A bifacial solar cell produces energy from both faces: the front collects direct sunlight, the rear collects light reflected by the ground and diffuse light from the sky. On paper the advantage is obvious; in practice the gain ranges from zero to more than 20% depending on how the module is built and where it is installed. In this guide we look at how the technology works, how it is measured, how much it really delivers and when it pays off, with the numbers you need to do the maths yourself.
How a bifacial cell works
In a conventional monofacial cell the rear is covered by a continuous layer of screen-printed aluminium: it acts as electrical contact and mirror, but it is opaque. Light arriving from behind cannot get in.
More recent architectures use a localised rear contact instead: in place of the full layer there is a grid of metal fingers, as on the front, and between them the silicon is exposed and passivated. This is the route taken by bifacial PERC cells (also called PERC+), which replace the full aluminium layer with an aluminium grid, and even more so by n-type cells such as TOPCon and heterojunction (HJT), which are symmetrical by design.
The result is a cell that, for the same format and front-side power, adds the production of the rear. The front remains the main face: it is where nameplate power is measured and what the system is sized on.
Technologies compared
| Technology | Wafer type | Typical bifaciality | Notes |
| Conventional Al-BSF | p | None | Full aluminium rear, monofacial |
| Bifacial PERC (PERC+) | p | 65-80% | Aluminium grid on the rear, the most common until a few years ago |
| TOPCon | n | 80-90% | Today's standard for utility-class modules |
| Heterojunction (HJT) | n | 85-95% | Symmetrical structure, very low temperature coefficient |
Values are indicative ranges from the technical literature: for a specific model the datasheet prevails, when the figure is stated.
The bifaciality factor and how it is measured
The bifaciality factor (φ) is the ratio between the power produced by the rear and by the front, measured separately under the same light. A bifaciality of 70% means that the rear, lit with 1000 W/m², would deliver 70% of the front-side power.
Measurement is governed by the technical specification IEC TS 60904-1-2, published in 2019 and updated in 2024. In addition to classic STC (1000 W/m² on the front only), it defines BNPI (bifacial nameplate irradiance): 1000 W/m² on the front and 135 W/m² on the rear, a condition representative of a raised module over ground of average albedo. Equivalent irradiance is calculated as:
G equivalent = 1000 + φ × 135 W/m²
With φ = 0.70 you get about 1095 W/m² equivalent, i.e. roughly +9.5% power compared with STC. With φ = 0.90 it is about +12%. That is why bifacial module datasheets show two or more power values: the STC one and those with a given rear-side gain.
Albedo: how much the ground matters
Albedo is the fraction of light a surface reflects. More than any other factor, it decides how much light reaches the rear.
| Surface | Indicative albedo |
| Asphalt, dark paving | about 10% |
| Grass | 15-26% |
| Light soil, light gravel | about 20-30% |
| White paint or membrane | 30% and above |
| Snow | 55-98% |
On a flat roof, a white membrane under the modules can be worth more than the choice of cell. In the mountains, snow makes bifacial modules an almost obvious choice, also because a raised, tilted module sheds snow sooner.
How much the rear really adds
Combining bifaciality, albedo and installation geometry, the annual energy gains observed in the field are in this order:
- Modules flush with the roof, a few centimetres above the tiles: minimal gain, a few percentage points. The rear sees almost only shade.
- Fixed ground mounts over grass or gravel, with the lower edge at least 50-100 cm above the ground: roughly 5-15%.
- Highly reflective surfaces (white membranes, light sand, snow) or single-axis trackers: gains can exceed 15-20%.
Details matter too: structural rails running behind the cells shade the rear, and module rows placed too close together shade each other's ground. Estimating the yield of a bifacial system requires software that models rear irradiance; for a first assessment, assuming 5% over grass and 10% over bright surfaces is a prudent starting point.
Where bifacial PV pays off
- Pergolas, canopies and carports: the module is raised, there is often light paving below, and glass-glass modules let light filter between the cells. See our page on PV panels for bioclimatic pergolas.
