The infinite shore-power cord.

An onboard supply of 120-volt alternating current used to be a luxury available only on large boats. Today, technology allows us to enjoy the benefit of AC power on a boat as small as a canoe, provided we understand and respect some simple facts about available sources of energy and the conversion of energy from one form to another.

 The basic reality of electrical energy, whether supplied as AC or DC current, is that it cannot be stored and therefore must be produced as it is consumed. There are two practical means for producing AC power on a boat. DC/AC inverters powered from the boat’s house battery bank can provide modest AC power for some hours and sufficient power to operate two- to three-kilowatt loads for a limited period of time. Power for air conditioners or other substantial loads that must operate for extended periods of time requires an engine-driven AC power source, such as a gasoline-fueled portable or marine genset, or a diesel-fueled marine genset.
Solid-state DC/AC inverters can be a wonderfully convenient source of AC power, provided both their capabilities and their limitations are understood and respected. The waveform of the AC current produced by an inverter may be a pure sine wave, identical to commercial power, or it may be a modified sine wave that may not be suitable for certain consumers. The amount of power an inverter can deliver may be significantly less than what is advertised. At the same time, the amount of energy wasted in the process of converting low-voltage DC to 120-volt AC is often more than the specification sheet discloses.


The waveform of commercial shore-power AC is a sine wave, (see page 51) characterized by a smooth transition from one polarity extreme to the other. In their most basic form, inverters deliver a waveform that switches abruptly from one polarity to the other, producing a square wave. This waveform is what an inverter you bought back in the 1970s and early 1980s produced. It was suitable for light bulbs and small appliances but created a lot of electrical noise if used to power a radio.

The waveform supplied from today’s inverters will most likely be a modification of a square wave, called a modified sine wave. This waveform is perfectly acceptable for small appliances and lights and for most radios and TV sets. However, modified square wave power may not be suitable for some of the new microwave ovens, powerful entertainment systems and some computers, and it is far from ideal for powering induction motors, including those used in air conditioners. All of these waveforms are theoretical; the actual shape of the waveform will depend in part on the type of load imposed on the inverter or genset.

There is no free lunch
In most applications, all of the energy delivered by an inverter must come from energy stored in the battery. Inverters are not 100 percent efficient, consuming more power from their supply source than they deliver to the AC load. A typical inverter is no more than 80 to 85 percent efficient and will consume approximately 1.2 watts of DC energy for every watt of AC produced. Inverter manufacturers suggest dividing the wattage of a device operated from an inverter by 10 to determine the current demand from a 12-volt battery. Doing so recognizes that the likely efficiency of the inverter will be about 83 percent.

 While this calculation is a good starting point, it does not tell the whole energy story. The process of recharging the battery imposes an additional energy loss of about 15 percent. Using the suggested efficiency factors, powering a 100-watt load from an inverter for one hour will drain about 10 amp-hours from the battery. The battery-charging system will have to deliver 11.5 amp-hours to restore the battery to its original state of charge. These facts of electrical life are not show-stoppers, just facts to keep in mind when it takes a bit longer than you think it should to recharge your battery bank.

The amount of energy used from a battery during inverter operation can be reduced by supplying supplemental energy from an external source, such as the propulsion engine’s alternator. So long as the output current from the alternator exceeds the current demand of the inverter, no energy will be taken from the battery. The battery need only supply demands that exceed alternator capacity.

Inverters suitable for use on small- and medium-size boats typically have power ratings from 20 to more than 3,000 watts. Inverter power ratings can be confusing. The need to remove heat from the switching devices is the Achilles heel of any inverter. The amount of heat depends on the inverter’s power rating. This rating complication can be the result of the time required to dissipate the heat. It can take some time for a device at room temperature to reach a stable temperature.

Some manufacturers take advantage of the slow buildup of heat to allow a high power rating for a short time, perhaps a few seconds or, in some cases, up to as much as five minutes. Regardless of an inverter’s power rating, it must be provided with a way to dissipate heat.

