate a tremendous amount of heat, which must be dissipated.
  • When an alternator heats up, its output diminishes. For systems design, the key output number is the hot-rated output of the alternator at the alternator rpm reached when the engine driving it is at normal operating speed.
  • Any alternator charging a large bank of well-discharged batteries is likely to run at its full rated output for an extended period. The case temperature can easily exceed 200°F (93.3°C), with internal temperatures significantly higher. Some alternators can take this; some cannot and will burn up. An alternator controller that incorporates alternator-temperature sensing, or another output-limiting mechanism, may be needed.
  • Powerful alternators, especially those with poor efficiency, create significant crankshaft-pulley side loads for any engine driving them. Sophisticated alternator load-management strategies may be needed, especially if an alternator is installed on a relatively low-powered engine.
In our modules on Alternators and Alternator Controllers of our Advanced Marine Electrics course, we delve deep into the process of selecting, installing and troubleshooting alternators on boats.

Battery Constraints

Even when all the above considerations are satisfied, pointing to a particular alternator choice, we may now find that the batteries cannot use its full capabilities. So next we must accommodate potential battery limitations. Consider these basic issues when assessing the ability of a given battery to function effectively in an alternator-based energy system:

Charge acceptance rates (CARs)

For all batteries, we can define a charge acceptance rate as the rate at which a battery can absorb energy at any particular state of charge (SoC). It is typically specified as a C-rate, where 1C represents a CAR equal to the battery’s rated capacity in Ah—i.e., for a 100-Ah battery the 1C rate is 100 amps; for a 200-Ah battery the 1C rate is 200 amps. For a 100-Ah battery a 0.5C rate is 50 amps, and for a 200-Ah battery it is 100 amps. And so on. In powerful alternator-based energy systems some designers want batteries that will accept charging currents at up to the 1C rate, and occasionally up to the 2C rate.

With AGM PbA batteries, both TPPL and carbon-foam, the CAR drops below the 1C rate once a battery is ~60% charged, and thereafter the CAR steadily declines. Depending on the design parameters for the system, this can be a significant factor impeding the system’s optimization. In contrast, it is often stated that li-ion batteries have very high C-rates to high SoC.

Although this is true for some li-ion batteries, especially those in the automotive world, it is not true for many li-ion batteries sold for marine applications. These may have recommended C-rates as low as 0.3C (e.g., 30 amps for a nominal 100-Ah battery). Although these batteries can often be charged faster, it reduces their life expectancy. An optimized marine energy system requires matching battery C-rates to charging devices and charging times.

Partial state of charge (pSoC) operation

Toward the end of a charge cycle the CAR of PbA batteries steadily declines. It can take an hour or two to go from ~60% SoC to 100% SoC. If this part of the charge cycle requires a main engine or generator to be run solely for battery charging, it is extraordinarily inefficient, and the real cost of the energy being fed to the batteries is very high. On the other hand, lead-acid batteries that are not regularly fully recharged suffer a loss of capacity and fail prematurely from sulfation.

Ideally, the design for an effective marine energy system using PbA batteries must protect against sulfation without requiring engine run hours beyond normal propulsion runtimes.

 Within the PbA world, the only battery more- or-less immune to sulfation is the carbon-foam Firefly battery. If other PbA batteries are used, solar power can sometimes be configured as a cost- effective mechanism to ward off sulfation. Note that li-ion batteries are immune to sulfation. Indeed, their life expectancy is extended if they are operated in a partial state of charge (i.e., without being fully recharged). 

 Made of carbon foam, they render the batteries nearly immune to sulfation. The lead surface grid, connects the plate to the battery’s busbars.

Two sides of the “magic” plates inside a Firefly battery. These negative plates are unlike anything found in other lead-acid (PbA) batteries.

The NMC and NCA chemistries will suffer a significant loss of life if maintained for extended periods of time in a full state of charge, such as when a boat is dockside and plugged into shore power. Specialized charging regimes are needed to prevent this. The LFP chemistry is significantly more robust but with a lower energy density (i.e., more volume and weight for a given capacity).