- Vertical fences and balustrades with their faces pointing east and west: one face catches the morning sun, the other the afternoon sun. Over bright ground, annual yield can approach that of a south-tilted module, but it is spread over two peaks and the module stays cleaner and free of snow.
- Agrivoltaics: vertical or elevated modules leave room for farm machinery and use light reflected by crops. See our agrivoltaics guide.
- Flat roofs with a white membrane and sufficiently high ballasted structures.
- Snowy areas and high-altitude systems.
On the other hand, paying a premium for a bifacial module on a pitched roof with flush mounting makes little sense: a good monofacial module performs practically the same there.
What changes in module construction
A bifacial cell only delivers its advantage if the module is transparent at the back. There are two solutions:
- Glass-glass: cells are encapsulated between two glass sheets. It is the most robust solution, with typically lower annual degradation and longer warranties, and the standard choice for pergolas and architectural applications.
- Transparent backsheet: lighter and cheaper, suitable when weight is a constraint.
Laminating a bifacial cell with an opaque white or black backsheet turns it into a monofacial cell: a common mistake in DIY builds. The junction box should also sit on the edge or be split into three small side boxes, so as not to shade the cells from behind.
Watch the electrical sizing
The rear mainly adds current, not voltage. Under BNPI conditions short-circuit current rises roughly in proportion to power: about 9-10% more with φ = 0.70, and over 20% on snow or white membranes. Take this into account when choosing:
- Maximum current per inverter MPPT input: with modern modules already close to the limits, the rear contribution can cause clipping;
- string fuses and cable cross-sections;
- charge controllers in off-grid systems, which should be sized on real current, not STC current.
Voltage, by contrast, barely changes: the maximum cold-weather string voltage calculation stays the same.
Building a custom bifacial module
If you want to experiment or build a bifacial module of unusual shape or power, you can start from the cells. Our bifacial monocrystalline 5BB solar cell, 156.75 mm reaches 5.40 Wp on the front with 22.1% efficiency, has 5 front busbars and soldering pads on the rear: it is connected in series with standard ribbon, as explained in our guide on how to solder solar cells. 36 cells in series give a 12 V nominal module of about 190 W from the front alone.
If you need the finished module instead, MR WATT designs and manufactures custom solar panels, including bifacial glass-glass modules for pergolas, balustrades and architectural integration.
Frequently asked questions
Do bifacial panels always produce more?
No. They only produce more if light reaches the rear: you need a glass-glass or transparent-backsheet module, clearance under the module and a reflective surface. On a pitched roof with flush mounting the gain is a few percentage points and rarely justifies the premium.
What percentage gain does bifacial give?
On ground mounts over grass or gravel the annual gain is roughly 5-15%; over white membranes, light sand or snow it can exceed 15-20%. It depends on cell bifaciality, ground albedo, module height and structural shading on the rear.
What does BNPI mean on a bifacial module datasheet?
It is the rating condition defined by IEC TS 60904-1-2: 1000 W/m² on the front and 135 W/m² on the rear at 25 °C. BNPI power is higher than STC power by roughly φ × 13.5%, where φ is the bifaciality factor. System sizing still starts from STC values.
Does a vertical bifacial module make sense on a fence?
Yes, if the fence runs north-south so that the modules face east and west. One face produces in the morning and the other in the afternoon, giving a two-peak production curve that suits self-consumption well. A vertical module collects little dirt and does not hold snow.
Can I build a panel with bifacial cells myself?
Yes, with busbar cells such as our bifacial 5BB, which are soldered with ribbon and an iron like conventional cells. The one rule not to get wrong is the back of the panel: it must be closed with glass or a transparent material, otherwise the cell works as a monofacial one.
Do bifacial cells run hotter?
No, they usually run cooler: a cell that lets infrared light pass through the rear and a raised, ventilated module operate at lower temperatures than a module flush with the roof. This reduces thermal losses, which for monocrystalline cells are around 0.3-0.4% for every degree above 25 °C.