An example of the difficulty of removing heat can be seen in some small inverters whose data sheet may state that it can power a maximum load of 50 watts, but the average load should not exceed 35 or 40 watts. This inverter may be able to safely dispose of the heat created when powering a 40-watt load but may overheat in a few seconds or a few minutes when operating at the maximum advertised power.

The horsepower race for the highest possible power rating creates even more confusion when trying to define the safe power output capacity of the larger portable and smaller fixed-mount inverters. These units may have multiple power ratings: continuous, normal, steady-state, long term and surge. The surge rating is usually a large number, perhaps as much as 200 percent of the continuous rating. In many cases the surge rating describes a power level that is available for only a few AC cycles, a fraction of a second or a few seconds at the most. The continuous power rating of some inverters is limited to five minutes. In most cases the steady-tate or long-term power rating is an accurate statement of the load that can be supplied for as long as the needed low-voltage DC is available and adequate cooling air is available.

On a table or bolted down?
Inverters are available as either portables or fixed-mount units. The portable units range in output from about 25 watts to as much as 700 watts of modified sine wave power. Prices range from about $20 to perhaps $130 for a 700-watt continuous, 1,000-watt surge capacity unit. The size and weight of an inverter should pose no problem, even for small boats.

Once the need for AC power exceeds about 700 watts, inverters become a bit too big to be considered portable. For example, some 3,000-watt units weigh as much as 100 lbs. All inverters benefit from being located close to the battery bank. Any voltage drop in the connecting wires results in a waste of energy. Ignoring efficiency losses, the input current is proportional to the ratio between the input and output voltages. A fully loaded 3,000-watt, 120-volt inverter will demand a 12-volt input current of 250 amps. Even if the total cable length between the battery and the inverter can be limited to 10 feet (5 feet per cable) size 1/0 cable would be the smallest recommended size. In an installation of this type, it would be prudent to use 3/0 or 4/0 cable to ensure minimum voltage drop.

Fixed-mount inverters are available as stand-alone devices or with built-in shore-power switching systems. When an automatic shore-power switching inverter is installed, the vessel’s shore-power flows through the inverter before being delivered to the boat’s AC distribution system. Whenever shore power is present, the inverter operates in standby mode, consuming an insignificant amount of power. If the shore power fails or is purposely disconnected, the inverter turns on in a fraction of a second (often 20 milliseconds or less), continuing to supply the current previously provided by the shore-power source.

Any work carried out on the electrical system of a boat equipped with an automatic power-switching inverter must be done with the greatest care and caution. As with shore power, the voltage produced by the inverter is enough to kill! Removing the shore-power cables from the power inlet will not necessarily eliminate the danger created by energized 120-volt circuits when an automatic switching inverter is installed. Labels warning of the presence of an automatic switching inverter should be posted at the electrical panel. When work is to be carried out on the AC power distribution system, the minimum required precaution is to disconnect the shore-power cable at both the shore and boat ends and turn off inverter’s main switch. Disconnecting the inverter’s positive battery cable is a worthwhile precaution.

Incorporating a battery charger in a built-in inverter makes great sense. The necessary AC shore-power feed is present. The large conductors that carry battery current to the inverter can carry charging current to the batteries. A few additional lines of code will enable the inverter’s microprocessor control to manage even the most sophisticated battery-charging protocol.

It all depends on the batteries
Success in using an inverter to supply substantial amounts of AC power depends on good battery management practices. It is very easy to become used to watching the TV set or using some other appliance and forget that there is a finite amount of energy available from the battery. Most inverters will automatically shut down if the battery voltage drops too far; however, this automatic cut-off voltage may be set to protect the inverter and may be far too low to protect the battery from excessive discharge. In some large inverters, the cut-off voltage may be a programmable parameter. A cut-off voltage of 12.1 to 12.2 volts will ensure that battery discharge is never excessive.

A bank of high-quality, deep-cycle batteries is required to support a high-power inverter. If properly cared for, flooded-cell, heavy-duty deep-cycle batteries are the best choice. This is the battery type used in the massive, land-based uninterruptible power systems that support communication networks. The more costly, usually shorter life absorbed glass mat (AGM) batteries are the best choice if you are unwilling or unable to maintain flooded-cell batteries.

