Power Generation

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WorldPower Generation

Power Generation

Electrical power generation on a sailing yacht is used for two purposes:

  • Charging the batteries which are used to power the on-board equipment (lighting, autopilot, refrigerator, freezer, etc).
  • Providing AC current to power on board AC systems such as refrigeration, air conditioning, water makers, power tools, etc.

Battery Charging

Charging a battery is much like pumping air into a tyre. The voltage in the battery equates to the air pressure in the tyre, and the voltage at the charging source equates to the air pressure in the pump. When the voltage in the battery is low, a high voltage at the charging source causes a significant amount of charge, or current measured in Amps, to flow into the battery. As the battery voltage increases (like increasing the air pressure in the tyre), the amount of current flowing into the battery reduces.

This continues until the difference between the voltage in the charging source and the voltage in the battery is too small to allow any more current to flow into the battery. The only way to continue charging the battery at that point is to increase the voltage at the charging source.

The effect of this is that multi-stage or "smart" chargers are required to bring a battery bank up to full charge and keep it there. Older style chargers simply poured current into the battery until the terminal voltage reached a certain level (and frequently used up excess power by dumping it into a resistor bank, converting it to heat).

Modern multi-stage chargers usually have a set number of phases, for example:

  • A "bulk charge" phase where the charger brings battery voltage up to a certain level by increasing voltage at a fixed current level.
  • A "top up" phase where the charger holds the voltage at a certain high level (say around 14.4V) until the current drops indicating that the battery can take no more charge.
  • A "float" phase where the battery voltage is held somewhere around 13.4V to 13.8V to replace any natural losses.

Some power generators will have this type of smart charging built into them, whereas others will simply dump charge into the battery at a fixed voltage level (e.g. around 13.4V or 13.8V) until the battery can take no more. Solar panel regulators tend to be of the first type, whereas engine alternators tend to be of the second type unless an external regulator is fitted.

AC Power Generation

DC to AC Inverter -

To run AC powered equipment on board requires either a genset or an inverter, generating the appropriate amount of power at 110V or 240V AC (the most commonly used voltages).

There are two different methods of providing AC power on board from power generating devices. These are:

  1. Run a direct feed from the AC output of a genset into the AC distribution board of the boat.
  2. Run a feed from an inverter running from the house batteries, and use any power generating devices simply to top up the house batteries.

Larger boats fitted with inbuilt gensets usually choose the first method, because the types of AC powered equipment that they use (e.g. fridges, air conditioners, etc) use more power than can efficiently be produced by an inverter.

Smaller boats without inbuilt gensets often choose the second method because the wiring is simpler. All of the power generation on board (including shore power via a battery charger) is used to charge the battery bank, and any AC powered equipment runs from the inverter. This often requires a larger capacity inverter than very small boats might carry, but to run a small computer or laptop, perhaps a battery charger for a cordless drill, and occasional use of galley items such as a blender or small microwave, this is often sufficient. This method also has the advantage that the voltage of the AC power on board is independent of any power generation or the shore power. For example, 110V shore power can be fed into a battery charger and this used to top up the batteries along with 12V solar panels, and a 240V inverter run from the battery bank giving 240V AC on board independent of the shore power voltage.

Another advantage of this method, usually best applicable to steel boats, is that an isolation transformer is not required.

Some boats use a hybrid system, where the AC distribution on board can be fed either from an inverter, from a genset, from a shore power system, or from a combination. This is usually fitted in conjunction with auto-start devices for the genset, where the genset can be automatically fired up should the load increase or the battery charge drop to the point where charging is required. This does have the disadvantage that the shore power system, the genset, and the AC distribution on board, all need to run from the same voltage.

Load Analysis

Before determining how much power generation you need to fit to a yacht, you need to estimate the power usage on board. There has been quite a lot written about this already so I will just summarise the main points:

  • Consider the load, in watts or amps (I find converting everything back to amps at 12V to be the most convenient) of all of the equipment on board. This includes autopilot, TV, fridge, freezer, stereo, portable battery chargers (I find that a significant current draw is charging of various phones and tablets on board using USB power drawn from a 12V to USB converter), laptop computers, etc.
  • For each item, multiply the number of amps by the number of hours in the day where the item is likely to be used. For example, fridges and freezers tend to run 24x7 if they are cycling mode, but for fewer hours if they are eutectic. A phone charger might only be used for 1-2 hours per day, but a navigation laptop and an autopilot might also be used full time while on passage.
  • Consider both on-passage and at-anchor loads. For example, on passage the autopilot might be running constantly while at anchor it might be turned off.

