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Article by Tor Pinney                                                                                                                                        Back to List of Tor's Tips


The Optimum Electrical Power System 
for the Cruising Sailboat

1989 Tor Pinney - All Rights Reserved


When a sailor forsakes dockside shore power for the wild blue yonder of the cruising world, his vessel becomes a self-sufficient living environment. It has to produce whatever energy is required to operate the various electrical and electronic accessories aboard. This entails periodically recharging the ship's storage batteries. Most sailors accomplish this by running the engine to drive the stock alternator. Boats with refrigeration, whether 12-volt or mechanical, run engines as much as two hours every day to keep the fridge and batteries topped off. To the long distance or liveaboard sailor, this translates into considerable fuel consumption and engine wear over the years. Running a large diesel engine without sufficient load on it will shorten its useful life, not to mention the noise it creates.

Ah, but there are better ways, alternatives to this common, inefficient method of deriving ship's electrical energy from running the ship's main propulsion engine. The energy we need is available from the sun, wind, and water, and from more efficient fossil-fueled engines. When combined in an integrated energy system, these contemporary sources can relieve the engine of double duty as a generator by providing 12-volt power for operating on board equipment, including refrigeration.

An "integrated energy system" for the cruising sailboat is simply a monitored combination of equipment that produces, stores, and distributes ample 12-volt, DC electricity to meet the needs of the vessel. The four components of this optimum yacht electrical power system are the sources, the storage batteries, the distribution, and the monitors.


12-Volt Energy Sources

Solar Panels

The most fundamental, readily available source of energy on Earth is the sun. The photovoltaic cell is an effective method for converting solar light energy into electrical energy. Multiple photocells (or solar cells) are laminated onto sheets of paper-thin stainless steel (for marine-grade solar panels) and sealed with a clear, protective coating of space-age polymers to form solar panels. Flat, lightweight, and durable, many of today's solar panels are well suited to use aboard boats.

There's a place and a need for one or more solar panels on nearly every cruising sailboat. Their function is to continuously and silently recharge the ship's batteries during hours of sunlight. For a vessel with minimal electrical gadgetry, located in a sunny region, this "trickle charge" may be all that's needed to keep batteries topped off. It's a valuable, virtually maintenance-free source of energy in an integrated energy system.

Solar panels are available today with either rigid or flexible housings. Rigid panels are marketed by a host of companies. They're usually between and 1 inches thick and come framed in many rectangular sizes, the handiest being anywhere from one to three feet wide, and from two to four feet long. On a boat, they can be screwed flat on any clear deck area or cabin top. They'll often need a bit of shimming to compensate for camber in the deck.

However, solar panels are most efficient when they're angled to face the sun directly. So it's a good idea to mount them in a manner that enables you to adjust the direction they face. At anchor, you could simply lean and lash a panel wherever it will get the most direct sunlight. But it's probably a better idea to gimbal-mount panels permanently so that they can pivot and/or tilt to optimize their angle to the sun. Stern rail mounting accomplishes this easily. Installing reflectors to direct more sunlight onto the panel surface can enhance solar panel performance.

Flexible solar panels offer even more possibilities for sensible installations on a sailboat. They come in a variety of sizes ranging from one to eight square feet. Their chief advantage over rigid panels is their ability to mount smoothly on curved or flexible surfaces such as a cabin trunk, a cambered deck, or a Bimini top. They're lightweight and quite thin - about inch - so they're never in the way. Best of all, they can drape over a curved dodger top or lash, snap or zip onto a sun awning. So while you're making shade, you're also making electricity. On the down side, flexible panels may not be as efficient than their rigid counterparts. Check the rated watts when comparing solar panels' output.

The amount of electricity a solar panel produces depends primarily on the size of the panel and the directness of the sunlight striking it. Manufacturers tend to advertise absolute maximum amperage output of their products based on perfect, controlled conditions where blazing, unobstructed, perpendicular sunrays strike flat panels. The amperage output is then measured right at the panel.

In real life, however, the sky is rarely cloudless, nor the sun at maximum strength and declination. Rigging and spars will sometimes cast shadows, decreasing a panel's output. Also, on board a boat the electricity must travel some distance through wiring and diodes before it trickles into the battery, further reducing amperage received. So when a panel is rated for, say, 3 amps (about 36 watts), it will actually yield a net average of 1.5 to 2.5 amps during the brightest part of the day. If a panel yields 2 amps for 5 hours a day, the batteries have absorbed 10 amp-hours - enough to play the radio and power your cabin lights that evening.

If you wire several panels together in series, the power output is multiplied. A boat in Florida was seen to have eight 33-watt flexible solar panels mounted on the sun awning. That's 8 panels X 2 amps (approximate average output) = 16 amps X 5 (bright sun) hours per day = about 80 amp hours. That's enough amps to power a hefty 12-volt cold-plate refrigerator/freezer.