Batteries must be well secured, sufficiently separated from one another (about half an inch) to dissipate the heat generated during charging, and kept clean. All batteries, regardless of type, must be recharged as promptly as possible. Deep-cycle batteries should not be discharged beyond 50 percent of their rating. In many instances, battery life will be enhanced by limiting discharge to about 43 percent of capacity, half of 85 percent of battery rating.


How much popcorn can you pop?
One of the more common uses to which we put our modern electronic miracles inverters and the magnetrons in microwave ovens is popping popcorn. Exploring a theoretical popcorn binge illustrates some of the facts of inverter use. How much popcorn you can pop depends on the combined impact of the maximum power demand of the microwave and the length of time it takes to pop the corn. A 1,000-watt-rated microwave will consume energy from the battery at approximately a 100-amp rate. A battery bank consisting of two new, fully charged Group 27 lead-acid flooded-cell batteries has a total presumed energy storage capacity of 200 amp-hours. Applying a 50 percent of capacity discharge limit, the total energy available before the need to recharge is 100 amp-hours, enough energy to run the microwave for one hour. However, high-current discharge reduces the amount of energy that can be extracted from the battery. Therefore, it may be best to assume that only 90 percent of the rated capacity will be available. Under this assumption, 90 amp-hours are available, enough to operate the microwave for about 54 minutes. Assuming it takes 3.5 minutes to pop a bag of popcorn, we could pop 15 bags of popcorn before a battery change or recharge would be required.

Our calculation is not complete until we compute the length of time it would take to recharge the batteries. Assuming the use of flooded-cell deep-cycle batteries whose charge current should average about 20 percent of capacity, the charge time would be on the order of five hours, presuming of course that the charge source (the propulsion engine’s 60-ampere-output alternator) could deliver a constant 40 amperes to the batteries. If we had taken the precaution of running the engine during our one-hour popcorn marathon, the energy provided from the engine’s alternator would have limited the discharge from the batteries to about 40 ampere-hours, making it possible to recharge the batteries in just over two hours, or to pop popcorn for about 2.25 hours, producing 35 bags of popcorn, enough for the entire flotilla (assuming the microwave did not give up along the way — few home units are designed for continuous duty).

Extending operating time/power
You can only withdraw what you have in the bank, and your battery bank does not allow overdrafts. Fortunately it is possible to reduce the inverter’s drain on the battery bank by supplying energy to the battery from an external source  an engine-driven alternator, a wind-powered generator or solar panels. If the amount of external energy is equal to the inverter’s drain, the battery serves only as a massively effective filter, assuring that the inverter is fed well-conditioned direct current.

For example, running a propulsion engine in neutral at a speed high enough to provide a reasonable percentage of the alternator’s rated output (perhaps 2,000 rpm) can provide enough energy to extend AC system operating time or limit the depth of discharge of the battery bank. This mode of operation is not particularly good for the engine (the typical alternator imposes too low a load on the engine); however, if such operating periods are of reasonable duration and followed by a brief period of running the engine in gear at a relatively high speed (perhaps 400 rpm below maximum), there is little likelihood of harming the engine. When doing this, you should monitor engine performance for any indication of a carbon buildup in the engine’s exhaust riser adjacent to the cooling-water injection point. New types of alternators may become available in the near future that will allow a propulsion engine to operate at idle speed while producing as much as 2.5 kw of DC power.

A small engine dedicated to powering a DC output alternator can be an attractive alternative to using the propulsion engine alternator to support the boat’s inverter system. For example, an eight-horsepower diesel engine can power a 200-ampere, 12-volt alternator. Since the voltage at a battery supplied by this alternator will remain well above 12 volts, it should be possible for an inverter to power a 2,500-watt load continuously. Plenty of manufacturers offer various combinations of small diesel engines with large DC output alternators and, when desired, refrigeration compressors, reverse-osmosis watermaker pumps and the like. The advantage of having a second fuel-supported energy source on a cruising boat can be compelling, especially if the engine can be hand-started.