The result will be a number of amp hours (Ah) of total load that you will to supply need per day. Ideally your daily power generation should be more than the daily load.

Here is a simple spreadsheet as an example of a power usage analysis:

Item Current Draw (Amps) Usage anchored (hours/day) Usage on passage (hours/day) Ah/day anchored Ah/day on passage
Fridge 5 2 2 10 10
Freezer 2.5 12 12 30 30
Autopilot 5 0 24 0 120
Lights 1 4 4 4 4
Instruments 1 0 24 0 24
Nav laptop 1.5 0 24 0 36
TV/Stereo 2 4 2 8 4
Tablet/phone charger 2 2 1 4 2
Total 56 230

Battery Capacity

Consider how long you might be comfortable running without any charging capacity -- for example if your primary solar regulator fails then you may have to run for 24 hours or longer while it is replaced.

In general I would have a battery capacity that was no less than 2x the daily load of all of the equipment that uses power. For example, if you find that your total daily load is around 200Ah, then I would have a battery bank that was no less than 400Ah.

See Boat Batteries for the different types of battery that you could use, but in the end it comes down to what you are prepared to pay for what you are prepared to have. Flooded lead-acid batteries are still the cheapest type of battery to have on board although their charge capacity per weight is lower than that of Lithium batteries. If you are prepared to pay the money for lithium then by all means do so!

Take into consideration that most battery types do not like to be dropped below a certain percentage of their full charge. For AGM and deep cycle flooded batteries this is around 50%, although lithium batteries can frequently be dropped to 30% of charge or less without significant damage.

Charging Capacity

All things being taken into consideration -- gensets, alternators, solar regulators, shore power, etc, it is best to aim for a charging capacity that is no less than 10% of your total battery capacity, and no more than 25%. For example, with a battery bank around 400Ah, the total charging capacity should be somewhere between 40A and 100A. Lower capacity will generally be unable to bring the battery bank up to full charge, and higher capacity will have a tendency either to overcharge batteries or waste power by attempting to put more current into the battery bank than it is able to absorb (although some battery types, e.g. AGM can charge at a higher rate).

Charging capacity needs to be examined both underway and at anchor. Some considerations include:

  • Anchoring is usually done in quiet sheltered anchorages -- where wind generators do not generate a significant amount of power.
  • Tow generators usually generate a steady power output while underway, but of course zero while at anchor.
  • While at anchor, solar panels may be more efficient than while underway. This is because the panels may be partially shaded by the sails while underway.
  • A petrol generator can usually be used while at anchor, but there may be nowhere to safely run it while underway.

A similar spreadsheet to the load analysis spreadsheet above can be used to estimate daily charging capacity while at anchor and on passage:

Item Capacity (Amps) Gen anchored (hours/day) Gen on passage (hours/day) Ah/day anchored Ah/day on passage
Solar Panels 12 6 6 72 72
Wind Generator 2 0 24 0 48
Tow Generator 3 0 24 0 72
Genset 25 2 0 50 0
Engine Alternator 20 0 2 0 40
Total 122 232

Of course you can adjust the hours/day ratings for each item as you see fit. Note that I'm being a bit generous with the solar panels here -- assuming an average day in the tropics, you may have to adjust your estimate of the number of hours during the day that you think the solar panels can generate at capacity depending on your latitude, the amount of sun / cloud cover, etc.

Engine Alternators

Alternator -

The theory behind an engine alternator is that a current flowing in a wire generates a magnetic field around the wire, and conversely, a wire moving inside a magnetic field has a current generated inside it.

An engine alternator works by having a rotating electromagnet (rotor) spun by a belt driven by the engine. The rotor creates a magnetic field, around which are placed some fixed outer windings made of wire (stator). The electromagnet in the rotor is powered by a small field current which creates a rotating magnetic field. This creates an alternating current in the stator by the magnetic field. A solid state rectifier mounted on the outer case of the alternator converts the AC into DC and a charge controller (or regulator) converts that DC into a current useful for charging the battery.


