What a bifacial panel actually is
A conventional solar panel collects light on its front face only. The rear is covered by an opaque, usually white, polymer backsheet. A bifacial panel replaces that backsheet with a transparent rear surface, normally a second sheet of glass, so the cells can also collect light that reaches them from behind. That rear light is almost entirely reflected light: sunlight that has bounced off the ground or surrounding surfaces and travelled back up to the underside of the array.
The idea is straightforward, but the extra energy is not free or guaranteed. It depends on the cell itself, on how much light the ground returns, and on how the array is mounted. Each of these can be measured, which is why bifacial performance is more predictable than it first appears.
Bifaciality factor
The first number to understand is the bifaciality factor: the ratio of the rear-side efficiency to the front-side efficiency under the same standard test conditions. A bifaciality factor of 0.80 means that, for equal light landing on each face, the rear produces 80 per cent as much power as the front. It is a property of the cell, not the installation, and it appears on the datasheet alongside the usual front-side ratings.
Bifaciality depends strongly on cell architecture. Older p-type PERC cells, adapted for bifacial use, sit around 0.70. Modern n-type cells do better: TOPCon is typically in the low to mid 0.80s, while heterojunction (HJT) cells, which are inherently close to symmetric, are usually quoted in the high 0.80s to about 0.95. If you are comparing modules, the bifaciality figure tells you how much the rear can contribute before any site factors are considered. For more on how these figures are presented, see reading a solar datasheet.
Albedo: the most important site variable
Albedo is the fraction of light a surface reflects, from 0 (fully absorbing) to 1 (fully reflecting). It governs how much light ever reaches the rear of the panel, and it is the single biggest installation-side lever on bifacial gain. Typical values vary widely: dark soil and asphalt sit around 0.10–0.17, grass around 0.20–0.25, dry sand around 0.30–0.40, concrete anywhere from about 0.25 to 0.55 depending on age and finish, and fresh snow as high as 0.80–0.90.
The relationship is close to proportional: across the usual range, rear-side energy rises roughly in step with albedo, so a brighter surface beneath the array returns markedly more light to the rear. This is why ground-mount operators sometimes lay light gravel or reflective fabric beneath rows, and why the same panel can perform very differently on a dark roof versus a pale concrete yard.
Mounting height and ground cover ratio
Two geometric factors decide how much of that reflected light actually lands on the rear. The first is height above the ground: panels mounted higher can "see" a wider patch of illuminated ground, and the rear receives more even, less shaded light. Field studies show rear irradiance rising noticeably as clearance increases from roughly half a metre to one or two metres, with gains levelling off beyond about 1.5 metres while structural cost keeps climbing.
The second is the ground cover ratio (GCR), the proportion of land area covered by panels. Tightly packed rows shade the ground between them and shade each other's undersides, suppressing rear gain. Spacing rows further apart raises the reflected light reaching each rear face, at the cost of needing more land. Both factors favour open, elevated arrays.
Realistic bifacial gain
"Bifacial gain" is the extra annual energy a bifacial system produces compared with an identical monofacial one. The honest ranges are wide and site-dependent. The US National Renewable Energy Laboratory measured roughly 7 per cent for fixed-tilt ground-mount over natural ground cover, and reports that such systems generally stay under 10 per cent unless the ground is made more reflective; the broader literature spans roughly 5 per cent to 30 per cent at the favourable end, over very high-albedo ground such as snow or bright gravel with well-elevated, widely spaced rows.
Typical rooftops are a different story. Most residential panels are fixed close to the roof, often little more than 10 centimetres of clearance, so very little light can reach the rear. On a dark roof the realistic gain is often just a couple of per cent; even a pale roof returns modestly at that height. Australian modelling reflects this spread: an Australian National University study found gains ranging from about 5 per cent to 23 per cent depending on tilt and roof reflectivity, but the upper figures assume tilt frames or elevated mounting with a bright surface beneath, not a flush dark roof. The practical conclusion for most homes is that bifacial helps most on flat roofs with tilt frames, carports, pergolas and ground mounts, where the rear can actually see reflected light.
Construction and durability
Because the rear must be transparent, bifacial modules are almost always glass-glass: tempered glass front and back rather than glass-and-backsheet. This construction tends to resist moisture ingress and ultraviolet degradation better than polymer backsheets, and many such modules carry slightly lower stated annual degradation and longer warranties. The trade-off is added weight, which matters more on rooftops than on the ground.
Where it makes sense
Bifacial technology earns its keep where the rear can be fed: elevated, well-spaced ground-mount arrays over reflective ground, and tilted or raised rooftop structures. On a standard flush rooftop the gain is usually small, and the decision often comes down to whether the glass-glass durability and price are competitive in their own right, rather than to the bifacial bonus.