Wiring solar panels is simple. The wires pass inside the boat through a watertight fitting available at most marine stores. Some solar panels have a built in diode, a device that allows current to flow in one direction only. If yours doesn't, then solder a diode in series in either wire somewhere before the positive and negative wires connect to the corresponding battery bus terminals. A Schottky diode is recommended because of its low voltage drop. This will prevent batteries discharging through the panels after dark. For safety, you should install both an in line fuse and an on-off switch between the solar panel and the distribution panel. You can wire in an ammeter to monitor the amount of solar energy flowing into your batteries.

Prices for marine-grade solar panels vary among. Larger panels cost proportionally more than smaller ones. Expect to pay $300 to $500 or more for 1 ft. x 4 ft., 40-watt rigid panels, and as little as around $100 for small sizes that produce one amp or less.

Wind Generators

Harnessing the wind is nothing new to a sailor! With a wind generator, you can convert wind's kinetic energy into DC current for around-the-clock battery charging. Of course, this is a more productive energy source in regions of steady breezes, such as the trade wind belt.

A wind generator is a DC generator driven by a propeller. When the wind spins the prop fast enough, the unit produces a trickle charge ranging from an amp or two, up to 10 or 15 amps, depending on wind speed and the propeller and generator size. A full-size ship's wind generator in the Virgin Islands, where the trade winds maintain a steady 15-knots much of the time, can easily produce 6 or 7 amps continuously, well over 100 amp-hours per day. That's enough to power radios, lights, radar, television, and a 12-volt refrigerator/freezer!

Beware of manufacturers' spec sheets that promise amperage output in very light (5- to 7- knot) breezes. The measurements are taken at the generator output terminal and only indicate what a virtually "dead" battery might absorb in these wind conditions. For DC current to be accepted and stored, the source voltage must be higher than the voltage in the receiving battery. A wind generator may well be generating current in light airs, but at only 9, 10 or 11 volts. So no amps will go into a battery that's only partially discharged to, say, 12 volts - which is the typical on board scenario - and no charging will occur. Most of the large-bladed wind generators don't actually do much charging until the wind exceeds 10 knots. The small bladed, European style units need a good deal more wind than that to produce useable energy.

The popular American-made wind generators feature two- or three-blade propellers with diameters of four to five feet. These are hefty generators with relatively good light air performance. Some European designs are smaller in size, weight, and blade diameter, with multiple-bladed props. These are less productive in light breezes, but unlike the large models, they can be left running in very high wind conditions.

There are several ways to mount a wind generator on a sailboat. It can be hoisted into the foretriangle area by a jib halyard while at anchor, positioned by guys. This allows the unit to pivot and track wind shifts. But it must be taken down and stowed somewhere every time you want to get underway, so this installation is more practical aboard a vessel that tends to remain anchored for long periods of time. Some skippers mount their wind generator permanently on the forward side of a mizzenmast, which eliminates the set-up and tear-down hassle, but will not allow the wind generator to pivot or face anything but a head wind. A bow pulpit mount is also rigid, with the added disadvantages of being vulnerable to damage and dangerous to crew.

The most effective solution is mounting on a pole. A pole mounted wind generator, normally located at the stern, can pivot to face any wind shift as your boat sails back and forth at anchor or lies to a current. Best of all, it's always in position to operate, even under sail. When sailing close hauled, a stern pole-mounted unit is especially well powered, getting strong apparent wind funneled into it by the mainsail. The ability to operate underway makes the pole mount most valuable to cruising sailors, who need to keep their batteries charged during long offshore passages. Though this increases the boat's windage somewhat, it's not enough to matter to a sailboat loaded for cruising. A further development of the pole mount arrangement is the bi-pole rig. Looking something like a football goal post on the stern of the boat, one pole supports the wind generator; the other, a radar dome or antennae dish. The horizontal crossbar between the two lends structural support. It's also a handy place to mount antennae, such as loran and sat nav, where masthead height isn't imperative. Or you can mount two wind generators and double the charge. The structure is stiffened by struts running from each pole to the boat's rail, either fore or aft, and by one post-to-post diagonal bar that creates the triangulation necessary for absolute strength on the athwartship plane.

Pole mounts are generally custom made and must be very sturdy. The installation is often strongest if stepped through the deck onto a block or plate secured to the hull below. With a little planning, pole mount assemblies can be engineered to disassemble by unbolting. This will be necessary for boats whose cruising itinerary includes the canals of Europe, for example, where masts are unstepped and the vessel's height clearance becomes critical. Poles (or bi-poles) can be either stainless steel, or schedule 40 aluminum, the latter being lighter and less expensive.

A word of caution: The spinning blade of a wind generator propeller can be very dangerous when mounted within reach. Brightly painted or reflective blade tips make the blade more noticeable to crewmembers.