Alternators are fitted with regulators that adjust the output voltage of the alternator by adjusting the input voltage to the field coil in the alternator rotor. If a higher voltage is applied to the rotor then the stator will produce a higher output voltage.

Because an inbuilt alternator reduces the output voltage of the alternator, as the battery becomes more fully charged the regulator will quickly reduce the output of the alternator to a fraction of the alternator's capacity, and the motor will be spinning for a long time to recharge the battery. This problem can be overcome by fitting an external regulator to the existing alternator. These are stand-alone electronic units that can drive the alternator as hard as possible to fully charge house batteries in the shortest possible time.

Companies such as Ample Power, Mastervolt and Sterling produce external alternator regulators, these usually require some form of modification to the existing alternator such as unscrewing the existing standard alternator regulator and brush holder assembly, which can be done by an auto electrician.

Generators (Gensets)

Gensets are the thing that all yachties love to hate, whether you're bothered by the purchase, installation and maintenance cost of a diesel genset or whether your night's sleep is disturbed by the noise from the genset of the boat next door. They have their advantages, however, those being:

  • They generate power whether it's sunny or not, windy or not, or whether there is any boat speed or not (unlike solar panels, and wind or tow generators). The only thing that they require is fuel.
  • Especially diesel gensets, they generate an amount of power that's impractical to generate via solar or wind on a space as small as a sailing vessel. This power can be used to run AC devices such as air conditioning, water makers or even an electric stove.
  • In comparison to running an engine alternator to charge batteries they are fuel efficient. Also they don't end up damaging the pistons or cylinders, which can happen when an engine is run for long periods at low revs just for the purpose of charging batteries.

Petrol Generators

Honda petrol genset -

A portable petrol powered generator can be used to generate a fair amount of power in a small space, and can be fairly fuel efficient. Petrol powered generators typically use between 0.25 and 1 litre of fuel for every hour that they run, and can easily power a 25A or 40A battery charger.

Petrol generators come in fixed or variable RPM types, with generating capacities usually between about 1kVA and 3kVA (for a larger capacity generator on board a yacht I would strongly consider a fixed diesel genset).

Fixed RPM generators are cheaper, but have to run at a fixed RPM to create AC power at 50 or 60Hz. They have a tendency to be somewhat noisy and run less reliably. They also generate either a square wave or a modified sine wave power output, which is suitable for running a battery charger but not ideal if you are going to run sensitive electronic equipment such as computers or laptops.

Variable RPM generators, otherwise known as inverter gensets, create 3 phase power which is rectified to DC power internally and then inverted to a fixed AC voltage and cycle afterwards. They generally create a better sine wave output and their fuel consumption is lower than a fixed RPM generator.

The Honda EU1000i/EU2000i generators are quite popular amongst cruising sailors especially as I have noticed in the South Pacific where spares for other types of generators might not be easily available. They have a reputation for being quite, reliable and efficient. They come in 120V AC and 240V AC versions.

Diesel Gensets

For higher power generation than a portable petrol generator, an inbuilt diesel genset is the answer. The smallest diesel gensets start at around 2kVA rated or continuous output and go up to about 6kVA in sizes still easily installable on a small yacht, whereas the most powerful portable petrol genset under 30kg is around 2.5kVA.

Installing a diesel genset is just like installing an engine -- with the same attention to detail. Routing fuel lines, routing the exhaust pipe (using waterlock system just like an engine exhaust, because the genset is normally mounted below the waterline), and connecting battery wiring all requires some attention.

As with petrol generators, diesel gensets are either fixed RPM or inverter types. Fixed RPM gensets run at a constant speed (usually either 3000rpm or 3600 depending on whether 50Hz or 60Hz AC is needed), whereas an inverter type only runs as fas as needed to power whatever is connected. Fixed RPM gensets have a reputation for being noisier than equivalent inverter gensets however it depends a lot on the capacity as well as mounting considerations such as noise baffling around the genset installation area.

Most small diesel gensets are single phase only, and so if you have a need to power a welding unit requiring three phase power it may be better to look at a mediu sized 3 phase portable petrol powered genset, perhaps in conjunction with an in built diesel genset.

