There seems to be many misconceptions regarding solar panels, particularly concerning marine installations on vessels. I’m thinking that maybe it’s because we all see solar panels on houses and assume that what applies to roof-top installations also applies to boats. Wrong!

Here’s some solar facts. Typical solar panels with silicon cells will:

• Produce electrical power whenever they are exposed to light, and this is proportional. Small amount of light - small power output.
• Produce full voltage even in very low light, often even indoors, but only if they are disconnected and not being asked to produce any power. Potential voltage output depends mainly on the number of cells. Each cell typically produces around 0.6 to 0.7 volts.
• Produce full current output only if there is a big enough load, the sunlight is good enough, and shading is non-existent. Potential current output depends on the size and type of cells.

In general; the quantity of cells determines voltage; the quality of cells determines amperage.

Now, what we are after in a boat solar application is to grab whatever power we can whenever there is available sunlight, and that means from dawn right through to dusk, not just at noontime. I often hear customers say that there is absolutely no shading at the back of their sailboat, but then when it is suggested that maybe in early to mid morning or mid to late afternoon, the presence of a humongous mast and associated rigging might shade a stern-mounted panel, they will humbly agree. From that sort of response one could deduce that many boaters are under the assumption that solar panels will really only work for a couple of hours each side of noon. Not so Horatio.

A while ago I debunked some common misconceptions about solar panels in another blog here, but it seems I missed one that has come to light several times recently, i.e. the myth that to have proper, serious, useable solar power on a boat one must use residential panels, as marine solar panels are just itsy-bitsy wimpy battery top-up trickle-chargers.

Nothing could be further from the truth!

Typical marine solar panels are comprised of a number of silicon cells (normally 32+) connected together electrically in a series string. Individual silicon cells produce only around 0.6v to 0.7v, and so enough of them have to be connected together in series to produce a voltage high enough to be able to charge a 12v battery.

A Charge Controller must be connected between the panel and the battery to reduce the panel output to a safe charging voltage. Some panels have less than the normal number of cells and produce less voltage than is required to charge a 12v battery, and these will require either a special boost controller, or for a number of them to be connected in series to produce a higher voltage.

The good news for those with flexible solar panels with a polymer top coating is that maintenance is minimal.

1. Keep the modules clean by washing them with fresh water to remove salt water deposits, bird droppings, dust particles or other debris that can lessen the chances of the sun's rays from reaching your solar cells.

The surface of the panels may be cleaned using neutral soap and water, wiping carefully and without using abrasive material. Denatured alcohol (methylated spirit) can be used to remove grease, etc.

When panel is clean and dry, apply a coating of a plastic protectorate such as "Plexus" or Novus Plastic Cleaner #1- available at your marine chandlery or online.

If contaminants build up or oxidation occurs over time, it may be necessary to polish the panel. The recommended polish is either ReJex or Novus brand polish, #2 (Fine scratch remover). It is recommended that you hand polish the panels, as a power polisher could burn the surface with too much friction. Follow the instructions on the product label.

2. Check the structural integrity of the installation and the electrical connections periodically.

3. Check the efficiency of the system using the monitoring functions in the charge regulators (LEDs or displays).

SunPower® back-contact solar cells are currently the highest efficiency cells available for use in everyday applications at 22%+ rated efficiency. But they can be expensive and difficult to purchase. Genuine high-grade SunPower® cells are only sold to prestigous manufacturers in a few select markets, and significant quantities, a minimum 10,000 cells at a time, must be purchased by the manufacturer to ensure a workable price for their customers.

It has recently come to light that some back-contact cells made by SunPower® have found their way on to secondary markets and into panels originating primarily in China. Rejected by SunPower®when they failed their rigoroustesting, these faulty cells cannot carry the prestigious SunPower® name due to their substandard quality. Unfortunatley, some promotional material we’ve seen brazenly disregards this fact.

Be wary of any solar panel with back-contact cells that does not explicitly make reference to the cells as being genuine, high-grade SunPower® cells.

An increasingly popular question we hear at Boat Shows is "What size solar panel do I need to run the refrigeration on my boat?" Of course, this begs the question "What is your refrigeration's current draw?" which in itself may not have a set answer.