An engine-driven AC output alternator, a genset, is the traditional way to produce AC power on a boat. The genset option has always been attractive where substantial amounts of AC power are required or where power is required for extended periods of time. Except for a few marine air conditioners specifically designed to work with small DC gensets, the majority of air conditioners must be supplied with 120-volt, 60- or 50-Hz AC power.

Serious power diesel gensets
There are large parts of the world where temperature and humidity are uncomfortably high and cooling breezes are notable by their absence — Florida and the Gulf of Mexico states, the U.S. Southeast and, for some, summer in Chesapeake Bay. Many boatowners in these areas consider air conditioning, refrigerators, freezers and even icemakers a virtual necessity. Their AC power requirements are not likely to be met with the use of an inverter or by portable, gasoline-fueled gensets. They need substantial AC power for a significant part of every day. A diesel genset is the best choice.

Gensets are available with power ratings ranging from about three to as many kilowatts as you want. Practical power output choices for sailboats in the 30- to perhaps 60-foot range are between four and 15 kw. The power rating of a genset in kilowatts is typically stated in terms of a 1.0-PF load. PF is electrical speak for “power factor” and describes the manner in which the load consumes power.

Starting an induction motor, such as those used in air-conditioning systems, imposes a momentary initial current demand on the AC power source that may exceed twice the nameplate running current for the unit. Low voltage during a start attempt can cause rapid overheating within the motor and can lead to motor failure, if done repeatedly. Genset power ratings must exceed the running current requirements of such loads by perhaps 20 percent to ensure proper motor starting performance.

The first task in sizing a genset is preparation of a list of all the AC power consumers onboard the boat or that are likely to find their way aboard in the foreseeable future. List the devices in descending order of power required. It is likely that the air-conditioning system will be the largest consumer, followed by the AC-powered reverse-osmosis watermaker. Heat-generating appliances consume substantial power. A toaster may consume more than 1,000 watts, an electric frying pan as much as 1,250 watts, and a hair dryer 1,800 watts. The boat’s domestic water heater typically is fitted with an electric heating element that consumes about 1,250 watts. Some motor-driven appliances, such as a vacuum sweeper, can consume 400 to 600 watts, a kitchen mixer 200 watts. The shore-power-operated battery charger may consume as much as 1,600 to 1,800 watts. The TV and audio systems used on boats less than about 70 feet long usually consume relatively modest amounts of AC energy. On most boats, the interior lighting system operates from 12 volts DC or from small, dedicated DC/AC inverters with a total power consumption that rarely exceeds 100 watts. Assume that the lighting demand must be met regardless of what other systems are in use. Turning off lights is rarely helpful in controlling the drain on the genset.

A workable power consumption estimate can be made by assuming that the “essential” systems will operate simultaneously, for example all air-conditioning units. Power may be provided for one or two additional consumer groups, such as all lighting plus the entertainment system. If boat size and/or budget constrain the capacity of the genset, it will be necessary to assume that an air-conditioning unit may have to be switched off when running the microwave or other similar appliance.

Consistently operating a diesel genset at too low a load can be harmful to the unit’s engine. It is better to install a genset that is slightly too small than one with a capacity equal to the vessel’s total possible AC load. A typical five-kilowatt genset can deliver a maximum running current of a little more than 41 amps at 120 volts. The genset will perform best when the continuous load is limited to about 85 to 90 percent of maximum, 35 to 37 amps.

Power, not noise
Regardless of all other factors, the most critical concern in choosing a genset is the amount of noise it creates in the saloon and sleeping quarters of the boat. A genset that creates more noise than the air conditioner is simply unacceptable to most boatowners. Unlike most powerboats and trawler yachts, sailboats usually have limited space for a genset and offer little in the way of sound-attenuating bulkheads between the equipment space and the accommodations. Except in the most unusual circumstances, any genset installed in a sailboat must have an integral sound shield. Many genset manufacturers provide sound-level specifications measured at a distance of 7 meters. The measured noise level increases with decreasing distance. For example, a genset rated at 54 dbA at 7 meters can be expected to produce a noise reading of 71 dbA at 1 meter. Choosing a genset whose noise level does not exceed approximately 71 dbA at 1 meter can go a long way toward creating a really acceptable installation.