There are sea water cooled and fresh water cooled gensets in both fixed RPM and inverter types. Fresh water cooled gensets have a sea water heat exchanger much like a diesel engine, these are the only types that I would consider in a yacht because sea water cooled gensets will generally have more corrosion issues with hot sea water running through the engine block. There are also small air cooled gensets available.

Nanni, Fischer Panda and Mase are well known companies that manufacture diesel gensets for marine use. Examine their product offerings before making a decision.

Solar Panels

Flat solar panel mounted on a doghouse roof -

Typically when we say "solar panel" on a boat we mean a photovoltaic module (PV module). A PV module is a packaged, connected assembly of solar cells in an array which can provide an output voltage (typically in the 12 - 24 volt range).

The main types of PV modules in use on boats include:

  • Polycrystalline modules packaged under glass
  • Monocrystalline modules packaged under glass
  • Thin film modules on a steel or flexible substrate, without a glass coating

The type of solar panel you want may depend on mounting considerations. For maximum efficiency per dollar spent, monocrystalline panels are the best choice. However thin film / flat panel type PV modules have their advantages especially in situations where they may need to be walked on.


Shade is the enemy of power generation with solar panels -- this should go without saying, however the effect of shade on a solar panel is more than you might think.

For example a typical solar panel rated at 12V nominal / 100W peak might produce 80W on a typical sunny day in a subtropical region at ideal temperatures, but shading 25% of the panel might reduce that not to 60W, but perhaps as low as 10W or 20W.

This means that mounting considerations are important when mounting solar panels. Mount them as high as possible or at least where there is the smallest possible amount of shade during the course of the day.

Solar panels don't work at night. Some people seem to forget this. A 100W rated solar panel will not generate as much power over 24 hours as will a 100W tow generator with the yacht cruising at 6 knots.

MPPT vs PWM Controllers

See Charge Controllers.

The choice of an MPPT vs PWM controller is much more important with solar panels than with any other type of power generation system.

Typically the maximum output of a solar panel will occur at a particular voltage, and that voltage will vary depending on sun conditions, ambient temperature, etc.

A PWM controller will pull the voltage of the panel down to a certain level, equivalent to the voltage required to charge the batteries. Typically this will be somewhere between 12.5 and 13.8 volts, whereas the maximum power point of a solar panel nominally rated at 12V may be closer to 15V or even 17V. An MPPT controller will allow the panel to output at its maximum power point and step the voltage so generated down to the voltage of the battery.

Using MPPT controllers it is also possible to run multiple solar panels in series. For example, 4 x 12V panels could be run together in series to produce an output voltage around 48 - 60 volts total, and the MPPT controller will step this down to the voltage required to charge the battery. Note that this requires a lot more current handling capacity inside the controller -- larger and more expensive MPPT controllers will do this better than smaller and cheaper controllers.

Wind Generators

Rutland 914i wind generator -

Wind generators, or wind turbine, convert wind energy into power that can be used for charging on board batteries. Typically wind generators either come with an inbuilt regulator or an external regulator, or can be fitted to an existing charge controller if there is sufficient capacity.

There are a range of different types of wind generator that can be fitted on board from a number of different manufacturers. Some examples are:

  • Ampair produce a 100W and a 300W generator. The 100W generator can be converted for use as a tow generator.
  • Marlec produce a range of wind generators, the most well known of which is the Rutland 914i. This has a reputation as being a quiet and reliable generator. Their previous model, the 913i, has created a number of similar cheap knock-off versions mostly made in China.
  • Silentwind range made in Germany includes a 400W generator.

When assessing wind generators don't just consider the "rated" output (e.g. 300W, 100W, 400W, etc). Look at the power generation curves for various wind speeds -- some wind generators sold as 400W won't actually output 400W of power unless they are exposed to 50 knots or more of wind, which doesn't happen often! Consider their generating capacity at 5 knots, 10 knots, 15 knots, and maybe 25 knots to get an idea of how much power a wind generator will actually generate. Some data points that I have been able to obtain from manufacturers are:

  • Silentwind 400W generator model generates 170W @ 22kts.
  • Rutland 914i generates 140W @ 11m/s (22kts)
  • Ampair 100 generates 54W @ 22kts. In comparison the same generator rigged for tow generator use generates 60W @ 6 knots of boat speed.
  • Ampair 300 generates 180W @ 22kts

Some wind generators have a reputation for being noisy -- check product reviews before purchase!