Many wind generators come with some kind of over-speed governing device or brake to protect the unit against damage in very high winds. Otherwise, they must be manually shut down. In preparation for extreme wind conditions, they should be taken down altogether. Generally, they should not be left running when the boat is unattended for long periods. It may be worthwhile to wire your wind generator through a regulator to prevent overcharging the batteries. Also, they need a diode, either installed in line or built into the unit itself, to prevent battery discharge in calms.

Wind generators, especially the larger units, create some noise and vibration. In moderate conditions, it's a whooshing sound that passes as unnoticed background noise. In high winds, it's often a louder fluttering, chopping, whirring noise. Some pole-mounted wind generators shudder momentarily when they pivot to track an apparent wind shift, though a new generation of balanced epoxy props has eliminated this vibration on at least one popular brand, the Windbugger. Maintenance on wind generators is minimal. Wooden propeller blades need occasional painting. Periodically, the brushes and bearings in the generator need replacing, just as in an automobile alternator, but for the most part wind generators just keep on working, night and day, to keep your ship's batteries charged. After the initial cost of purchase, usually ranging from $800 to $1100, your ship's electrical energy is as free as the wind.

Water Generators

Water generators can be a valuable addition to the cruising sailor's integrated energy system. They are the least common alternate energy source, perhaps because of the inevitable drag created by towing that extra propeller in the water, 25 to 35 lbs. at 6 knots of boat speed. Or perhaps it's because most boats spend the majority of their time in anchorages rather than under sail.

A water generator is a DC generator activated by a propeller towed in the water. The prop is allowed to freewheel when the boat is underway. There are four basic ways to set this up, and there are variations: (1)Ttrailing a propeller astern on a cable or braided line. The passing water spins the prop, which twists the cable and the cable turns the generator mounted on the stern rail. On some models the prop is attached to the generator and a cable tows the entire unit; (2) Mounting a style of water generator that looks like a small outboard motor, its lower gear case and propeller submerged. (3) Installing a dedicated propeller shaft through the hull of your boat and letting it freewheel when sailing to turn the generator. (4) Connecting a generator by way of a belt, bicycle chain, or gear drive to the ship's main prop shaft and allowing that to freewheel when sailing. The trailing portion of the first, towed type may be damaged or taken by large fish. The latter two options require custom design and installation.

Many wind generators readily convert to water generators, propelled by the cable and prop method. In fact, wind and water generators are much the same in terms of cost and maintenance, being the same machine with different drives. Because the fluid coupling between water and propeller is much greater than between wind and propeller, water propulsion yields about double the amps per knot of speed, generating about 5 amps at 6 knots of boat speed. To get the same 5 amps from the wind requires about a 12-knot breeze. For a sailboat making an ocean passage, running down the trades at 6 knots plus, that adds up to an extra 120 daily amp hours for the taking! On a cloudy day, on a dead run with little apparent wind, when the solar and wind energy just isn't coming through, the water generator will run everything aboard as long as you keep sailing.

Like the wind generator, the water generator can be shut off when your batteries are topped off. Or it can be wired through a regulator, eliminating this requirement.

Generators and alternators are very similar machines. Both produce DC current for charging batteries. Generators are more rugged than alternators; alternators are the more efficient of the two. If you'd like to build your own wind or water generator from an ordinary alternator, the book, The 12-Volt Doctor's Practical Handbook by Ed Beyn (Spa Creek Instruments, Annapolis, MD), describes how to do it.

Engine Alternators

The alternate energy sources we've discussed so far are all important ingredients in the integrated energy system. They will greatly reduce, if not entirely eliminate, the need to run the engine in order to charge batteries. But the main engine's alternator can and should be a ready source of electrical energy, particularly when you're running the engine to propel the boat anyway.

Most marine diesel engines come standard with too small an alternator to cope with the large capacity battery bank of the liveaboard cruising sailboat. It may well be that with solar, wind, and water energy sources, you often won't need more than a small alternator. But in harbor conditions of overcast skies and light winds, the alternator becomes your only means of recharging the batteries away from the dock. So a more efficient system is worth considering.

For years now we've been hearing about switches that enable us to manually over-ride the alternator's regulator for quick-charging the batteries. One of the functions the regulator is to control the voltage output of the alternator or generator by regulating the current in the field coil. This eliminates the risk of overcharging and perhaps destroying the battery. When you over-ride this function, the alternator can pump full charge into the batteries without automatically tapering off as the battery becomes charged.

A variable rheostat can replace this on-off switch, allowing you to increase or decrease the field current and, therefore, the charging rate. But this system requires constant monitoring. You can damage your alternator by overheating it. The chief danger, however, is that you only have to forget just once to switch back to automatic regulation and your batteries will be boiled and permanently damaged. That's a pretty stiff penalty to pay! Even with an automatic shut-off wired in, the variable rheostat is, at best, an old fashioned, partial solution in our quest for more efficient battery charging. There is an optimum charging curve which demands precisely decreasing current at specific battery levels during the charging cycle. Human control, even devoting full attention during the process, is going to be less than perfect.