Gensets are not noted for their light weight. Even a modest five-kilowatt unit will weigh at least 350 lbs with its sound shield. The unit must be fastened securely to a substantial part of the boat’s structure.

The space in which the genset is installed must provide the highest possible degree of sound attenuation. Sound behaves like water and will leak through any crack. Sealing all joints is more important than the amount or type of sound-dampening material used. The genset space must allow an adequate supply of air, both for combustion and in most cases for alternator cooling.

The cooling systems used in most gensets are identical to those used in propulsion engines, a fresh water-coolant mixture circulates through the engine that is in turn cooled by seawater flowing through a heat exchanger with a wet exhaust system. The passage of current through the copper-wire windings in the genset’s alternator creates heat. Most genset alternators are cooled by a forced airflow created by their integral cooling fans. A few use water-cooled alternators, thereby avoiding the discharge of heated air into the compartment in which the genset is installed. A few manufacturers extract heat from air-cooled alternators by installing an air-to-water heat exchanger, a radiator, within the genset’s sound shield. Seawater circulated through the heat exchanger removes accumulated heat, eliminating the need to discharge heated air from the unit.

The supply of seawater to the genset and for any air-conditioning equipment must pass through a seawater strainer. A clogged seawater strainer can be a much more common and annoying problem for both the genset and the air-conditioning system, since they ingest grass and debris when the boat is stationary. Some boatowners solve this problem by installing a self-cleaning seawater strainer, like a Groco Hydromatic unit, using it as a sea chest to supply filtered seawater for the genset, air-conditioning system cooling pump, the boat’s heads, etc.

The genset’s exhaust system must be designed carefully. The majority of genset installations in sailboats and trawler yachts place the unit below the waterline. A wet exhaust/waterlock muffler is required to dispose of the exhaust gas and cooling water safely. In addition, a properly elevated ventilated loop will be required in the hose that carries the cooling seawater from the engine’s exhaust manifold to the water-injection fitting on the exhaust line or riser. In some sailboats, the exhaust must exit the boat on the side rather than in the preferred position on the stern. Depending on the particulars of the installation, it may not be possible to run the discharge from the waterlock to an elevation high enough to ensure that seawater cannot flow into the exhaust when the boat is heeled to the maximum. The problem is compounded since the genset is not likely to be operating when sailing at a large angle of heel. In such installations, it may be necessary to install a full-flow manual shutoff valve on the genset’s exhaust line. This valve should be readily accessible and located as close to the hull side as practical. A large placard should be posted immediately next to the genset control panel as a constant reminder to open the valve before starting the engine and to close the valve when the genset is shut down. Use of an automatic check valve in the exhaust line is not recommended. The exhaust system back pressure should be checked against the genset manufacturer’s specification. Excessive back pressure is particularly harmful to turbocharged genset engines.

The AC power leads from the genset must be connected to the vessel’s AC power distribution system through a properly rated two-pole AC circuit breaker connected to the boat’s AC power transfer switch. This switch must positively prevent the output of the genset from being connected to the shore-power inlet of the boat. Any attempt to become a co-generator for the shoreside AC power system will, at the minimum, end in disaster for the genset. Unless you are well versed in the requirements of the ABYC Section E-11 recommendations for shore-power systems and genset wiring, this work is best left to qualified experts.

Best of both worlds
Insofar as AC power is concerned, the best of both worlds can be a combination of a well designed shore-power system, a modest DC/AC inverter and a genset. When away from shore, loads such as interior lighting can be supplied from small, low-cost dedicated inverters that are switched off when the lights are not in use, thereby avoiding even their minimal standby power drain. A modest-size inverter capable of delivering perhaps 300 to 500 watts of modified sine-wave power can run the entertainment system, laptop computers and small appliances. Larger loads, including air-conditioning systems are supplied by the genset. Genset installations in sailboats must be operated with due regard to the heel angle of the boat when under sail. The genset’s engine will usually carry a specification stating the maximum allowable continuous inclination angle.

Contributing Editor Chuck Husick is a sailor, pilot, former company president and freelance writer based in Florida.

By Ocean Navigator