Switching to Solar Offshore

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Solar panel technology is better than ever, but shade-free locations remain a problem aboard a sailboat

Solar panel technology is better than ever, but shade-free locations remain a problem aboard a sailboat

No sensible bluewater sailor would consider setting off on a long cruise these days without some means of generating power other than by burning fossil fuels. The good news is that solar energy is becoming less expensive by the day, making it an obvious answer for providing the natural power needed to keep your yacht’s batteries topped off.

Like most things technical in the marine world, however, there is a lot of ambiguous and even misleading information being spouted by folks eager to make a buck. This is especially true on-line, with myriad claims coming from people who know little of the actual makeup and workings of solar cells. The purpose of this guide is to help you to understand the basics of how solar panels work and which will be the best for you.

The Basics

An example of a flexible solar panel, as manufactured by Solara

An example of a flexible solar panel, as manufactured by Solara

The solar, or photovoltaic (PV), “cells” that allow a solar panel to generate power are made from silicon crystals that can be either monocrystalline or polycrystalline in their makeup. The former consists of a slice of a single, large crystal and is the most productive, producing around 140 to 170 watts (W) per square meter. The latter is comprised of a selection of smaller crystals joined together, which are less energy efficient, producing 120 to 135 watts of power for the same area.

A photovoltaic panel comprises a number of these silicon cells, typically between 36 and 40, which are connected together in series to produce a voltage of between 18 to 24V in open-circuit (no load) condition; the more cells the more power. In operation, a panel’s output will also depend on a number of factors, including the amount, the angle and the intensity of the sun’s rays hitting it. Panels are commonly rated from 10W (or Wp, i.e., watts peak power) to 250W.

Monocrystalline panels are typically housed in a rugged aluminum frame and, being glass-covered, are more susceptible to impact damage. They should therefore not be walked on, although they will be fine if mounted on a cockpit gantry or rigid bimini.

For mounting directly on deck areas, semi-flexible, resin-coated panels that can be directly adhered a using mastic are usually more suitable. Although they produce only 70 to 80 percent as much output as similar-sized rigid panels, their resilience makes them a very practical compromise.

There are also a number of very flexible, amorphous “thin-film” panels available on the market designed for hanging over biminis, sail covers etc. They can even be rolled up for stowing. However, the output from these panels is limited, and they’re really only good for things like starter-battery maintenance.

Rigid solar panels shouldn’t be stepped on and thus do best on a bracket; the arrangement shown is also nicely shade-free

Rigid solar panels shouldn’t be stepped on and thus do best on a bracket; the arrangement shown is also nicely shade-free

How much power?

It’s not easy to calculate exactly how much power can be harvested using a particular set of solar panels. Though some of the confusion is due to misinformation from poorly worded sales information, the problem also stems from the many other factors and complexities that must also be taken into account.

Again, a PV panel is nearly always labeled according to its peak power output. However, this figure is about as reliable as the fuel consumption figures quoted by the automobile industry. In reality, a 100W panel, for example, is likely only going to supply around 60 percent of its peak output if mounted horizontally, due to the shading that almost inevitably occurs aboard a sailboat and the fact the sun is rarely hitting it at a right angle.

This figure can increase to 80 percent if the panel is tilted toward the sun and adjusted every hour. However, even this level is rarely achieved, because at anchor a boat can easily swing through an arc of 180 degrees due to wind or tidal shifts. (It is for this reason that, in practice, most owners simply leave their panels horizontal.)

In addition to shading and panel angle, another major factor in solar panel effectiveness is the quality of the panel’s assembly. In a well-constructed panel, each cell is isolated from the next by a series of diodes (one-way electrical valves). That way if any of the cells is shaded they won’t drag down the others with it, and the panel’s overall output will drop only slightly. However, in a panel containing non-isolated cells, output will drop dramatically even if only a small area is shaded. It should go without saying that whatever way you choose to install your solar panels, they need to be mounted away from such obvious shade sources masts, booms and sails.

Another important factor in panel efficiency is overheating. If a PV panel gets too hot, which can easily happen, especially if it is mounted directly onto a flat surface without at least an inch-wide gap to provide air circulation, its output will drop noticeably. Of course, semi-flexible panels adhered directly to a GRP surface won’t have any kind of air flowing beneath them, one of the reasons they generate less energy than their rigid counterparts.

Cabling and Circuit Protection

Another frequent cause of reduced output from a PV array is inadequate wiring. Because the resistance of a wire conductor decreases as its cross-sectional area is increased, go as big as is practicable to ensure the least voltage drop, especially with longer cable runs.

Traditionally, panels have been wired in parallel, with diodes inserted to prevent one panel from discharging into another should one of the two be shaded, and to stop your batteries from discharging out through them at night. More recently, however, with the advent of smart-charge controllers, there can sometimes be a benefit to wiring two or more identical panels in series, thereby presenting a higher voltage to the controller. Series wiring can also sometimes makes sense in the case of a long cable run (greater than 10ft), as it will reduce the effect of the voltage drop. (A quick reminder: as is the case with batteries, wiring multiple PV panels in series increases the voltage but keeps the current the same, whereas wiring them in parallel does the opposite.)

