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Bifacial PV

Bifacial PV photovoltaics solar energy Albedo module yield Agrivoltaics solar park

Bifacial PV refers to photovoltaic modules that convert light into electricity on both the front and the rear side. In conventional monofacial modules, the rear side is covered with an opaque backsheet and contributes practically nothing to the yield. In bifacial modules, the rear side remains light-active. It mainly uses light reflected from the ground, snow, water, gravel, light-coloured roofing membranes or concrete, as well as diffuse sky light. Bifacial technology therefore belongs to photovoltaics, but it changes how modules are evaluated and systems are planned.

An illustrative picture: a monofacial module is like a sheet of paper written on only from the front. A bifacial module can be written on from the back as well. How much actually ends up there, however, depends on how much light reaches the rear side at all. Under a low-mounted module over a dark surface, little arrives; under a high-mounted module over light gravel or snow, considerably more does.

Bifaciality factor and power ratings

The central module parameter is the bifaciality factor (symbol φ). The technical specification IEC TS 60904-1-2 defines how it is measured. For maximum power, it is the ratio of the rear-side power to the front-side power, each under single-sided illumination at standard conditions. Factors for short-circuit current and open-circuit voltage are determined analogously. A φ of 0.8 means: at the same irradiance, the rear side delivers about 80 percent of the front-side power. In operation, however, the rear side usually receives only a small fraction of the front-side irradiance. The annual energy gain is therefore much smaller than φ.

The values depend on the cell technology. According to the ITRPV 2025 technology roadmap, typical values in 2025 were about 87 percent for heterojunction cells (SHJ), about 80 percent for TOPCon and about 70 percent for PERC and back-contact cells (IBC). These figures are industry averages from a manufacturer survey. For a specific module, the datasheet is what counts.

The rated power of a module in watt-peak is determined under standard test conditions (STC): 1000 W/m² irradiance, 25 °C cell temperature and a standardised solar spectrum. For bifacial modules, the STC value refers to the front side, while the rear side remains unilluminated. In addition, IEC TS 60904-1-2 defines the so-called Bifacial Nameplate Irradiance (BNPI): 1000 W/m² on the front and 135 W/m² on the rear. Many datasheets therefore state, alongside the STC power, a power at BNPI or a table of power values for different rear-side gains. Following the approach of the European Commission's PVGIS calculator, the BNPI power can be approximated as STC power times (1 + φ × 0.135). A 500 W module with φ = 0.8 would thus reach about 554 W at BNPI.

The 135 W/m² is a reference point, not a yield guarantee. According to PVGIS, the BNPI condition applies to a free-standing system with an albedo of 0.2, a tilt of 37 degrees and one metre of ground clearance. It does not fit building-integrated or roof-parallel systems with a covered rear side, nor vertically mounted modules with east-west orientation. For such systems, PVGIS suggests separate calculations for both sides of the module.

Why the surroundings are part of the yield calculation

For bifacial systems, it is not enough to know the irradiation at the site and the module area. The ground becomes part of the system. What matters is the albedo, that is, the share of incident sunlight that a surface reflects. An albedo of 0.2 means that about 20 percent is reflected. In studies, this value applies for example to short grass; aged concrete is at about 0.25. Fresh snow reflects considerably more and can temporarily enable very high rear-side yields. Moist soil, asphalt or dense vegetation reflect less. Albedo can change over the seasons and the operating life, for example when light-coloured gravel becomes overgrown or when changing crops stand on an agri-PV area.

Besides the average, the distribution of the reflected light also matters. The ground has the strongest effect directly beneath and between the module rows. It is precisely there that the modules cast shadows themselves. Ground clearance, tilt, row spacing and packing density (ground coverage ratio) are therefore decisive. With dense packing, the rows shade each other, which reduces the rear-side gain and also the irradiance on the front. Frame parts, torque tubes of trackers, junction boxes and cables on the rear add further shading. Bifacial PV is therefore less a property of the module than of the entire system design.

Typical magnitudes of the energy gain

Reliable figures come from field comparisons of bifacial and monofacial systems installed side by side. The US research laboratory NREL (now the National Laboratory of the Rockies) determined, on a model basis, a bifacial energy gain of 4 to 15 percent for ground-mounted systems with single-axis tracking. It depends on the module type and the ground albedo; the global average was around 9 percent. About 7 to 9 percent were measured at test installations. This gain comes on top of the 15 to 25 percent that single-axis tracking already provides compared with fixed racking. For individual high-albedo configurations, gains of around 20 percent were also calculated. The figures refer to specific sites and designs and cannot simply be transferred to Central Europe, fixed racking or rooftop systems.

