What "bifacial" means
A conventional solar panel has solar cells that face the sun and an opaque backsheet behind them. A bifacial panel uses cells that can generate electricity from light striking either face, and it replaces the opaque backsheet with glass or a transparent backsheet. The front side still does most of the work, but the rear side adds a bonus from light reflected off the ground, nearby surfaces and the sky.
Where the extra light comes from
The rear of a panel sees mostly reflected and scattered light. How much depends on three things:
- Albedo of the surface underneath. Albedo is how much light a surface reflects. Fresh snow, white gravel and light-coloured roofing membranes reflect far more than dark soil, asphalt or dark shingles.
- Mounting height and spacing. Panels raised higher above the ground, with space between rows, let more reflected light reach the back. A panel bolted tight against a roof sees almost nothing behind it.
- Shading of the rear side. Racking rails, junction boxes and frames can block part of the back, so mounting hardware is designed to keep the rear as open as possible.
Because these factors vary so much from site to site, the rear-side gain is not a fixed number. Manufacturers usually publish a "bifaciality" figure, which compares how efficiently the back converts light relative to the front, and system designers model the expected gain for a specific installation.
Construction differences
Many bifacial modules use a glass-glass design: a sheet of glass on the front and another on the back, with the cells sealed in between. This construction is stiff and resists moisture well, which is one reason some manufacturers offer long warranties on these panels. Glass-glass modules can also be heavier than standard panels, which matters for racking and roof loading. Some bifacial products use a clear polymer backsheet instead to save weight.
Several common cell technologies, including TOPCon and heterojunction cells, are naturally well suited to bifacial designs because both of their surfaces can collect current efficiently.
Where bifacial panels make the most sense
Ground-mounted arrays. Utility-scale solar farms and large ground mounts are the natural home for bifacial panels. Rows can be raised, spaced and paired with single-axis trackers, and the ground surface can even be managed to improve reflectivity.
Carports, pergolas and awnings. Panels mounted overhead with open space and a light-coloured surface below can benefit, and the see-through look of some glass-glass modules is a design bonus.
Flat commercial roofs. Tilted racks on a white roof membrane can give the rear side useful reflected light.
Typical pitched home roofs. Here the benefit is usually small. Panels sit a few inches above shingles that reflect little light, so the back side has little to work with. Bifacial panels still work fine as ordinary panels in this situation; you just should not pay a premium expecting a large rear-side gain.
Trade-offs to consider
- Cost: pricing has narrowed as bifacial cells became mainstream, but compare the price per watt of front-side rating, not a rating that already includes an assumed rear gain.
- Weight and handling: glass-glass modules can be heavier and need racking rated for them.
- Datasheet reading: some datasheets list output under several rear-gain scenarios. Make sure quotes compare like with like.
- Modelling: realistic production estimates need site-specific assumptions about albedo and height. Ask how the installer modelled it.
Questions to ask
- What rear-side gain did you assume for my site, and why?
- How high above the surface will the panels sit, and what is the surface?
- Is the production estimate based on the front-side rating or a boosted figure?
- Is the racking rated for the module weight, and does it shade the rear side?
The bottom line
Bifacial panels are a genuine technical improvement that pays off most where reflected light can reach the back: raised ground mounts, carports and reflective flat roofs. On an ordinary pitched roof they behave much like good conventional panels, so judge them on price, warranty and front-side performance there.
General information only, not engineering advice. Confirm production estimates and structural requirements with a qualified installer.