Today's solution is solid state electronics and high output alternators with self-governing regulators, such as the series made by Balmar Products of Seattle, Washington. These units measure the condition of each battery and automatically regulate charge according to the ideal charging curve, a more efficient and much safer alternative to the old manual control methods.

Mounting a non-standard regulator may require some minor customizing of the engine brackets. It's possible that the increased side load on the fan belt may cause bearings to wear rapidly. Before increasing the size of your alternator, consult the engine manufacturer for approval.

When you upgrade your boat's alternator, leave the original unit mounted in place, if possible, to facilitate a temporary switchover if the big one ever fails. Otherwise, clean the old one, spray it thoroughly with light oil, wrap it in plastic, and store it away in a dry locker. It is always wise for a cruising sailboat to carry a spare alternator, as well as replacement brushes and bearings for both units aboard.

The Power Charger

If you need more battery charge than you're getting from the sun, wind, and water generators, there is a diesel powered alternative to running the engine. Balmar Products markets the Power Charger, comprised of a four-horsepower Yanmar diesel engine that drives a large (100-amp or 140-amp) Balmar alternator. It's compact, weighs just 65 lbs. and burns only a pint of diesel fuel per hour. For boats with large power consumption devices like DC refrigeration and microwave oven, the Power Charger may be a valuable part of an integrated, ample charging system. From the spare pulley provided, you could drive a mechanical refrigerator compressor at the same time, or a scuba compressor, or auxiliary, large capacity water pump for a deck wash and/or emergency bilge pump. Balmar offers an optional water desalinator that runs off this unit, producing 20 gallons of fresh water per hour while you charge your batteries and fridge!

Installation isn't too difficult, but the Power Charger really needs a water cooled (and muffled) exhaust system to be tolerable, not the dry exhaust offered as standard. Until a water-cooled exhaust system is made available, installing one yourself is a complex custom job.


AC Power Sources

While a 12-volt DC electrical system satisfies most of the needs of most cruising boats, there are certainly occasions when 120-volt AC power is desirable. It's handy for operating hand tools such as drills and saber saws, galley appliances such as microwave ovens, blenders, and coffee makers. On large boats the conveniences of air conditioning, washers, dryers and water makers may be considered important. Here's an overview of several different ways to obtain 120-volt AC power aboard your boat.

Besides shore power, which of course is restricted to dockside use, there are four basic ways to have AC power aboard a sailboat: (1) Carry a portable gasoline or diesel AC generator or install a heavy duty gen-set; (2) Install an inverter to make AC power from your ship's DC batteries; (3) Install an engine driven AC generator; (4) Install an AC electric generator powered by the ship's DC batteries. A fifth type, seldom if ever seen on boats, is the emergency standby generator system that operates on LPG or LNG. Manufactured by Winco in 5,000- and 8,000-watt models, prices are about $3,400 and $4,100, available from the Hamilton Ferris Company.

Before buying, make a detailed list of all electrical appliances aboard and the number of watts consumed. Include lights stereos, TV's, hair dryers, tools, air conditioning and so on. Electric motors (circular saws, air compressors) may require up to five times their normal operating wattage during start-up; this is called surge and must be factored into your calculations. List normal watts and surge watts separately. Though you probably won't run all appliances at the same time, the power rating of your AC source (inverter, gen-set, and so on) should exceed by a safe margin the number of watts you expect to use.

Portable Generators and Gen Sets

For many small boat cruisers, fossil-fueled portable generators provide all the intermittent AC required. Numerous manufacturers such as Honda, Yamaha, Tanaka and Nissan sell small, lightweight (20 pounds and up) and inexpensive ($350 and up) generators to the RV market. Yanmar makes a line of portable diesel generators, starting at two kilowatts and 127 pounds. While most are not made of marine-grade materials, common-sense maintenance and weather protection enable them to survive at sea for many years. The units stow handily in cockpit lockers. They are perhaps most useful for powering hand tools and as emergency back-ups for battery charging.

Larger boats with electric stoves, air conditioning and other luxury appliances have little choice but to permanently install a gen-set. Because electrical equipment in the United States is designed to operate with a fixed frequency of 60 Hz, the frequency output of the generator must be fixed at 1200, 1800, or 3600 rpm. The slower-turning engines are quieter but heavier than the fast-turning models. The 1800-rpm, four-pole set is a good compromise for many boats. It makes sense to use the same fuel (gas or diesel) and exhaust type (air- or water-cooled) as the main engine.