The current consumption of any refrigeration system is meaningless unless all the conditions are specified, i.e. box temperature, ambient temperature, water temperature, compressor speed, voltage, etc. The manufacturer's figures are simply average numbers; some manufacturers, like Frigoboat, attempt to give a true average figure, while others use their figures more as a marketing tool.

Making use of variable compressor speed, as Frigoboat does with the Merlin II and Guardian speed controls, and others do with the Danfoss/Secop AEO control module, brings even higher efficiency and lower overall power consumption in air- and keel-cooled systems, but is counter-productive in a pumped-water system. This is because the pump adds 25% to 35% extra current draw, and so it is best to run the compressor at full speed and get the job done as quickly as possible. The most efficient system is one using the Keel Cooler; no pump, no fan, and variable compressor speed can be used to gain even higher efficiency.

Daily consumption is measured in amp/hours per day, and you can get an idea of what to expect by using a watts meter, like Watts Up, on your refrigerator leads. These meters can tell you how many amp/hours per day a device is using, and it's best to use it over several days to get a daily average. The running consumption will vary dependent on conditions, as will the on/off cycle time, so you will need to be able to come up with an average daily amp/hr draw.

As a general rule of thumb, a solar panel with SunPower® cells will give approximately 1/3 of its rated wattage as a daily yield in amp/hrs. A panel with regular monocrystalline cells will produce about 1/4 of its wattage as daily amp/hrs, and a polycrystalline panel produces around 1/5 of its wattage rating in amp/hrs per day.

Let's take an example:

Mounting solar panels on hand rails around a boat's perimeter allows for some flexibility of use, plus the opportunity to angle the panels if you happen to be a dedicated twiddler with too much time on your hands. But when using standard aluminum-framed glass panels, necessity dictates that the clamps that secure the panel to the rail must be on the center-line lengthwise.

Glass panels are pretty heavy, and too much twisting and torsion will either "soften up" the aluminum frame or cause the glass to crack and/or shatter. Most panels of this type are designed to be installed on solid, permanent structures like roofs (specialized marine panels like the Solara Ultra series being an exception), and it is assumed that these land-based platforms will not heave, pitch, twist, or generally rock-and-roll (barring earthquakes) like a small/medium sized boat often does.

With this in mind, having a balanced mounting point is essential when using glass panels in rail-mount applications if regularly having to replace panels is to be avoided. But having a balanced mount, i.e. with the rail clamps on the center-line lengthwise, means that half of the panel's width will be intruding into the walkway, cockpit, etc. when deployed, and the wider the panel the more it will become an obstacle to movement.

Choosing a narrow panel is one way around this dilemma. Here's one way to do it:

We hear it all the time – flexible solar panels are not worth buying, they only last a short time, they never give out the watts they say they will, they are not as durable as glass panels, the list goes on and on of the laments by those who bought a solar panel that didn’t live up to its manufacturer’s hype.

The published wattage rating of solar panels is determined by testing with a machine that flashes a light with an intensity of 1,000 watts per square meter on to the panel as if the sun were directly overhead. This flash test, performed under standard temperature and air quality conditions, gives a theoretical maximum power output that might be possible from that panel under the most ideal conditions.

This may sound like a cheap marketing ploy, but in fact is done from a safety aspect, as the cable and safety devices (fuses, breakers, etc.) must be sized in accordance with the maximum power that the panel(s) might produce, especially in multiple panel arrays. Typically, a panel will produce nowhere near its rated output in normal use, except maybe occasionally and then only very briefly. Considering the above, and the multitude of ever-changing conditions in a real deployment, it is not practical to simply use the wattage rating of a solar panel as an indicator of what power output to expect over the course of a complete solar day; i.e. from sun-up to sun-down. So how can we estimate what size panels, and how many, we might need to satisfy our daily amp/hour consumption?

First it is necessary to differentiate between the three most common types of silicon cells in general use.

I think we all get the idea of what a “By-Pass” is, whether it’s a by-pass road to divert traffic around a city, or a heart by-pass operation to channel blood supply around a restricted artery. So what does a By-Pass Diode do in a solar panel? Obviously it must divert something around something, but what, why, and how?

Right away, let’s quell the myth that By-Pass Diodes are there primarily to improve performance under shaded conditions.

