After a recent 5-day cruise aboard our trawler where we had to continually power up the generator to supply electricity to the 115 volt ac freezer to maintain temperatures, I began to consider alternatives. We put over 90 hours on the boat generator in just 5 days; time for another oil change! I installed an inverter shortly afterwards and that improved the run time for the generator significantly, but there was still work to be done. I still needed to keep the batteries charged to operate the inverter. Then it occurred to me, why not explore installing solar panels on boats.
Solar panels have been successfully used since the mid 1950s, originally used in manned space exploration. They have been dropping in price since about 2004 when their popularity really took off. And now with the Green movement afoot, solar panels are as popular as ever. So I set out to research them and determine how to buy and install one; I was in for a shock.
You can find many retail suppliers online that will sell you a solar panel but nowhere could I find a detailed description of how to determine what to buy and how to install it; much less aboard a yacht. So this article was born as I made my way through the process; thus is a truly a learn-as-you-go article.
What is a Solar Panel and How Do They Work?
Solar panels are in theory any panel that uses the sun’s thermal energy to produce electricity. A solar panel can be described as a photovoltaic panel, the term used in the industry, for panels designed to produce electricity from the rays of the sun. Despite the category of solar panel being discussed, almost all solar panels are flat. This is because the face of the panel needs to be at a 90 degree angle from the sun’s rays for the most favorable angle to absorb the sun’s rays.
Solar panels are able to take in energy from the sun through an array of solar cells on their surface. Much like how a plant is able to soak up energy from the sun for photosynthesis, solar cells perform in a comparable manner. As the sun’s rays hit the solar cells on a photovoltaic panel, the power is transferred to a silicon semiconductor. The power is then changed into (dc) direct current electricity and then passed through connecting wires to finally enter a storage battery.
Types of Solar Panels
Types of panels most normally used in boating applications have either multicrystalline or amorphous thin-film cells. Multicrystalline panels are the oldest technology available and also the most powerful. When sized appropriately and matched to suitable batteries, these are the panels to use for operating large loads such as refrigeration.
Amorphous thin film solar panels are only about 50% as effective as multicrystalline panels, but can be bought in flexible forms so they can roll or fold, or correspond to the shape of a yacht cabin top or bimini. They don’t normally have enough output for significant energy replenishment, but can be used to trickle charge a battery bank.
How Much Power Do Solar Cells Make?
Generally, we measure solar panels by wattage and that is how we buy them. You can buy solar panels for boats as small as 10 watts to as large as 200 watts or even larger. But it is easier to understand when we convert watts to amperage.
We arrive at these values by multiplying the number of hours the panel spends in full sun (usually defined as 5 per day in Florida) times the panel’s wattage.
For a 195 watt solar panel the output would be 195 x 5 hrs = 975 watts/day. Taking it step further, 975 watts/12 volts = 81.25 amps per day.
Before you consider what size panel to buy for your boat, you will need to complete an energy budget to determine what sources of power consumption you have aboard while at anchor. I use the at anchor situation as this is where you will use the most energy; under power and your boat will be able to supply its needs without any problems.
Example, if you have 3 interior lights that draw 2 amps each and you leave them on for 4 hours per night, your consumption would be 3 x 2 x 4 = 24 AH/Day. We are not concerned by running lights and electronics as they will not likely be operable while at anchor.
DC Load Table
|Fresh Water Pump|
|Other||Total Amp Hours|
Inverter Load Table
Inverter loads also use DC power but they are powering AC appliances and equipment. If you need to convert watts to amps use (12watts/12 volts = 1amp).
|Other||Total Amp Hours|
Calculate your total daily energy consumption AH/per day
Solar Energy Production Table
Alternative sources of power such as solar panels can replace the amp/hrs drawn from the batteries. But like the energy budget that calculated your usage you will also need to calculate your re-supply of amp hours. Remember the formula – (12 watts/12 volts = 1 amp). But keep in mind, the formula is only a gauge; absolute accuracy can only be where the panel output is constant and a solar panel may at times operate inefficiently due to shading by clouds.
|Solar Panel 1||5|
|Solar Panel 2||5|
|Total Amp Hrs Production|
Compare the daily energy consumption in AH/Day to the solar energy production. Your solar energy production should be greater than the consumption. If not, select a larger wattage panel and recalculate. Always purchase more solar panel output than you will think you will need; some planners recommend at least 30% in excess.