Contrary to common belief, the number of blades on a wind generator does not affect its output. The output capacity of a wind generator is mostly dictated by the overall diameter of the blades.

Tow Generators

Tow Generator -- click for larger view

There are few manufacturers of dedicated tow generators (water turbines) around, which is a pity because a properly set up tow generator can produce a lot of power on a passage -- frequently more than a wind generator.

The main ones are:

  • Aquair 100 by Ampair. This is a generator that can be rigged either way -- as a wind generator or as a tow generator. The unit can have a water turbine attached on a long line in tow generator mode, or can have a tail and blades added to operate as a wind turbine. The unit is rated at 100W which means around 8A at 12V. Delatbabel -- I successfully used one of these for 5000+ miles of sailing in the South Pacific, including deep in the southern ocean where there was little or no power coming in via the solar panels. I got a steady 4-6A out of it at cruising speed.
  • Ferris Waterpower 200 from the USA is sold as a tow generator but can also be converted to a wind generator. This one claims to generate 200Ah per day which means approximately 8A at 12V, also approximately 100W.
  • Eclectic Energy in the UK make 2 types of tow generator, one being the DuoGen 3 which is a convertible wind/water generator and the other being the Sail-Gen which uses the same components but is a dedicated tow generator.

Charge Controllers

Solar Charge Controller -

Old style charge controllers were frequently no more than a shunt regulator, designed to pull the output voltage of a solar module down to an acceptable voltage for charging batteries (such as 13.8V). Any excess charging capacity was just burned off as heat.

These days series charge controllers have taken over from the old shunt regulator style. Newer technologies such as smart chargers and MPPT charge controllers have become more prevalent and also reduced in price.

PWM Controllers

Until a couple of years ago, most charge controllers seen on the market were PWM type charge controllers. The PWM controller is in essence a switch that connects a solar array to a battery. The result is that the voltage of the array will be pulled down to near that of the battery.

More complex PWM controllers had various added features, such as the ability to split charge between two battery banks, to add a "load" output which could be disabled automatically by the controller when the battery voltage dropped to too low a level, digital readouts, etc.

However the basic principle remains that a PWM controller, even a well made one, does not allow a solar panel to run at its most efficient level. The main advantage that PWM controllers have is that they are cheap and their circuitry is reasonably simple in comparison to an equivalent MPPT controller.

MPPT Controllers

The output voltage of a solar cell can be varied by varying the load resistance in ohms. As the load increases on the cell, the voltage will increase, however for most of the operating range of the cell the current remains constant, which means that the power increases. At a particular point the current starts to decrease as the cell cannot handle the load that's applied, and at around this point the maximum Power of the cell (calculated by multiplying the voltage times the current) is reached. This is known as the maximum power point of the cell.

A Maximum power point tracking (MPPT) controller contains some circuitry to sample this power curve of the cell and settle on a load that allows the cell to operate at its maximum power point voltage, rather than at the voltage required to charge the battery. It then uses a DC to DC converter (actually a set of circuits including a high frequency transformer and a rectifier) to step the maximum power point voltage down to the charging voltage.

MPPT controllers have reduced in price significantly over the last few years, so that the cost difference between a PWM controller and an MPPT controller is now not as much as it used to be.

One issue with cheap MPPT controllers is that they frequently do not have sufficient current handling capacity in the transformer to be able to efficiently step the voltage down and continue to work reliably over a long period of time. A saying that's often quoted in the electronics and computer industries is fast, cheap, reliable -- pick any two. If you're relying on a cheap MPPT controller while on a long passage then it might be wise to have a similarly cheap but reliable PWM controller of about the same capacity stashed away as a backup.

See MPPT on wikipedia.

AC Battery Chargers

Battery Charger -

A battery charger (typically running from 110V or 240V AC) is used to charge house and/or engine batteries from an AC power source. Typically this is used to charge on board house batteries from shore power systems while docked at a marina berth, however it can also be used as a more efficient means of charging house batteries from small gensets incapable of producing large DC currents.

e.g. a Honda 1000i genset can typically output 8A at 12V but can also output 1000W at 240V, which is around 4A. A battery charger providing 40A to the house batteries can usually run fine from the AC output of such a generator, which means that the same amount of generator fuel is used to provide 40A of battery charging via AC as would be used to provide 8A of charging via DC.

