Articles, Catamaran Electrical Systems

PV solar panels are now an accepted part of a cruising boat’s electrical power inventory, and rightly so—they produce consistent power year after year with no noise, smell, or moving parts. They are especially popular on multihulls, where panel mounting options are numerous.

Unlike solar panels used for heating, PV (“photovoltaic”) panels convert light energy directly into electricity through the use of thin, specially treated silicon cells. When exposed to sunlight, each cell in a panel produces about 0.5 volts, regardless of cell size. Roughly 36 cells are connected in series to create sufficient voltage (17-19 volts) to charge a 12-volt battery even when light levels are low. When a sun-drenched PV panel is connected to a battery, current flows. How much current relates to cell size as well as how much light is reaching the cells.

PV panels are rated according to their “peak” output in watts (ie. their best performance in favorable conditions). Shading, clouds, and facing away from the sun all contribute to power loss. Multiple PV panels can be wired in parallel according to desired current output, available mounting space and budget.

“Standard” PV panels have sturdy tempered glass covers, aluminum perimeter frames for rigidity, and long warranties against power loss (10-20 years!). “Marine” PV panels have polymer covers and no perimeter frames. Both are completely weatherproof and able to withstand tough conditions at sea. Standard panels are most cost efficient in terms of $/watt, but they cannot be walked on, are quite a bit heavier, and don’t have as many mounting options. Marine panels are lightweight, can be walked on, and can be mounted in a variety of ways—some models are even fully flexible for mounting to biminis and dodgers.

Despite laboratory breakthroughs using exotic materials, they haven’t been able to improve commercially available panels much in the last decade, proof that this technology has arrived. PV advances have come mainly in the form of the public’s acceptance, fully flexible panels (thin film technology), a lower panel cost/watt, and the advent of blocking diodes between cells on all fully flexible (UniSolar models) and some standard panels (Solavolt’s 75 & 85 watt panels). Blocking diodes prevent total power loss when part of the panel is shaded, a great asset on a sailboat. Mounting methods have also become increasingly creative.

In addition to the panels, you’ll want to have a simple controller to prevent battery overcharge and some way of monitoring panel output. PV solar panels work well on their own for small electrical loads, or in combination with wind & water generators or engine-driven power sources. In the next issue we’ll focus on wind generators, another renewable source of power.

Part II – Solar Panels – Going to Extremes

It used to be that a 40- to 50-watt photovoltaic (PV) solar panel was considered a sizeable piece of gear to fit on a sailboat, especially if it was a standard panel with glass cover and aluminum frame. Sailor’s wanting to supplement their battery charging with solar power often spent long hours agonizing about how and where to mount one or two of these “large” panels. (*Note—A 50-watt panel typically covers about 4.5 square feet, and they come in various configurations; some manufacturers favor longer, narrower panels, while others favor wider, shorter versions.)

Within the last year or so, however, attitudes toward solar panel size and how much they can contribute to a total cruising electrical load appear to have changed dramatically. Most sailors, boat manufacturers, and boat designers now seem to be comfortable with solar panels of all sizes, and are making provisions for solar power to be a major part of their battery charging mix. When 50-watt solar panels first came out, they seemed huge. Now sailor’s I talk with don’t bat an eye at the prospect of mounting multiples of 50-, 60-, 80-, even 120-watt panels on their multihulls. These days, it’s not unusual for me to set up a multihull solar charging system capable of supplying 70 to 140 amp-hours a day, using 240 to 480 watts of solar power. Now that’s a serious amount of electricity! Of course, I still meet a few people at the shows who think solar power is only suitable for trickle charging batteries, but then these unenlightened individuals invariably sail monohulls.

What is interesting to me is that the price per watt of the larger standard solar panels is roughly the same, regardless of size or rating. This means that you can use the panels best-suited to the mounting space you have. For instance, if (4) 60-watt panels fit your space better than (2) 120-watt panels, you won’t pay much, if any, more for the 60-watt panels. You’ll still pay more for thin marine panels that you can walk on, but then they offer advantages in terms of less weight, lower profile, and more mounting possibilities.

Why are large solar systems now so popular on boats? One answer lies in their inherent simplicity. They are by far the most benign power source on the planet, and simplicity is essential for successful cruising. There are no sounds or smells to contend with as they silently, reliably transform sunlight into electricity, day after day for decades. Solar panels also have no moving parts, and the standard panels on the market have absolutely nothing to wear out or maintain—manufacturers of standard panels are presently giving 10 to 12-year warranties against power loss. (*Note—The polymer covers on marine “walkable” panels may eventually fade or delaminate, and the thin film cells of fully flexible panels degrade slightly over time—marine panel warranty periods are more like 3 years because of the increased likelihood of abuse.