Minimum charge rates

Some batteries—carbon-foam and to some extent the TPPL—can suffer a slow but progressive loss of effective capacity if repeatedly discharged to low states of charge and then recharged at relatively slow rates (e.g., below 0.3C). These batteries like to be charged fast, with minimum charge rates of 0.4C if charging commences with the battery in a low (less than 30%) SoC. In carbon-foam batteries this loss of capacity from slow charging resembles sulfation; but unlike sulfation, the capacity can be recovered with a normal full charge cycle, beginning with a charge rate of 0.4C or higher and then continuing with a steadily declining CAR until the battery is 100% charged.

If you want to gain a deeper knowledge about battery construction, efficiency, rating and everything else you need to know to size and setup a suitable battery for your boat, check out our Boat Electrics 101 online course.

Peak discharge rates 

Remember the bow and stern thrusters? If PbA batteries are hit by a short-term high-rate discharge, so long as any fuses and/or circuit breakers in the system have a high enough amp rating to not blow or be tripped, the only effect will be a sagging voltage. The voltage on li-ion batteries will not sag in the same way. However, the battery management system (BMS) associated with almost all li-ion batteries will likely include a current (amp) sensor, and there may also be an internal or external fuse.

If the current sensor is subjected to a discharge rate that exceeds its upper set point, it will disconnect the battery from the boat with potentially catastrophic results for onboard electronics—similarly if a fuse blows. The boat’s electrical system will also suddenly go dead, likely at an inconvenient moment.

If a disconnect occurs, the BMS must be wired (most are not) to first shut down all charging devices to prevent the possibility of a high-voltage spike on the boat’s wiring. Ideally, an energy system’s design must ensure that the highest possible peak loads on the system can be met without the battery disconnecting in any way, which is not always the case with li-ion installations.

Temperature extremes

Ambient temperatures in boats can vary enormously—from a laid-up boat in northern states, with temperatures going as low as –40°F (–40°C), to a closed-up boat in the tropics, with temperatures going well above 100°F (37.8°C). Localized engine room temperatures can exceed 180°F (82.2°C). PbA and li-ion batteries can tolerate these temperature extremes when in storage mode, so long as PbA batteries are fully charged in extreme cold temperatures, but all will suffer a significant loss of life at higher temperatures.

As a general rule of thumb, for every 18°F (10°C) temperature rise above 77°F (25°C), PbA life expectancy is cut in half.

PbA can also tolerate these kinds of temperature extremes in use, although with a significant loss of performance at lower temperatures and loss of life at higher temperatures.

Li-ion is not as tolerant. Most li-ion batteries cannot be used if the battery temperature is much below 32°F (0°C) and should not be charged if the battery temperature rises above 113°F (45°C), although higher charging temperatures can sometimes be traded for a reduction in life expectancy.

Any attempt to charge li-ion with a battery temperature above 140°F (60°C) is likely to push internal cell temperatures uncomfortably close to thresholds at which the battery can start to get out of control. (First, the SEI layer begins to break down; temperatures rise, and then if the separators don’t shut things down and instead melt, the battery suffers thermal runaway.) 

Note that the internal temperature of all batteries can be significantly higher than the ambient temperature because of heat generated within the battery during normal charge and discharge cycles.

Cycle life 

All PbA and li-ion batteries have a discharge-recharge cycle life that varies with depth of discharge, temperature, and other factors. The cycle life of even the best PbA batteries is typically significantly less than that of all li-ion batteries, while the cycle life of the NMC and NCA variants of li-ion is typically significantly less than that of LFP. In designing any DC- based energy system, it’s important to understand the different factors at play in a given application to select batteries with an appropriate cycle life.

The large bank of Lithionics lithium-ion batteries on a 57' (17.4m) catamaran allows the owners to enjoy the comforts of home without needing a generator on board.