Battery chargers vary in size and capacity from 10A or less up to 100A or more. Usually a battery charger should be sized to approximately 10% of the total house battery capacity, so a 400A battery bank should be equipped with a 40A charger. A smaller charger will run OK but will take longer to charge the batteries, while a larger charger will not be able to run at full capacity because of the inability of typical deep cycle batteries to absorb charge at a faster rate.

Battery chargers are becoming more common and cheaper these days with companies like CTEK and others producing them at commodity prices. Auto shops and marine chandlers will usually have a range of different capacity chargers.

Desirable Features

Most AC battery chargers have a temperature sensor which can read the ambient temperature. The charger then adjusts the charge rate to compensate for changes in battery temperature. This can be significant with larger battery banks.

Other desirable features in a battery charger are thermal overload protection, reverse polarity protection against accidental reversal of the output leads, short circuit protection, output overload protection and some kind of charge status readout (LEDs or LCD display).

A smart move might be a battery charger that accepts multiple input voltages -- Victron and Xantrex make battery chargers that accept any input voltage in the range 90V - 270V and can therefore be plugged in to shore power anywhere in the world and still provide the same charging capacity to your battery banks.

Combined Charger/Inverters

One reasonably recent introduction is the combined charger/inverter. This combines the functions of an AC battery charger with an inverter, having one AC input, one AC output, and an input/output for the house battery bank. Companies like Magnum Energy, Victron Energy and Mastervolt produce a range of inverter/chargers with different input and output capacities.

DC Battery Chargers

A DC battery charger is a method of charging one battery bank from another battery bank.

In the past this has typically been achieved by using diode splitters, voltage sensitive relays (which link the two battery banks together when a charging voltage is applied, and disconnects them when there is no charging voltage), relays, or simple manual switches. All of these methods have their disadvantages -- diodes can cause voltage drops and generate heat, and relays can cause large inrush currents when a fully charged battery is connected to a depleted battery, and can cycle (switch on and off repeatedly) when the charge voltage is fluctuating as frequently happens from wind and tow generators.

More recent innovations from companies such as Balmar, CTEC and Redarc have created DC to DC series chargers, which perform various functions to effectively charge an engine battery from a house battery bank. They include features such as current limiters, multi-stage charging, etc.

InterVOLT is another company in the market that has created a DC to DC charging unit that provides a stable output voltage regardless of the input voltage. It can even provide sufficient charge to an engine battery to allow the engine battery to start the engine when both the house battery and the engine battery are effectively flat -- providing a 13.8V output voltage and charging current from an input voltage as low as 10.75 volts.

Shore Power Considerations

Whether you run shore power into your yacht just for charging the battery bank, or for running AC appliances as well, there are some things that need to be taken into consideration.

AC voltage

Most ocean going sailing vessels are designed to sail in many places around the world, and it's a fact of life that there are different voltages present at shore power installations at various marinas.

Almost every country in the world uses 220V or 240V AC power these days -- the main exception being in the Americas and Caribbean where 110V is more common . If you have a yacht that's wired for 240V and arrive at a marina that's powered for 110V then you have 2 main options:

  • Run the shore power only into a battery charger (one that is capable of dealing with multiple different voltages), and run all onboard AC appliances from an inverter that delivers the correct voltage.
  • Run a transformer to step up / step down the shore power voltage to whatever is required on board.

Do not rely on marinas world-wide to carry the correct transformers for your yacht, you should decide what type of transformer you require and then carry one with you if you need to.

Fuses and RCDs

Any AC safety issue can be solved using fuses or a Residual Current Device (RCD), sometimes known as a Ground Fault Current Interrupter (GFCI). A fuse will blow if there is an oversupply of current to the boat or to an electrical component on board. A RCD will trip in case of a short circuit or current leakage to ground. Current leakage to ground is undesirable for many reasons:

  • It can cause large amounts of galvanic corrosion on any metal parts of the boat that are exposed to ground.
  • It can leak potentially lethal currents into the water -- current leakage in AC systems has been implicated in deaths amongst swimmers in marinas in the past.