In most boats the gen-set may be mounted in the engine room. In any case, it must be ventilated, sealed from the living cabins, convenient for fuel and raw-water hook-ups, and mounted on sturdy structural members. Unless you're very knowledgeable about such installations, this is a job best left to professionals. Many of the major manufacturers, including Norther Lights, Onan, Kohler, Westerbeke and Medalist Universal Motors, publish useful manuals to help size, select, install and maintain their products.


An inverter is an electrical device that changes DC from the ship's batteries to AC and boosts voltage from 12 to 120. Unlike some older models, the sophisticated modern inverter, such as the Heart Interface Power Inverter, produces a smooth sine wave suitable for running TV's and computers. It is a good method of obtaining intermittent AC power, but of course it is limited by the capacity of the battery bank - the more ampere-hours in the bank, the longer the inverter can be run. Unlike the generator, it is not a power source, but simply a means of changing electrical current from one form to another. Some principal manufacturers are Balmar, Dytek, Heart, IMI/Kenyon and Trace; prices range from about $100 (for a small 100-watt model) to $2,500 (for a powerful 2,000-watt model with battery charger). Optional add-ons increase usefulness and cost. A power inverter is a convenient addition to the integrated energy system on a cruising boat. And best of all, it doesn't rely on fossil fuels.

Engine-Driven AC Generators

While a gen-set is merely an AC generator directly geared to an internal combustion engine, the engine-driven AC generator just uses the main engine instead of an auxiliary, dedicated one. Engine-driven AC generators such as those made by Auto-Gen mount on or near the main propulsion engine and are driven by belts and pulleys. Therefore, AC power is available whenever the engine is run. Auto-Gen's units are available from 2.5 kilowatts to 6.5 kilowatts and cost from about $2,000 to $2,700.

AC Electric Generators

Like an inverter, the AC electric generator "makes" AC from DC, but not by the same means. The AC electric generator uses battery power to run a small electric motor, which then turns the generator. Honeywell is one of several manufacturers; its units produce from 500 to 1,600 watts. Again, such a device is limited by the ampere-hour capacity of the battery bank.

There is no single "right" product for cruising boats. Just as an integration of two or more 12-volt solar, wind and water generators helps to meet the varying conditions found under way and at anchor, a combination of AC power sources offers versatility. For example, if the gen-set is operated only when the main engine is shut off, an inverter or engine-driven AC generator can provide AC under way. It really depends on the number and type of appliances installed aboard your boat, and how you use them.

Shore Power

Many cruising sailors spend as little time as possible dockside. Nevertheless, we may at times tie up and plug in the "umbilical cord," so shore power should figure into our ship's integrated energy system.

For our purposes, the 125-volt, AC electricity brought aboard through the dockside power cable needs to be converted or rectified into Direct Current at 12 volts. In many countries and throughout Europe, a boat wired for American voltage will need a 220/110 step-down voltage transformer to avail itself of shore power. This current can then charge the boat's batteries to supply all our electrical needs while we're plugged in.

What's called for here is a marine AC converter/battery charger. This differs from an ordinary automotive battery charger in two important ways: (1) The design ensures electrical isolation between the 125- volt AC circuit and the battery circuit, which prevents stray currents that pose shock hazards aboard a boat and could set up corrosive electricity in the surrounding water, and (2) The converter produces a non-trickle type charge delivery, which avoids the risk of damaging batteries by overcharging. The marine charger automatically shuts itself off when batteries are fully charged, and switches itself back on when battery voltage drops.

Remember to shut down any alternate energy sources, like solar and wind, which aren't self-regulating. Overcharging batteries is one of the quickest ways to destroy them!


Storage Batteries

Batteries are the heart of the integrated energy system. Because the living environment of a boat requires relatively low amperage doled out over a long period of time, we use "deep cycle" batteries - batteries designed for gradual discharge - to power our lights, radios, and so on. Deep cycle batteries are built more heavily than standard batteries, which are designed to deliver lots of amps in brief bursts such as for starting an engine.

The first consideration for the battery bank in a successful energy system is its capacity. To some extent, capacity is determined by how much space, weight and money we can afford. Most stock boats do not have enough battery capacity for liveaboard cruising. As a rule of thumb, the ship's batteries should be able to supply nominal electrical energy for four 24-hour days of live-aboard consumption without recharging.

To compute how much energy you typically consume, see the Sidebar "Typical Loads For Accessories". For each item, multiply the amperage it consumes by the number of hours you use that device per day to give you the ampere-hours (amp-hours) drawn. For example, you may burn the anchor light for 11 hours per night. It draws 1.2 amps. So the consumption is 13.2 amp-hours (1.2 amps X 11 hours). You might play the stereo for a few hours in the evening: 1 amp x 3 hours = 3 amp-hours. Total up all the amp-hours you've calculated, and you have your approximate daily electrical energy consumption. Consider that your usage is quite different during offshore passages, where running lights, radar, and navigational devices are operating for long periods. Depending on your cruising style, you might average this in. Now, multiply the total daily amp-hours times four. If your batteries can deliver this total without requiring recharging, you've got an adequate battery bank aboard.

