What a bifacial module is
A bifacial photovoltaic module can convert irradiance reaching both the front and rear of its cells. The rear is exposed through a transparent layer, which may be glass or a transparent backsheet. This construction detail is separate from the cell technology: PERC, TOPCon and HJT cells can all appear in bifacial products.
Most rear-side irradiance comes from light reflected by the ground, roof or nearby surfaces, plus diffuse light from the sky and surroundings. The rear contribution is therefore a property of the complete array and site, not a fixed bonus attached to the module name.
Bifaciality is not bifacial gain
Bifaciality expresses rear-side performance relative to front-side performance under defined test conditions. Bifacial gain is the extra energy or power produced in a particular system compared with its chosen monofacial reference. They are not interchangeable.
The exact module datasheet may provide bifaciality information and additional electrical ratings for assumed rear-side irradiance. Use those declared values and conditions. Do not assign a technology-wide bifaciality factor to a product that has not declared one.
The system variables
IEA PVPS identifies albedo, diffuse-light conditions, mounting height, row spacing and module spacing among the variables that affect rear-side yield. Tilt, ground-cover ratio, array size, edge effects, obstructions and rear-side shading also matter. Rear irradiance is commonly non-uniform, so two modules in the same array may not receive identical rear illumination.
Albedo describes the share of incident light reflected by a surface, but a nominal albedo value is not a complete yield model. Surface condition can change with moisture, soiling, vegetation, ageing or snow, and geometry determines how much reflected light the rear can actually see.
Keep gain claims bounded
There is no honest universal bifacial-gain percentage. In one bounded context, IEA PVPS reports that single-axis tracker fields over typical natural ground with albedo around 0.2 to 0.3 generally remain below 10% bifacial gain. That is not a rooftop allowance or a promise for another tracker site. The same report found that no single parameter correlated well across all of the field systems it reviewed and noted substantial differences between modelling approaches.
For a roof, carport or ground mount, model the actual surface, geometry and product. If those inputs are unavailable, leave the expected gain blank rather than borrowing a favourable figure from a different installation.
Electrical design must include the rear contribution
Rear irradiance can increase current. IEA PVPS notes that designers may need to adjust conductor, fuse and inverter sizing for bifacial arrays. Follow the exact module's declared bifacial electrical data and the project's applicable design process; do not add a generic percentage to current without a supported basis.
Field checklist
- Confirm the exact module model, rear construction and declared bifacial parameters.
- Record the surface, clearance, tilt, spacing, gaps and rear-side obstructions used in the yield model.
- Check voltage and current limits using the manufacturer's bifacial electrical information.
- Check module mass, mounting zones, clamps and load ratings; bifacial does not always mean glass-glass.
- Read the exact product and performance warranties. Do not infer durability or warranty length from transparent rear construction.