Even if only a small amount of current, perhaps a small fraction of an amp, leaks between on board AC and earth, it's possible for that leakage to set up a significant amount of galvanic corrosion in a fairly short space of time. RCDs protect against this type of leakage.

Isolation Transformers

An isolation transformer eliminates any electrical continuity between AC shore power and the boat. Especially in steel hulled boats this is considered a significant advantage because it prevents any galvanic corrosion caused by a direct connection between the on-board earth and the on-shore earth in an AC system.

Connecting the ground wire of the shore-side supply to the metal parts of the boat will result in galvanic corrosion if there is a potential difference between the on-shore earth and the water. In some marinas this difference has been measured to as much as 40 volts!

Bringing only the live and neutral wire on board results in an unsafe situation because RCDs will not work nor will a fuse blow in case of a short circuit to a metal part on the boat.

The main disadvantage of isolation transformers is that they are bulky, expensive, and generate quite a bit of heat.

The effect of an isolation transformer is to create an on-board AC system and a shore-side AC system that are electrically disconnected. This allows a greater degree of protection from galvanic corrosion than just RCDs by themselves.

The obvious alternative to isolation transformers and RCDs is to only run a battery charger from the on-shore AC supply and to run all on-board AC electrics from a suitably sized inverter.

Shore Power Connectors

110V systems

Especially in the USA and other countries in the Americas where 110/150/120V AC is the norm, there are a wide range of different sockets that you might find at a marina to plug into your shore power system. Because the voltage in these systems is lower, you need more current to carry the same amount of electrical power. A 10A fitting is not sufficient and you frequently find a number of different sockets in use for 15A, 20A, 30A and 50A connections.

Two things need to be taken into consideration:

  • The current carrying capacity of your shore power cable. If you're putting more amps down your shore power cable then you need to have a heavier cable. Of course the number of amps will depend on what you have boat-side that's drawing current. That could be one or more of battery chargers, washing machines, fridges, TVs, power tools, etc. You won't hurt anyone by having a heavier cable running a lower current than it is rated for, but a cable that's rated for 10A that's in fact carrying 20A could be a problem.
  • The rated capacity of the connector on the end of your shore power cable. Although it's frequently the case that your cable can carry the rated current, the connector may not be able to. If you've got a 30A connection on shore and you're running from a pigtail into a 15A connector on the end of a 30A rated cable, then the connector may be a problem.

In most cases it probably pays to have a single shore power cable and connector that's capable of carrying sufficient current for your boat. Whether that's 15A, 30A or 50A, choose a cable and connector that will work.

When you arrive at a marina that doesn't have sufficient current carrying capacity to feed to your boat it may be the case that you need a Y adapter so that you can take two shore cables and run them into one boat side inlet.

This article from West Marine discusses the various shore side and boat side power options.

240V systems

15A caravan style plug -

In 240V countries the choice is much simpler -- the shore side power connection will typically be 10A/13A (household), 15A or 30A (marine/outdoor/caravan rated). On board you would typically have a 10A input although 15A and 30A connections for larger boats can occasionally be found. It's then just a matter of getting the right sort of cable.

In most countries running 240V you will frequently see shore side power sockets requiring plugs looking like the one in the picture to the right. This is probably the one that's most useful to carry on board, although be aware that it does come in 15A and 30A versions (with the 30A version being slightly larger). I did find one marina in French Polynesia that only had 30A sockets but most have 15A sockets.

Frequently it's possible to run to a cable with bare wire ends, and find sufficient connectors that can be attached and detached with set screws or similar. It may alternatively be preferable to use a pigtail adapter -- in this case you would have one shore power cable with this type of connector and a number of different pig tail adapters to choose from so that you could pick the correct adapter for whatever marina you are in.

Be aware that New Zealand has some odd regulations in terms of connecting boats to shore power -- you need something called an electrical warrant of fitness from an electrician before you can even be supplied with the correct cable, and pigtail adapters will usually not be allowed.

UK pigtail adapter -

For example in the UK some marinas have 13A household sockets instead of the 15A caravan sockets. A pigtail adapter like the one shown to the right can be purchased from caravan and camping supply shops or dedicated electrical retailers which allows connecting your shore power cable with a standard marine 15A plug to the 13A socket on shore.