Distinction from efficiency, glass-glass modules and tracking

The efficiency of a module describes how effectively it converts light on a given area into electrical power. Bifaciality, by contrast, describes that a second light-active side can be used. A bifacial module can have a high or low front-side efficiency. The additional benefit arises only when the rear side receives enough light.

Glass-glass modules are not automatically bifacial. Many bifacial modules are built as glass-glass modules, but some have a transparent backsheet instead. Conversely, there are glass-glass modules without a significantly active rear side. The ITRPV also points out that bifacial cells are now also installed in monofacial modules. Whether a module is bifacial is therefore determined by the module construction and not by the cell alone. Bifaciality also says nothing about durability, warranty or fire protection, even though these questions arise together when selecting modules.

A tracker orients modules toward the sun over the course of the day and mainly increases the irradiance on the front side. Bifacial modules can be used on fixed racking or on trackers. The combination with single-axis tracking is common in large-scale plants, because the modules there are usually mounted high above the ground and spaced apart. Whether this pays off must be calculated for each site.

Electrical design

The rear-side gain increases current and power on the DC side. This must be taken into account when designing strings, cables and inverters, so that current-carrying capacities and the input limits of the inverters are respected. With high irradiance on both sides, DC power can at times exceed what a single-sided design of the same rated power would lead one to expect. A high DC/AC ratio then leads more often to clipping: the inverter limits the power. This is not necessarily a fault, but it changes the economic calculation.

Significance for the power system and the market

Bifacial PV mainly increases the specific yield per installed kilowatt-peak and per unit area. Land, grid connections, permits and capital are scarce. More kilowatt-hours at the same site spread fixed costs such as project development, grid connection, fencing, access roads, operations management and land lease over a larger quantity of electricity. This can lower the levelised cost of electricity.

However, additional kilowatt-hours are not always worth the same. In systems with a lot of photovoltaics, the market values of solar power often fall in hours of high simultaneous feed-in. For south-facing and tracked systems, a large part of the additional yield falls in hours that are already high-yielding. Vertically mounted bifacial systems with east-west orientation, for example along field edges or transport routes, instead generate more in the morning and late afternoon. There, it is mainly the generation profile that changes, not just the annual quantity. Whether a flatter midday profile actually relieves the grid depends on local load, feed-in rules, storage and inverter design.

Typical misinterpretations

  • Energy gain as a constant: figures such as 5, 10 or 20 percent are incomplete without site, ground surface, ground clearance, tilt, row spacing and shading. On a dark, densely packed, low-mounted roof, the rear-side gain can be very small.

  • Bifaciality factor equals energy gain: a φ of 80 percent does not mean 80 percent more electricity. It only describes how sensitive the rear side is compared with the front side.

  • BNPI power as rated power: the power at 1000 plus 135 W/m² is a reference value under defined mounting conditions. For comparisons, it must be clear whether STC power, BNPI power, annual yield, specific yield per kilowatt-peak or yield per area is being discussed.

  • Energy gain equals firm capacity: even with an active rear side, photovoltaics remain dependent on weather and time of day. Night hours and a Dunkelflaute (prolonged dark, low-wind period) remain. For security of supply, what counts is the contribution in critical hours and to covering the residual load, not the annual energy.

Economic and institutional context

Bifacial cells are now the industry standard. According to ITRPV 2025, they have a market share of about 90 percent, which is expected to rise to around 95 percent over the next ten years. The main driver is n-type TOPCon technology, which in 2024 overtook PERC technology for the first time. For large-scale plants, bifaciality is thus no longer a feature of individual premium projects. Economic viability nevertheless remains project-specific: the additional yield must be weighed against possible additional costs for higher racking, larger row spacing, ground maintenance, yield assessments and an adapted inverter design.

For banks, developers and operators, a reliable yield forecast is important. A rear-side gain that arises only under ideal assumptions can distort financing. Sound assessments therefore model albedo, geometry, rear-side shading, soiling and degradation separately and, wherever possible, justify the chosen albedo with measurements. A manufacturer's performance warranty refers to the module and is not a commitment to a particular bifacial energy gain at the site.

In legal and regulatory terms: support rules, tenders and grid connection commitments are, depending on the country, tied to installed capacity, feed-in capacity or area. How bifacial modules are to be treated in this respect is governed by the respective national law. In general, however, more energy yield does not automatically increase the permissible feed-in capacity at the grid connection point. If the connection is limited, part of the additional yield can only be used with storage, self-consumption, curtailment concepts or a different system configuration.

Bifacial PV shows that a solar module cannot be evaluated in isolation. The yield arises from the interplay of module technology, irradiation, ground reflection, mounting structure, inverter and grid connection. Bifacial photovoltaics are therefore not better across the board. Their benefit depends on how well module, site and system planning fit together.

Sources

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Edited by eCMS on 11.11.2026 15:40

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