Batteries cannot deliver their rated capacity. To do so would drain them "flat", and that's a sure way to shorten the life of a lead-acid battery. It is healthiest for batteries to be recharged once they've reached their 50% capacity. The 50% discharge point of deep cycle, 12-volt batteries is 12.2 volts. We monitor this from an accurate voltmeter. But even 50% is too much to expect, because our batteries are not always charged to capacity. In fact, after normal charging, they're only at about 85%. So, in order to limit discharge to the 50% level, we have just 35% of the rated capacity of our bank available for use without recharging (85% charged - 50% discharge level = 35% available).

This means we want our 4-day amp-hours to equal 35% of our batteries' rated capacity. Let's assume a 50 amp-hour daily usage aboard. That's 200 amp-hours for four days. This indicates a battery bank rated for about 572 amp-hours (35% of 572 = 200.2), requiring three or four large, deep cycle batteries. With this kind of deep cycle battery capacity, there really isn't a need for a separate, standard engine start battery.

Do yourself a great favor and invest in top quality batteries. Their price may be higher at purchase, but their cost will ultimately be lower when averaged out over years of continued use beyond the life span of cheaper brands - not to mention the reliability factor.

Batteries are best mounted low in the boat because they're so heavy and can effect boat stability and trim. However, they must also sit above high bilge-water levels. The less distance between energy sources, batteries, and loads (distribution panel and appliances), the better, because power is lost in long wires. Batteries shouldn't be allowed to get very hot, as they will if installed in most engine compartments. Heating will shorten their life considerably. They need to be secured so that they can't possibly come loose, even if the boat is turned upside down and shaken. If they're lead-acid batteries, they need to be in strong battery boxes that will catch any acid that leaks, and they should be ventilated. Be sure the installation is readily accessible for routine maintenance.

You have the option of using pairs of 6-volt batteries, connected in series, instead of single 12-volt batteries. These may be easier to move around and install. There's the added advantage that if one cell goes bad, ruining the battery, you only have to replace half as much battery.

The lead-acid battery is the most common 12-volt battery type. But today there is an alternative: gel packed Dryfit Prevailer batteries made by Sonnenschein Batteries (Cheshire, Connecticut). They have several advantages over lead-acid batteries: They cannot spill acid, they don't form dangerous gases while charging, they're totally maintenance-free (never needing water added), and, according to the manufacturer, they can be discharged flat without harm. The only way to damage these military-designed batteries is by overcharging them. In size, weight, price, and longevity they're comparable to top quality lead-acid batteries.


Typical Loads for Accessories

Estimated power consumption of some common on-board devices


Anchor Light 1.2
Anchor Windlass 75.0
Autopilot 4.0
Bilge Blower 2.5
Bilge Pump 4.0
Cabin Fan 1.2
Cabin Light (incandescent) 1.5
CB Radio (receive) 1.0
Compass Light 0.1
Deck Wash Pump 10.0
Depth Sounder 0.1
Fluorescent Light 0.5
Forced Air Heater 7.0
Foredeck Light 1.7
Fresh Water Pump 8.0
Fuel Pump 3.0
Head Pump 18.0
Horn 3.0
Inverter 1.4
Knot meter 0.1
Loran 1.2
Masthead Tri-color 2.0
Microwave (via inverter) 95.0
Propane Electric Shut-Off 0.7
Radar 3.7
Recording Depth Sounder 0.5
Refrigeration (cycling) 25.0
Running Lights 3.5
Sat Nav (average) 0.3
Spreader Lights 6.0
Spotlight 10.0
Steaming Light 1.0
SSB Radio (receive) 2.0
SSB Radio (transmit) 25.0
Strobe Light 0.7
Tape Player 1.0
VHF Radio (receive) 0.3
VHF Radio (transmit) 4.5
Water Maker (small 12-volt) 3.6
Weather Fax 2.4
Wind Speed indicator 0.1

To estimate the amperage draw of other 12-volt devices, divide their rated watts by 12.



If the batteries are the heart of the integrated energy system, then the distribution system is the brain. Distribution of electricity aboard happens in two stages: First, source energy is consigned to one or more of the batteries. Then the stored energy in the batteries is directed to each device as needed.

Source energy other than that of the alternator (i.e., current from solar panels, wind and water generators, battery chargers) may be channeled through switches enabling us manually to determine the target battery, the battery to receive that charge. But that's not necessary. Each of these trickle charges can simply be wired directly to one specific battery. If, for example, we have dedicated one battery to power a 12-volt refrigerator compressor, usually the heaviest load aboard, we can expect this battery to be depleted daily. It's going to need almost constant replenishment of energy. If the wind generator is the most consistent producer aboard, we can wire it to this refrigerator battery (we'll call it battery #1). In breezy conditions, the Windbugger will generate 100 amp-hours or more daily, enough to keep Battery #1 topped off.

