Bifacial solar panels: a guide for homeowners
If you have a solar installation where sunlight can reach the back of the array, bifacial solar panels can give a nice boost in output.
Normal solar panels generate electricity from sunlight that hits the front of the panel, but bifacial panels can use light that hits the back of the panel too.
Bifacial panels do this by having a transparent layer at the back of the panel, where a standard panel would have an opaque backsheet. At the top of this article, you can see bifacial solar panels that are mounted on a carport. These panels are made by Lumos, a solar manufacturer based in Colorado, and are “glass-on-glass” panels that are constructed from two glass layers: one each at the front and back. It’s why you can see light in the spaces between each solar cell.
Frameless glass-on-glass bifacial panels like these are designed for projects where aesthetics are important, but bifacial panels with standard frames are available too.
Most home solar projects won’t benefit from bifacial panels: installations on sloped roofs don’t have enough light behind the panels. However, if you have a flat roof or a ground-mounted installation, especially one with a tracking systems, bifacial panels can increase your energy production and potentially be more cost-effective than using a larger number of conventional panels.
Comparison of conventional and bifacial solar panel construction
Here’s a cross section of a conventional solar panel:
The backsheet is a layer that sits behind the photovoltaic cells, and doesn’t generate electricity. It’s usually made of plastic and serves to protect the solar cells, provide a surface on which the cells and wiring can be mounted, and act as an electrical insulator.
In a bifacial solar panel, the opaque backsheet is replaced with a transparent layer, such as glass or a clear backsheet. This transparent layer allows the solar cells to receive light from both the front and back.
Backsheets are usually made of plastic, which is lightweight and inexpensive. For solar panels used in architectural projects (like the canopy pictured above), the plastic backsheet and aluminum frame may be replaced with a second sheet of glass. Sandwiching the solar components between two strong layers of glass provides enough rigidity that the aluminum frame can be eliminated.
How do bifacial solar panels work?
Solar cells are already photovoltaic from both the front and rear, so no special cells are required to make bifacial panels. It simply requires a transparent backside, and possibly a wiring design that minimizes shading. (For example, SunPower’s interdigitated back contact cells, which cover the entire rear of the cell with conductive material, would not work in a bifacial panel.)
Bifacial panels work in situations where light can reflect off the surface underneath and hit the rear of the panel. This means bifacial panels can be installed on ground-mounted arrays, but they’ll also work on flat roofs with tilted racks. Here’s an example of what a tilted array on a flat roof looks like:
In solar installations where light can hit the back of the array, a bifacial panel can generate up to 25% more electricity compared to a conventional solar panel. The actual increase depends on the brightness of the surface underneath. Good surfaces are light colored soil, grass (which is surprisingly bright) and white colored roofing materials, such as thermoplastic polyolefin (TPO).
How much more electricity can bifacial solar panels generate?
While a 25% boost is considered a best-case scenario for bifacial panels, the National Renewable Energy Laboratory (PDF) estimates that the average gain for a bifacial installation is closer to 9%.
The power gain depends on the amount of tilt and clearance under the panels. In many roof mounted arrays, there is little tilt or only a few inches of space under the panels. This might not allow enough light underneath for bifacial panels to be much benefit.
The time of year also matters because as the sun is lower in the sky in winter, less light may be able to reach underneath the panels.
One surprising situation where bifacial panels are a benefit? Snow on the ground! Because snow is so reflective, bifacial panels can be a big benefit in the winter and help offset losses due to snow cover on the front of the panels.
Albedo: quantifying the reflectivity of the ground
Because the amount of light reflected from a surface is key to whether bifacial panels will be beneficial to a solar installation, it’s worthwhile to understand the concept of albedo.
The albedo of a surface is a measure of its reflectivity, where zero albedo means that no light is reflected, and an albedo of 1 means that all of the light hitting the surface is reflected.
Here’s the albedo of some different surfaces (from Wikipedia):
|Conifer forest (summer)||0.08 to 0.15|
|Deciduous forest||0.15 to 0.18|
You can multiply these numbers by 100 to convert to a percentage. For example, green grass reflects 25% of the light that hits it. As you can see, some surfaces have a surprising amount of reflectivity. I don’t know what the albedo of a white TPO roof is, but I imagine that it would lie somewhere between new concrete and fresh snow, making it a great surface for a bifacial installation.
