Bifacial gain is site-dependent
A bifacial module can convert light reaching both the front and rear cell surfaces, but the extra energy is not a fixed percentage printed on the module nameplate. Rear-side irradiance depends on ground reflectance, array height, tilt, row spacing, tracker geometry and how much of the rear surface is shaded by rails, cables and structural members. The same module can therefore produce materially different bifacial gain at two sites.
Albedo is only one part of the calculation
Bright concrete, light gravel or snow can reflect more light than dark soil or vegetation, but high albedo does not guarantee strong rear irradiance if the array is mounted very close to the surface or rows shade one another. Seasonal ground conditions also change. A design model should use realistic site albedo and geometry rather than an optimistic generic bifacial multiplier.
Mechanical design can affect the rear side
Torque tubes, purlins, cable trays, junction boxes and neighboring rows can cast persistent shadows on the back of a module. Some system layouts are optimized to reduce this obstruction. For rooftop installations, the roof membrane color and module standoff height can matter, while ground-mounted and tracker projects have different rear-irradiance patterns.
Model total energy, not a marketing percentage
When comparing bifacial products, record front-side nameplate power, bifaciality factor, mounting geometry and the yield-model assumptions used for the project. Treat modeled rear gain as a site-energy input rather than guaranteed module output. A credible proposal should be able to explain where its bifacial gain assumption came from and how sensitive the economics are if actual rear irradiance is lower.