Now, suppose the wind is up and the wind generator is cranking out a steady 9 amps - over 200 amp hours in 24 hours. That's more than the fridge battery can use. We'll want to share some of those spare amps with other batteries. We could use some to top off the reserve or starter battery (#2) if we've dedicated one, or to supplement the "house" bank (#3). This is done through a pair of master battery selector switches, through which we can connect the #1 with #2, or #1 with #3, or #2 with #3 (see Wiring Diagram). Electricity will flow from whichever bank has the higher voltage to the bank that has the lower voltage. Like water, voltage seeks its own level. Therefore, soon after two batteries are connected through the switch, they'll level off at the mean voltage. In our example we'll connect the wind-charged fridge battery (#1) to the house bank (#3). First they'll level off. Then, as the wind generator's trickle charge enters Battery #1, half of it will bleed over to #3, keeping them both charged to the same level.

Conversely, when the refrigerator compressor cycles on while #1 and #3 are connected, then it will be drawing from both batteries simultaneously. There are times when this may be desirable. The main thing is that we are in control.

The one energy source that ought to be "automatic" is the alternator. This can and should be self-distributing as well as self-regulating. By wiring the alternator through isolators, its charge is directed to all batteries. A dual isolator enables you to charge 2 separate battery banks simultaneously. A pair of dual isolators charges 4 banks, regardless of the battery selector switch positions. The isolators determine which batteries need how much of the charge, and distribute the alternator's amperage output accordingly. So if the house bank is low when you fire up the engine, but the refrigerator battery is well charged from alternate sources, then the alternator will jam maximum amps into the hungry #3 battery, while sending only a trickle to #1. Finally, as all batteries approach a charged condition, the self-regulating alternator gradually reduces its output to zero.

Once energy is directed to and stored in the batteries, it can then travel to the ship's electrical panel, stage two of the distribution system. The modern panel includes rows of breaker switches, each one labeled for its load device. A word of warning: Many panels have AC and DC on the same board, albeit in separate rows. If you're ever poking around behind such a panel while plugged into shore power, and accidentally cross an AC terminal with a DC terminal with your screwdriver, a massive dose of electricity may be instantly distributed into you, or, through the negative grounded engine, into the surrounding water. Someone could get electrocuted! All 120-volt AC breaker panels should be physically separated from 12-volt DC panels. They rarely are, so be careful!

Many boat manufacturers locate the electrical panel in the nav station. Frankly, there are more useful things - navigational electronics, radios, etc. - that deserve this prime space. Panels can be placed in almost any convenient spot away from possible spray from hatches; the nearer to the batteries, the better.

From the panel, myriad wires run throughout the boat to the appliances. They should be color coded and recorded in diagram form for future reference. Neat, tie-wrapped bundles of wire, well secured to bulkheads along their route, are the mark of a professional installation. Most devices call for an in-line fuse of proper amperage, a smart precaution even if the panel's breakers perform the same protective function. In general, unless you're especially well versed in wiring, you should hire, or at least consult, a professional marine electrician regarding wire sizes, insulation, connections, diodes, fuses, switches, and safety precautions. Faulty or improper wiring is a prime cause of fires on boats.

From Top Left

A Power Charger will guarantee ample, efficient recharging for the #1 (refrigerator) battery and, through the switches, the #2 and #3 banks as well. The wind generator also feeds directly to Battery #1.

The main engine's large alternator charges all battery banks, each according to need, distributed by a pair of dual isolators.

Battery #2 is a reserve battery. It sometimes functions as an engine starter battery, and sometimes as a second refrigerator battery, as determined by the switches' positions.

Solar and water generators feed the #3 (house) battery bank. This house bank, which is shown as a pair of 12-volt batteries in parallel, also receives the battery charger's input when shore power is available. When shore power is disconnected, the power inverter can supply 120-volt AC to the ship's electrical sockets, drawing 12-0volt current from the #3 bank.

The #4 battery is dedicated to an electric anchor windlass. It is mounted far from the rest of the system, requiring long wire runs. For this reason, and since it normally is used with the engine running, Battery #4 is only charged by the engine alternator and cannot interact with the other banks through the switches, as can Batteries #1, #2 and #3.

The Switches

The system requires two four-position battery switches. Switch A receives Batteries #1 and #2. The output of this switch is used for engine starting. If Switch A is pointing to #1, then that battery will be called upon to start the engine (#1 is already feeding the refrigerator directly). The 1&2 setting combines these two batteries in parallel, for boosting the refrigerator battery's capacity, or for engine starting with two batteries. Set to #2 alone, it uses the reserve battery for starting.