Similarly in Australia you will find that most marinas have household style 10A or 15A sockets present rather than the caravan style sockets. These can be connected to using a pigtail adapter, although that may restrict you to using a 10A plug on your pigtail -- Australian 15A sockets will take a 10A plug, but a 15A plug won't fit into a 10A socket.

Hybrid systems

It's not uncommon to find, in the USA especially, marinas that have both 110V and 220/240V sockets. Make sure that you plug into the correct one!

When you are travelling you will frequently find that the voltage on shore is not the same as the voltage that your yacht was designed to take.

One option is to find a step up or step down transformer that will convert the available shore power to the required power for your yacht. For example, you could plug a 110V boat into a 240V system using a step down transformer. Assuming that you have a transformer in a rain-proof housing, the following suggestions should be taken into account:

  • If you are stepping down voltage, e.g. from 240V to 110V, then place the step down transformer as close to your boat as possible (or on board).
  • If you are stepping up voltage, e.g. from 110V to 240V, then place the step down transformer as far from your boat, and as close to the shore power outlet, as possible.

This is because once you have stepped up voltage the current will be lower, and once you have stepped down voltage the current will be higher. The current loss (and heat generated) in an electrical cable will be more if there is a higher current and less if there is a lower current -- basically higher voltages transmit better than lower voltages.

Another option is to move to a system where the shore power only feeds a battery charger, and that battery charger is capable of taking a range of voltages. See under Battery Chargers for more details. That means you can run any voltage into your shore power leads and it will be turned into the appropriate charging voltage for your batteries, without having to worry about transformers.



  • Kurt Kupper of Aquavolt has written an ongoing series of articles for Afloat Magazine titled Boat Electrics which cover some of the above points in detail.
  • Miner Brotherton, Ed Sherman, The 12V Bible for Boats, International Marine, ISBN 0713667036
  • Nigel Calder, Boatowner's Mechanical and Electrical Manual: How To Maintain, Repair, and Improve Your Boat's Essential Systems — 3rd Edition, International Marine/McGraw-Hill, ISBN 0071432388


Also See


We welcome users' contributions to the Wiki. Please click on Comments to view other users' comments, add your own personal experiences or recommend any changes to this page following your visit.

  • Delatbabel -- I have 4 solar panels aboard my boat:
    • 2 x flat 80W panels, "Lensun" brand, on top of the doghouse roof. I purchased these via eBay. These run in parallel into a 15A MPPT controller.
    • 1 x monocrystalline panel, 75W Kyocera, This runs in parallel with a wind generator and a tow generator and goes into a 15A MPPT controller.
    • 1 x polycrystalline panel, 200W Solraiser. This runs into a into a "Morningstar" brand PWM controller.
The panels generally give pretty good output. I'm impressed by the Lensun panels, I walk on these a lot because I need to get up on the doghouse roof to set and unset the main sail, and they still produce quite a lot of power. I'm less happy with the Solraiser panel, the peak output from it appears to be about 4A and it is a lower voltage panel (peak output at 12V compared to say 15V for some other panels) meaning that it's less easy to run its full output via an MPPT controller into a 12V system. 2 of these in series might have been a better option. It seems to do much better running into a PWM controller but really only gets up to full power when the battery voltage is quite low.
In addition to the above I have a 25A battery charger, a 1000W petrol generator, a 100W wind generator and a 100W tow generator. Despite this I occasionally need to run the engine while on passage (I don't run the petrol generator on passage because there is nowhere to safely put it, while at anchor I have it on deck), so I'm considering an external alternator regulator to reduce the amount of engine run time.
  • Webb Chiles is currently (as of 2014) sailing the world in a 24 foot ultralight sailboat that has no power generation facilities other than deck mounted flat solar panels. His only auxiliary power, used for entering and exiting marinas when required, is a Torqueedo electric outboard engine. He charges the battery for the outboard from the solar panel output as well as his house batteries. Last I spoke he was considering adding a tow generator but had not decided whether the added power generation was worth the extra weight.
  • At the other end of the scale, I recently spent time aboard a 75 foot sailing vessel that runs a 6kVA diesel genset constantly (24x7) in order to generate sufficient power to run all of the onboard electrics. The genset is mounted in the engine bay and has baffles around it for noise reduction.

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Names: Delatbabel


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