Another reason for the surge in solar panel use is the popularity of hardtops and tubular frameworks at the stern or over Biminis. These provide unobstructed, out-of-the-way mounting space for large areas of panels. If you need to be able to walk on your hardtop occasionally, consider using multiples of a low-profile marine panel such as the Solarex MSX30 watt. If you choose glass-covered standard panels for use on a hardtop, try to keep some open deck space on either side of the boom for emergency use. My advice to those in the process of setting up an electrical power system on a boat is to first do what you can with solar power within the confines of your budget and the mounting space you have or are able to create. In my next column I’ll describe the process of planning a complete solar electrical power system for the new owner of a Maine Cat 30.

Part III – Mounting Solar Panels

Most multihull sailors are aware that photovoltaic (PV) solar panels can be a great way to charge batteries on a catamaran or trimaran. These technological marvels are able to convert light into electricity with no noise or moving parts, and they can do it with greater efficiency (over 16{87af57bf33b759b13edf1201e0aac8ff568782d54202a219d5fee60abad8e986}) than standard incandescent light bulbs can change electricity into light (10{87af57bf33b759b13edf1201e0aac8ff568782d54202a219d5fee60abad8e986} efficient). The advantages of solar panels are numerous and a multihull provides an almost ideal platform for solar power, but even multihull sailors face the sometimes daunting task of finding suitable places to mount solar modules.

Mounting solar panels requires creativity, craftsmanship, and acceptance of the fact that every type of boat and every boat owner imposes limits to how much solar power should be included on board. Once you establish how much solar power is the right amount for your situation, a little creativity can help you find practical mounting solutions that don’t detract from the boat’s appearance while allowing the panels to have good average exposure to the sun. Craftsmanship in constructing the mounting hardware and installing the panels makes for a rugged yet visually pleasing installation.

There are several ways to determine the right amount of solar panels for your situation. One approach is to simply include one or two medium-size solar panels (50 to 100 watts of total power) in your battery charging mix. This allows for a gentle introduction to solar panel operation, as well as an opportunity to see how solar panels affect your battery charging needs and the appearance of your boat. You can always add more panels in future, one of the many benefits to solar as a charging source; this is particularly easy if your charge control is sized to handle additional current. Another approach to determining how much solar power to have on board is to put as many solar panels on your boat as you have space and money for, since in most situations you’ll probably run out of one of those before satisfying all your electrical needs. Whichever approach you take, the first step is to review the various types of solar panels on the market and their mounting options.

There are three basic types of panels: 1) standard models with a glass cover and aluminum perimeter frame; 2) semi-flexible marine panels with a polymer cover and a rigid backing plate; and 3) fully flexible models. When planning for solar panels, keep in mind that for best performance the panels should have good average exposure to direct sunlight throughout the day. Unlike with home solar installations, there is usually no “south” side to a boat (except perhaps when anchored in the trade winds), so your best option for permanent mounting is to install the panels horizontally, facing up. You can increase performance substantially if your mount allows you to pivot the panels into the sun, but be aware that getting those extra amps can become tedious, and you can actually decrease solar performance if you forget and leave the panels pivoted in one direction. Another consideration when mounting panels is that thick, dark shadows can diminish solar output dramatically, although fully flexible panels have blocking diodes between strings of cells that minimize this effect.

With this in mind, let’s look at the various mounting options.

Deck Mount

The classic solar panel mounting location is on some convenient, unobstructed portion of the boat’s cabin top or deck. Catamarans usually have lots of cabin top space to work with, while trimaran amas offer an unobstructed solar mounting location away from regular foot traffic. Semi-flexible panels made for the marine market, such as the Solarex MSXL-series, are best for deck mounting in areas where the crew may be walking. These panels can actually be walked on without harm, although I recommend you avoid mounting them in areas of high foot traffic. Since these panels are thin and can assume a slight curve, they can be fastened directly to the deck, passing the fasteners through rubber grommets in the corners of the panels (see sketch A). The grommets protect the deck and keep the panels just above the deck surface, allowing for some necessary air flow. Fully flexible panels can also be mounted on the deck, although they are put to better use on soft surfaces such as biminis and dodgers. Standard solar panels can not be walked on, so if you do deck mount them it’s best to do so on hardtops or other places well away from normal walkways. On hardtops, make sure to leave ample space on either side of the boom for access to the mainsail and to prevent the panels being shaded by the boom and sail cover.

The best way to deck mount a standard solar panel is to provide two parallel rails as shown in sketch B. These rails can be fashioned from teak and secured to the deck with screws, or made of fiberglass and faired into a glass deck surface. The latter technique is a favorite with many catamaran builders. Either way, construct the rails so the panel can be recessed yet still have airflow beneath to maximize panel performance. The panel can be fastened to the rails with screws or with pivoting clips that allow the panel to be easily removed and made portable in port so you can keep it faced into the sun.