Example – 100 watt solar panel/ 12 volts = 8.3 amp x 5 hours = 41.66 AH/Day production.
Mounting Your Solar Panel
Now that you have your solar panel, how do you go about installing solar panels on boats? As we said before, mounting the panel 90 degrees to the sun is optimum. You will get the best energy production this way. But on boats, finding a suitable location is tricky at best. Some boaters place them on brackets placed on the rails, other place them on top of the bimini, and I have seen them placed on the yacht dingy stanchions. But where ever you choose to mount them, remember that to get the most out of them they should be in the open, free from any shading from booms, yacht radar arches, or cabin structures. Remember that at anchor, the boat will turn to the sun twice daily due to the tides.
I chose to mount our panel on the top of the trawler sundeck hardtop in a horizontal manner. Here it will get the best view of the sun and be clear from the radar arch shading as the trawler turns at anchor. The angle toward the sun in not exactly at 90 degrees but it will have to do. I chose a 195 watt panel so I have almost a 50% reserve capacity in my panel to make up for the slight inefficiency of the sun’s angle. We bought the panel from Sun Electronics in Miami, as they had the best pricing I could find anywhere online. But remember, panels must be shipped via freight as they are heavily packed to reduce the chance of damage so be sure to calculate those costs in your purchase.
What is the best method to attach the panel to your boat? There are a number of manufacturers of solar panel mounting rails and supports but almost all of them are designed for roof or ground mounting. West Marine does carry a product for mounting small panels to the rails. Many boaters make their own mounts.
I found a mount made by Sunsei called a Sunsei Glue Mounting Kit that is attached to the boat and panel using 3M 5200 Marine Adhesive. The mount allows the panel to be mounted with about 2 inches of clearance beneath the panel for ventilation. I did not have to drill any holes in the hardtop either. You can find these mounts at Amazon.com.
Wiring Your Panel
Marine electrical wiring is very technical and hazardous; if you are not comfortable in doing this part of the task, please consult a qualified marine electrician. Your panel will be pre-wired for attaching to your boat but you will need to supply the connecting cables that will also be sold by your panel supplier; they are referred to as MC4 cables. The cables will come in different lengths suitable for your needs with a male and female connector attached; you cut one connector off.
In addition, you will also need a controller. The controller regulates the electrical flow from the panel to your batteries keeping your batteries charged yet preventing over charging. Some controllers are simple but others have LED displays showing the rates of charge etc. The more whistles and bells the more expensive it will be. Your panel supplier will be able to recommend a controller that will meet your needs.
I chose a controller made by Specialty Concepts. It is simple yet does the job. And the folks at the company are a big help in helping you choose the right model for your panel. When you contact them, the will need to know what size panel (wattage) you have and what the voltage is. I also bought my controller from the people at Sun Electronics in Miami.
The people at Specialty Concepts have also calculated how temperature will affect current flow and recommend that their controllers not be placed in engine rooms as the heat produced will reduce the controller efficiency by about 25%. I located mine in the electrical panel under the lower helm.
And lastly, you will need the appropriately sized cables to run from the controller to your batteries and a fuse to hook the controller to the battery bank. In selecting the correct fuse, you will need to find the short circuit current for your panel and rate the breaker at 125% of that number. This will give you the amperage of the breaker you will need. Your controller operating manual will have guidance on these too.
Having a solar panel to maintain your batteries seems like a great idea but you’ll need to have a way to monitor your batteries. I chose to also install a Trimetric 2025RV Battery Monitor. This smart device is wired into your battery bank to give a real time measurement of voltage going in to the bank, amps being used by your boat, the percent full charge on the bank, and the amp hours used since the last charge.
So we have now installed a 195 watt solar panel coupled with a 1800 watt inverter and a battery bank with 443 amp hours. We ran our tests this week at anchor with clear skies.
Using the worksheets attached I concluded that our power consumption is 112.5 amp hours per day. The freezer alone is the biggest draw using 60 of the amp hours followed by the refrigerator. Did you know that a standard anchor light uses 18 amp hours per night?
The battery monitor told me that our actual draw from the battery bank was only 65 amp hours which means we got the remaining from the sunlight, a full 42% was from the sun.
Now we can run the generator for about an hour to bring the battery bank up to full charge.