Series or Parallel - Which current flow option do you make when installing solar panels or battery banks?

There seems to be an increasing need to explain the differences between series and parallel electrical circuits recently, especially regarding battery and solar panel installations. So this week’s blog will be an effort to better explain the advantages and disadvantages of each configuration, and where better to start than with the water analogy … again.

Did you know that there is an official measurement for a hairs breadth? Well, according to my conversion tables:

one hairs breadth = 100 microns (micrometers, millionths of a meter)

So the thickness of a regular silicon solar cell, at around 200 microns, or 2 hairs breadths, is pretty darn thin! And when you consider that the SunPower® back-contact cells that are used by Solara and Solbian are even thinner, at about 1.5 hairs breadth, and that these cells are essentially glass in nature, you will no doubt appreciate that they will need careful handling.

The SunPower® cells used in Solbian flexible panels are high grade cells that are purchased guaranteed free from physical defects, but careless handling in storage, shipping or on site, together with improper installation, can initiate cracks in the cells which will be invisible to the naked eye.

In the majority of applications these cracks will be comparatively harmless and may not cause any problems other than a small loss of power, but

Standard solar panels produce much higher voltages than are safe to feed directly to a battery, so a solar controller or regulator must be connected between them.

Two types of controllers are available; PWM (Pulse Width Modulation) and MPPT (Maximum Power Point Tracking).

PWM Controllers - When charging, these controllers feed the power from the panel straight through to the battery until
the battery voltage reaches a predetermined Acceptance level. It will then keep the voltage at that level by pulsing
the panel voltage on and off to keep the battery voltage constant.

During the Bulk stage of charging, the amperage delivered to the battery is slightly less than the amps from the panel due to losses in the electronics.

MPPT Controllers - These controllers take the best mix of amps and volts from the panel to give the maximum power.
They then track this Maximum Power Point as conditions change to ensure the highest power is extracted from the panel
at all times.

The DC panel output is then inverted into very high frequency AC and then converted back down to DC to feed to the battery. The result of that inversion/conversion process is that more amps can be delivered to the battery than were produced from the panel, resulting in reduced charging times.

That is a significant advantage over PWM controllers, and more than justifies the higher cost of the MPPT models.

After several discussions recently with customers contemplating adding solar to their electrically powered vessels, there is still a lot of confusion about exactly how much power can be realistically expected from solar panels. I have an inkling that these customers are so honed in to watts and kilowatts from their dealings with propulsion that they assume that all watts are equal.

But solar watts are a different animal.

The watt is a measure of power and is normally derived electrically from multiplying volts times amps (W = V x A), or amps squared times resistance (W = I² x R).

So, using simple math, if we have a 5,000 watt (5 kilowatt, or 5kW) DC electric propulsion motor running on 100 volts, we would expect it to be drawing 50 amps at full load. If the same sized motor was designed to run at 50 volts then the current draw would be double that at 100 amps.

For reasons of wire sizing and cost of ancillary equipment we’d want to keep the amps as low as possible, so the higher the voltage the better (except for having highly lethal voltages in damp environments). As you can see, the numbers are all very simply calculated and it’s all cut and dried, and it had better stay very dry indeed!

Solar watts are different, and I’m referring here to the wattage rating of the panel(s).

When you want solar on your boat... You Want Solar On Your Boat! Now, where you put it depends on what real estate you have available that is relatively free of potential shading and yet still gives you room to move around.

Those of you with a sail boat need to be aware of the shadows cast by your mast, boom, and rigging. If you have a power boat, be aware of the shadows cast by your radar, arch, antennas, water toys, helicopter, etc.

So, let’s look at what others have done.

1. Dingy davits are popular locations for either glass panels or lighter weight semi-flexible panels. The davits themselves should be specifically designed and fabricated to carry the extra weight. Notice the clever person who put lightweight panels in-line with the davits to allow easy access to the dingy and swim ladder.

We had another highly successful show in Miami this year, with a lot of interest in our solar offerings, but we still find it necessary to spend considerable time with interested parties having to explain the what’s what of solar power for boats. Evidently there is still a lot of misinformation out there on the subject, so this seems like a good time to re-hash one of our most popular blogs from many moons ago. Here follows a list of ten myths and busts in an effort to set the record straight.