Hatch covers seem like a convenient place to mount solar panels, but there are several drawbacks: 1) hatch covers located underneath the boom are usually shaded by the boom and mainsail; 2) it can be difficult to find panels that are the right shape; and 3) if the hatch is transparent or translucent a solar panel will tend to block the light and view.

In the last issue we reviewed the various types of solar panels currently on the market and began a discussion of how to mount them on catamarans and trimarans, beginning with the classic Deck Mount. In this issue we’ll review several other good mounting options for solar chargers.

Rail Mount

sketch C. Panels mounted in this manner can be rotated about the rail to increase performance or to tilt the panel out of a passageway. Rail mounts also allow solar panels to be easily removed.

There are several variations of the Rail Mount theme:

Stanchion Mount

Some solar suppliers offer an angled piece of stainless steel tubing that attaches to a stanchion with a double set of rail clamps as shown in sketch D. Solar panels are mounted to the outer part of the tubing using a regular rail mount clamp set. In this arrangement the panel can be rotated about the tubing, and the tubing can be rotated about the stanchion, allowing the panel to face directly into the sun regardless of boat position or time of day. This rig is especially useful when in port.

Stanchion-to-Stanchion Mount

Another clever rail solar panel mounting system is a kit that creates a rigid section of rail tubing at the top lifeline between two stanchions as shown in sketch E. The lifeline passes through the tubing, which is attached to each stanchion with special adapters. This arrangement allows you to mount even large solar panels where they can be rotated to seek the sun or to tilt them outboard out of the passageway.

Davit mount

Davits provide a convenient place to mount standard solar panels. A davit mount kit features of a piece of stainless steel tubing installed between the davits, as shown in sketch F. This rail not only allows a solar panel to be mounted with a standard solar rail mount kit, it also gives the davits better resistance to lateral movement.

Arch Mount

Mounting solar panels on a stern arch can be a variation of a deck mount or a rail mount, depending on your method of attachment. The simplest approach is to weld or bolt horizontal struts the width of a solar panel onto the top rail of the arch, as shown in sketch G. The solar panel bolts directly to the strut in a fixed position. Another approach is to weld a smaller diameter rail (7/8″ or 1″) over the top bar of the arch. The upper rail provides a convenient place to attach standard rail mount hardware, which allows the panel to pivot into the sun, and to be easily removed if necessary.

Bimini and Dodger Mount

Biminis and dodgers are great places to attach fully flexible solar panels. Fully flexible panels are less efficient, which means they take up more space for a given power output, but that extra space is often available on large areas of canvas overhead. Fully flexible panels can be attached using their corner grommets or by actually sewing them into the bimini or dodger fabric. When choosing a mounting location, prevent the panels from being directly shaded by the boom.

For those trying to maximize their solar charging potential, rigid panels can be mounted on a separate metal tube frame installed over a conventional bimini. Alternatively, you can skip the soft bimini and simply create a “solar bimini” consisting of two arrays of standard solar panels, one each side of the boom. For additional weatherproofing, a strong yet lightweight rigid hardtop can complement or even replace the metal tube frame. This arrangement is especially practical on catamarans or beamy monohulls.

With a healthy dose of ingenuity and a desire to maximize the contribution from clean, renewable charging sources, solar panels can become standard gear on any blue water sailboat.

Other components

To make your solar installation complete, you’ll need the following components:

Wire. Properly sized 2-conductor wire rated for marine use. Even though the current is relatively low, the length of run can be substantial. Size the output wire for no more than a 3-5{87af57bf33b759b13edf1201e0aac8ff568782d54202a219d5fee60abad8e986} voltage drop.
Deck plug. A high quality deck plug allows you to lead the solar output below decks to the charge control. Make sure the plug is rated for the maximum amperage expected.

Charge Control

A charge control sized for the maximum amount solar current for present and future needs. New solar controls on the market have Pulse-Width-Modulated (PMW) circuitry for efficient charging, and either analog or digital monitoring of solar current. Some type of current monitoring is essential for accurate tracking of solar performance.


Only a few panels on the market have built-in blocking diodes that allow unshaded portions of the panel to keep working near full capacity, or a main diode to prevent reverse leakage at night. Most charge controllers supply reverse leakage protection, but you may need additional diodes, especially if you have panels mounted port and starboard wired in parallel. Diodes allows an unshaded panel(s) to produce full power if the other panel(s) become shaded.

Fuse and disconnect or circuit breaker. A properly sized fuse or circuit breaker should be placed as near the battery being charged as possible; some convenient method of electrically disconnecting the panels makes for a good installation.


About the Author

Independent Energy Guide Kevin Jeffrey is a long-time multihull sailor, independent energy consultant, author and book publisher. He is the author of Independent Energy Guide, a valuable resource for cruising mutihull sailors, and is the publisher of Adventuring With Children by Nan Jeffrey and the first three editions of the Sailor’s Multihull Guide. Sailors Multihull Guide