That said, while a decent controller will have no problem handling the increased voltage from four or even five panels wired in series, this approach may still be problematic given the shading found on sailboats. Simply put, it can be very difficult finding enough space to ensure all your panels will be in the sun at the same time. Therefore, if you must wire your panels in series, it’s best to split them up into a set of panels to either side of the boat: i.e., run one series bank to port and another to starboard. If you go with this option you should also fit a separate charge controller to each bank.

In addition to the built-in diode protection on each panel, a fuse rated just above the maximum total current available from the array should be fitted between the charge controller and the batteries (as opposed to between the PV panels and the controller). This way other charging devices can be connected in parallel at the battery, and the solar array can be left connected even when you are plugged into dock power and/or the battery charger is operating.

A solar, or photovoltaic, panel contains many electrically connected solar “cells”

A solar, or photovoltaic, panel contains many electrically connected solar “cells”

Planning your system

Mass production has drastically reduced the cost of solar panels over the past decade. However, as with most “marine” branded items, PV panels specifically intended for boats are higher-priced than those intended for domestic installation, because of the simple fact that the marine environment is such a harsh one.

When calculating what solar kit to buy, it’s important to be realistic about how much actual power you will be getting from each panel. Yes, a 120W/12V monocrystalline PV panel could, in theory, produce a peak power output of 10 amps when positioned at a right angle to strong direct sunlight and kept as cool as possible. However, again, this is very unlikely unless you install a sophisticated sun-tracking mechanism and there’s a nice cool breeze blowing.

Roughly speaking, the average energy available from a fixed, horizontally-mounted 120W panel over a 12-hour day is likely to be somewhere between 15 and 20 percent, providing around 215 to 290 watt-hours (Wh) and 18-24 amp hour (Ah) of the stated peak power in northern latitudes. In southern climes, figure 35 percent of rated peak power, or around 500 Wh and a little over 40 Ah over the course of a day. If you consume, on average, 100Ah/day you would, therefore, need three or four 120W panels to be able to rely on solar power alone.

Charge control

All solar arrays above 20W require a charge controller in order to safely regulate and extract the most from their output. As with the panels themselves, there’s a choice between the many cheapo cloned units available online and various different well-designed, carefully built regulators on the market.

Put simply, there are two types of controllers: Pulse Width Modulation (PWM) and Maximum Power Point Tracking (MPPT). It’s a bit like the difference between an older-style battery charger with its massively heavy transformer and a modern, lightweight solid-state device. PWM devices are very basic, hence very cheap. However, while a good PWM will get the job done, a “smart” MPPT controller will be much better at maximizing your array’s output by tracking the peaks and troughs and calculating the optimum output for the actual conditions. In fact, choosing the latter could give you up to 20 percent more useable power from the same array.

As a side note, if you also have a wind generator, don’t be tempted to put its output through the same controller, since most controllers can’t cope with two inputs fluctuating out of synch. Granted, there are a few genuine multi-input controllers out there that offer separate regulating circuits for these two types of inputs, but they are rare and expensive.

Some of the latest PV panels (Solbian All-in-One for instance) are also available with an integral controller. In the case of the Solbian model, the slim MPPT controller replaces one of the solar cells, making the panel ideal for single panel installations. However, they’re not really suitable for multiple panel arrays and can’t be connected in series to increase the voltage available.

There’s plenty of room for an extensive solar panel array aboard this performance cruiser

There’s plenty of room for an extensive solar panel array aboard this performance cruiser

Final Thoughts

Unless you throw thousands of dollars at it, the effectiveness of any solar energy installation will be a compromise. True, you could spend a small fortune fitting bi-directional tracking mounts that follow the sun’s azimuth continuously. You could also build an ugly rack across half the boat on which to mount the panels clear of any likely shadow. At the end of the day, though, you still won’t be able to control the movement of your boat or the amount of cloud cover. It’s therefore probably better to keep your installation fairly simple, but buy the best quality panels and charge controller you can afford. 

Solar Panel Basics

• More cells per panel means more voltage/output

• Rigid panels with monocrystalline cells are more efficient than semi-flexible panels with polycrystalline cells

• Mount panels where they won’t be shaded

• Allow a reasonable gap for adequate airflow underneath your panels

• An adjustable mount will allow you to keep your panels pointing toward the sun, but has its limitations due to boat movement

• A quality Maximum Power Point Tracking (MPPT) charge controller will pay for itself by boosting your array’s output and efficiency


Coastal Climate Control

Genasun (controllers)

Gioco Solutions

Global Solar




Nature Power

Solarland USA


Victron Energy

MHS Summer 2019



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