Often the price of a bifacial panel is very close to that of a monofacial panel, making it worthwhile to use bifacial panels on a ground installation even if the surface underneath is just dirt.
Understanding the efficiency numbers of a bifacial panel
If you want to know how much of a power boost you’ll get from a bifacial panel, you’ll need to look at the manufacturer’s datasheet.
The listed power rating of the panel shouldn’t change for a bifacial panel. For example, if you have a 400 watt panel, that power rating will be the same whether it’s bifacial or monofacial. Instead, the manufacturer should have a separate section in the datasheet that lists the potential power gain from the rear of the panel.
For example, here’s an excerpt from the specifications for the Eagle 78TR G4b, a bifacial commercial-sized panel by JinkoSolar. The base power rating for this panel is 465 watts and 20.43% efficiency.
|Rear side gain||Maximum Power||Efficiency|
This means that if the rear side of the panel adds a 5% boost, the panel will have a total efficiency rating of 21.46%, and up to 25.54% if the rear boost is 25% (which about as much as you can expect in a real world setting). The highest efficiency monofacial panels currently on the market have around 22% efficiency, which this panel exceeds with good light conditions.
If the back of the panel is completely dark, there will be no boost and no advantage to having a bifacial panel.
Every manufacturer will list this specification slightly differently, so be sure to read the datasheet carefully.
Frameless glass-on-glass bifacial solar panels
Frameless glass-on-glass solar panels are used in projects where solar panels feature prominently in the design. Because they’re frameless, they have a special rail system designed by the manufacturer that is often designed to hide the cabling and give the installation a neat appearance.
Here’s an example of a parking canopy that prominently features frameless glass-on-glass bifacial solar panels:
Without the frame between each panel, it almost looks like the parking canopy is made of one giant solar panel.
These panels look great, but they’re definitely intended for specialized applications and aren’t meant as a substitute for more conventional bifacial panels.Still, if you think you might want a statement design for a backyard pergola or carport, these panels can look great. You can check out my article on solar canopies and carports for some more examples.
Which companies make bifacial solar panels?
Many of the popular solar manufacturers have one or more bifacial products in their lineup. I’ve made a list of some of them, but it is by no means complete.
- Auxin Solar
- Canadian Solar
- Hanwah QCells
- Trina Solar
- Lumos Solar
Bifacial solar panels are exempt from Section 201 tariffs on solar panels
In 2018, President Trump placed tariffs on imported solar panels under Section 201 of the Trade Act of 1974. These tariffs were renewed for another 4 years by President Biden in 2022.
These tariffs have reduced the use of imported solar panels by the solar industry and caused some price increases. However, bifacial panels are exempt from this law. This means that bifacial panels are available from the manufacturers listed above, many of which are made in southeast Asia. Without these tariffs, bifacial panels may actually have a price advantage in some cases.
According to a study by the NREL, bifacial panels may add on average 5-6 cents per watt to the cost of a panel, without taking into account the effect of tariffs. This is quite a small price premium, and one that can be more than balanced out by increased energy production.
Are bifacial solar panels worth it for homeowners?
While utility-scale solar projects that are ground-mounted will often benefit from bifacial panels, most home solar projects are mounted on sloped roofs, which don’t have any reflected light under the panels.
Here are some situations where home installations can benefit from bifacial panels:
- You have a flat roof, especially one with a light colored surface.
- Your flat roof has a small amount of available space.
- You want to use ground mounts with or without a tracker.
- The aesthetics of the panels are important.
Flat roofs where the panels are significantly tilted are a good candidate for bifacial panels, but this is especially true for installations where the roof is small or the available space is limited by fire codes. In some municipalities, fire codes prevent you from installing solar panels to the edge of the roof, and require a few feet of clear space. This can be a real limiting problem in an urban setting where houses (and roof sizes) tend to be smaller.
For ground mounts, bifacial panels will often make sense because the relative cost of the racking equipment is high compared to a roof-mounted array. This is especially true if you decide to include a tracker.
Finally, if you have a project where the solar panels are featured prominently in the design, such as a solar awning, then the sleek look of glass-on-glass panels