Charge Controllers

By now it should be clear that numerous variables must be balanced against each other in an optimized, reliable, and cost-effective marine energy system. The primary management tool in this part of the equation is the charge controller for any alternators, battery chargers, and other charging devices in the system, aided and abetted by various voltage and current (amps) sensors and systems monitors.

The most basic controllers respond solely to the output voltage of a charging device, measured at the charging device. A step up in sophistication comes with voltage measurement at the batteries being charged. More sophisticated controllers incorporate amp measurements into control algorithms. Many include temperature sensing at battery banks and sometimes alternators. The most sophisticated alternator controllers incorporate various engine data—rpm, load, temperature, and fuel consumption.

The more powerful an alternator-based energy system, and the faster batteries are being charged, the more important it is to have a sophisticated charge controller that responds to multiple voltage, current, temperature sensors, and engine data. Some are now capable of developing a generic engine-power curve, a propeller-power- absorption curve, and a fuel-efficiency map, and then managing the loads created by the alternator to achieve an optimum balance of electrical energy output and fuel efficiency. A completely different control strategy is applied if the engine is not in gear.

An alternator-based energy system cannot be optimized without appropriate controllers that incorporate algorithms matched to the application and duty cycle. These systems also include a sophisticated systems monitor that tracks battery state-of-charge and numerous other performance parameters. For more information, check out our module on alternator voltage regulators and controllers, which is part of our Advanced Marine Electrics program.

Solar, Wind, and Water Generators

Regardless of how well an energy system is optimized, any time a fossil-fueled engine is run solely to generate electrical energy on a boat, the real cost per kWh of this energy is extraordinarily high. In this context, pretty much any source of non-fossil-fuel energy is a good investment. The primary alternative sources available to boat owners are solar power, wind power, water generators, and, on sailboats under sail, regenerative energy from a freewheeling propeller.

Solar power is an especially recommended option for any boat that cruises away from shore power for any length of time. Key features that should be looked for in a solar panel are efficiency (to optimize the amount of energy delivered from a given surface area), rugged construction suitable for the marine environment, and bypass diodes if the rated output is above 50 watts to 60 watts—to prevent hot spots and cell burning in the event of shading and panel damage.

In our Boat Electrics 101 online course we have an entire bonus module covering the field of alternative energy sources in. If you're planning to leave the dockside power supply for a longer time, you should definitely check it out!

In PbA systems, the inclusion of solar power generally results in batteries being maintained at a higher average state of charge, which significantly extends battery life. It may also be possible to configure the photovoltaic system to provide the low-level charge rate necessary to periodically achieve the 100% state of charge that is frequently necessary to hold sulfation at bay, once again extending battery life. Battery-replacement-cost savings are an often overlooked benefit from any solar installation.

Remarkably Powerful Systems

I put together my first substantial DC-based energy system almost 40 years ago, built around a high-output alternator, an early version of a multi- step charge controller, and a large bank of wet-cell PbA batteries. It met our modest electrical needs at the time, but only with long hours of chronically inefficient low-load engine runtime and batteries that died prematurely.

We have the same basic components on our current boat, but what a dramatic change in capabilities. This past summer, with our latest-generation DC-based energy system coupled to lithium-ion batteries, we not only enjoyed the comforts of home but did so for the first time without constantly obsessing over battery state of charge, which has been an undercurrent to our cruising for the past four decades.

With today’s technology, and careful design and installation, it really is possible to deliver superb DC-based energy systems, but of course they do not come cheap. Whether you are willing to pay for them is another story. To explore the options, viability, and details of installing such a system in your boat, make sure to sign up to our Marine Electrical Online courses!

About the author 

Nigel Calder

Nigel is often referred to as THE guru when it comes to technical systems on boats.

He is a long-time member of the American Boat and Yacht Council (ABYC) electrical Project Technical Committee (PTC) which writes the standards for recreational boat systems in the USA, and has also been involved in European standards development.

Nigel is best known for his Boatowner’s Mechanical and Electrical Manual (now in its 4th edition), and his Marine Diesel Engines (in its 3rd edition), both considered the definitive English-language works in their field.

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