The output of Switch A also goes to the A-terminal on Switch B. Switch B is also wired to the #3 battery bank. In the middle (A&3) position, Switch B connects the #3 bank with whatever Switch A is set to. So, if Switch A is set on #1, and Switch B is set on A&3, then battery banks #1 and #3 are connected. This would be a useful setting when shore power is connected, enabling the battery charger's charge to flow into #3, then onward into #1 to keep the refrigerator battery topped off as well as the house bank. Output of Switch B goes to the distribution panel.


The alternate energy monitor illustrated uses a pair of meters - a DC amp meter and a DC voltmeter - to monitor three alternate energy sources. The meters display the output of whichever source the four-position switch is set to: wind generator, solar panels, or water generator.

The Balmar digital meter displays the precise voltage of any one of three battery banks. It also displays the number of amperes flowing into or out of each battery, and how many amps are being produced by either of two sources (one source normally being the engine's alternator).

The distribution panel shown also houses four meters:

120-volt AC Load Current - how many amps of AC we are consuming
120-volt AC Line Voltage - how much AC voltage is being supplied
12-volt DC Load Current - the total DC amps we are using
12-volt DC Battery Condition - approximate battery voltage level


Monitors and Controls

The monitors are the eyes of the integrated energy system. By observing how much current passes in and out of the system, and how much we have stored, we can more effectively control and conserve electricity aboard. We can also protect the system from damage due to negligence.

Monitors, for our purposes, are meters. There are two basic types we can use: analog (needle-in-a-window) meters; and the newer, extremely accurate, solid state digital meters with LCD readouts. The latter are the better choice where accuracy is desired, such as observing battery voltage.

Monitoring the incoming source energy can be accomplished with simple needle meters. One AC voltmeter can display shore power when we're plugged in dockside and an AC ammeter will indicate how much load we're putting on the AC system. This will help us avoid overloading the 30-amp circuit that moderate-size sailboats commonly use. Away from the dock, the same AC meters will indicate voltage provided by, and amps drawn from the power inverter, if we've installed one.


We want to know how much energy our alternate sources are providing. DC volt meters, wired to the solar, wind, and water generators, illustrate at what point these units are developing enough voltage to start pumping amps into the batteries. Even more useful are DC ammeters. Wired to the source generators, they register how many amps each unit is producing. Knowing this, we can easily judge whether these sources are keeping pace with our consumption, and which ones are helping the most at what times. A single voltmeter and a single amp meter can monitor all three alternate energy sources one at a time if you wire in a three-way switch to each meter.

Similarly, one analog voltmeter can monitor all the batteries if a switch is wired in line to connect the meter with each battery separately, one at a time. An off position in the switch will conserve power, avoiding the inevitable small draw of a meter in constant operation. One DC ammeter should monitor the total amperage consumption aboard.

It's imperative that we watch the batteries closely. Neglecting them will surely result in damage. We need to know when our batteries reach the 50% discharge point (12.2 volts) to avoid deep-cycle discharge, which shortens battery life. Conversely, we must guard against overcharging that can ruin a battery.

The old fashioned way to determine lead-acid battery condition is by measuring the specific gravity of the sulfuric acid and water electrolyte with a hydrometer, a glass tube with a calibrated float inside and a rubber suction bulb on top. This is still a good gauge for identifying a bad cell. But the measurement is affected by temperature variations, making it a less than perfect means of determining voltage. An accurate voltmeter is needed for this. In the analog meter category, an "expanded-scale suppressed-zero" meter is the best choice. It ignores voltages below 10 or 11, because a battery is completely dead and depleted of useful charge at that point anyway. Instead, this meter's scale makes it easier to read exact fractions of volts by only displaying levels between, say, 11 and 16.

The most accurate meters are the digital models. Balmar makes one of the best. It will display the voltage level of three separate batteries (measured to the 100th of a volt!), and the amperage passing in and out of each. It also monitors the amperage output of two energy sources, one normally being the alternator. The selector button lets us choose what information is displayed on the LCD screen. With this monitor system, it's easy to check the voltage of each battery with a glance, several times daily. We can also see instantly whether a battery is gaining or losing amps, as loads that are consuming power compete with sources putting energy back in.

As you get used to making quick, regular surveys of your ship's energy components, you will find your confidence increasing with your ability to monitor and control your integrated energy system.



Today, more and more sailors are extending their cruising range and, correspondingly, the length of time that their boats must provide a complete living environment. Electrical energy systems aboard small and mid-size sailboats have come a long way to keep pace with the growing demand for civilized amenities afloat. We can have for our convenience, our comfort, and our safety ample 12-volt and 120-volt power for all normal marine and household uses. An integrated energy system will provide it continuously and efficiently. More power to you!

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