For a few years now there has been a lot of chat in various forums about bifacial solar panels with some larger sailboats and catamarans installing them onto their stern arches. Recently we've been receiving more enquiries about bifacial solar panels and so wanted to answer some of those questions in more detail.
What is a bifacial panel?
First off what exactly is a bifacial solar panel, great question. Essentially a bifacial panel can collect power on both the front and back sides of the solar panel, whereas a regular solar panel only collects power on the front. In essence this makes regular solar panels 'monofacial' - but that's not a term you will see very often!
Bifacial panels are not new technology, there's no hocus pocus or secret sauce, essentially all the manufacturer is doing is putting solar cells onto the back of the panel so as to capture some of the reflected light onto the backside of the panels. Some bifacial panels even have a clear backsheet to allow light to pass through the panel between the cells. It's a pretty cool aesthetic that you may have already seen installed on a carport or shade canopy in a garden.
What do you mean by they only capture some of the light on the backside?
It's not like the same amount of sunlight that hits the front of a solar panel will also hit the back of a solar panel. On your boat there is likely a dinghy hanging on some davits or the mounting for the panels itself will block some light. But one of the key factors that has a significant impact on the performance of a bifacial solar panel is albedo, or the amount of sunlight a surface reflects.
Mounting rails can block the backside of a bifacial solar panel
So what is Albedo?
Albedo is the amount of light that a surface reflects and values range from 0 to 1, with 0 meaning no light is reflected and 1 meaning all light is reflected:
Black color surfaces have an albedo close to 0 (meaning they absorb most of the light)
White color surfaces have an albedo close to 1 (meaning they reflect most of the light)
These days you may see this as a percentage so darker color surfaces will be closer to 0% and lighter color surfaces closer to 100%. So bifacial panels will work a lot better on say the white roof of an RV compared to hanging off the stern of a boat over the dark ocean. Here are some examples of albedo ratings:
Surface | Albedo rating | Albedo % |
Asphalt | 0.04 | 4% |
Ocean | 0.06 | 6% |
Green Grass | 0.25 | 25% |
Desert sand | 0.40 | 40% |
Fresh Snow | 0.80 | 80% |
As you can see from the table above the ocean has a very low albedo of 0.06 or 6%!! So that's only around 6% of light that will bounce back up off the water and head to the back side of your bifacial solar panel. Maybe even less actually hits the panels as there are many other factors affecting albedo; it's blocked by the dinghy hanging off your davits, even waves on the surface of the water can affect albedo aswell as time of year, angle of the sun etc.
How is the wattage on a bifacial panel rated?
When you see a regular solar panel for sale it will have a rated wattage, for example the Maxeon 470W, which means it can produce 470Watts from the front side (only side it has solar cells) given certain conditions (called STC or Standard Test Conditions)
When you see a bifacial panel for sale it will also has a rated wattage and it will also only be for the front side of solar cells, eg. the Renogy 220W bifacial panel means 220W from the frontside. What about the back side? Somewhere advertised (or listed on the spec sheet) will be the 'yield gain' or 'reverse gain' of the back side of the panel which is often written as a %.
Again using the Renogy 220W bifacial as an example it says, "Produces energy from both the front and back sides, increasing output by up to 30%." So that's 220W on the front side and up to 30% (or 66W) from the back side for a maximum panel wattage of 286W.
Sounds great, but the clincher is our friend albedo. Using the table above with bifacial panels installed on a ground mount or an RV roof maybe you could see up to a 30% gain but panels hanging off the back of your boat over water with an albedo of 0.06 or 6% that's now only a reverse gain of 6% or 13W for a maximum panel wattage of 233W, eesh doesn't sound as cool!
Does Sun Powered Yachts have any bifacial solar panels available?
No we don't have any bifacial solar panels available
Does Maxeon make a bifacial panel?
Maxeon did make a bifacial solar panel under the SunPower brand back in 2020, the SunPower Performance 5 panel, but it's not a panel we have access to. It's a commercial solar panel designed specifically for large-scale power plant applications (think multi megawatt solar farms!) where "roof space" for panels is not a problem and you have a much higher albedo from the land.
Summary
One of our mantras is "more solar is always better" and in theory a bifacial panel does sound like a great way to get more solar and I can see certain advantages for an RV or ground mount system but on a boat it's not a huge gain so is it really worth it? Probably not.
Probably if you cleaned your regular solar panels more often you would see more gain than from installing bifacial solar panels.
Factor in the extra weight of a bifacial panel (as it has a layer of glass on the backside too), extra cost per watt, low albedo of water, backside of the panel being blocked by mounting, dinghy etc and it's really not that much of a gain. In fact. if you probably cleaned your regular solar panels more often you would see more gain than from installing bifacial panels!
Have any questions about going solar aboard your boat? Get in touch with us today.
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