Bifacial panels can turn light reflected onto their rear face into extra electricity. How much they gain depends on the surface below, mounting height and tilt, which is why they suit ground mounts and carports far better than a typical roof. A bifacial panel has solar cells that can absorb light from both faces: direct and diffuse sunlight on the front, and light reflected or scattered onto the back. The extra output from the rear, called bifacial gain, depends almost entirely on how much light reaches the back of the panel. On an elevated ground mount over a pale surface, that gain can be worthwhile. On a panel mounted close to a dark roof, the rear sees very little light and the gain is small. Albedo, mounting height and row spacing explain the difference.
How a panel can use light from behind
A conventional monofacial panel has an opaque backsheet, and each cell has a solid metal layer across its rear, so light from behind is blocked. A bifacial design makes two changes:
- A transparent rear: either a second sheet of glass (glass-glass) or a clear backsheet in place of the usual white or black film.
- Cells that collect from both sides: the solid rear metal is replaced with a fine grid of contacts, similar to the front, so photons entering from behind can also generate current.
The way absorbed light becomes current is the same on both faces, as explained in how solar power works. The difference is how much light arrives at each face.
Datasheets describe the rear's performance relative to the front with a bifaciality factor. If a panel's bifaciality is 80%, its rear converts light at roughly 80% of the front's effectiveness. Commonly quoted values for current bifacial panels fall roughly between 70% and 90%, depending on cell type. That figure is a property of the panel; the actual gain in the field depends on the installation.
Albedo: how reflective is the surface?
Albedo is the fraction of incoming sunlight a surface reflects, from 0 (absorbs everything) to 1 (reflects everything). It is the biggest single influence on how much light reaches a panel's rear. Approximate typical ranges are:
| Surface | Typical albedo | Rear-side potential |
|---|---|---|
| Dark asphalt or dark roofing | About 0.05-0.15 | Low |
| Green grass or bare soil | About 0.15-0.25 | Modest |
| Weathered concrete or dry grass | About 0.2-0.35 | Moderate |
| Light gravel or pale sand | About 0.3-0.45 | Good |
| White membrane or white-painted surface | About 0.6-0.8 when clean | High |
Albedo also changes over time. Grass browns in summer, pale gravel darkens with dirt and moss, and a white membrane loses reflectivity as it weathers. Sound design estimates use a realistic long-term value rather than the best-case figure for a brand-new surface.
Mounting height, tilt and row spacing
A reflective surface only helps if the reflected light can actually reach the back of the panels. Several geometric factors control that:
- Height above the surface. Raising panels lets reflected light spread more evenly across the rear. Panels a few centimetres above a roof have almost no view of sunlit ground.
- Tilt. A tilted panel's rear looks out over a wider patch of ground and sky. Nearly flat panels mostly see the shaded area directly beneath them.
- The array's own shadow. The ground directly under an array is shaded, so it reflects little. Height and spacing allow more sunlit ground to contribute.
- Row spacing. Closely packed rows shade the ground between them and block each other's rear view.
- Rear obstructions. Rails, cross-beams and junction boxes behind the cells cast rear shadows, so bifacial mounting keeps them clear of cells where possible.
- Uneven rear light. Light on the back is rarely uniform, and cells wired in series are held back by the weakest, so some theoretical gain is lost to mismatch.
Where bifacial panels make sense, and where they don't
| Installation | Rear-side conditions | Likely bifacial benefit |
|---|---|---|
| Flush-mounted on a pitched tile or metal roof | Panel close to the roof; rear sees a dark, shaded surface | Small, often close to negligible |
| Tilt frames on a flat roof with a pale membrane | Some height over a reflective surface | Moderate |
| Ground mount over pale gravel or dry grass | Good height, open rows, reflective ground | Often worthwhile |
| Carport or canopy over pale concrete | High clearance and open sides | Often worthwhile, reduced when vehicles are parked below |
| Vertical, fence-style arrays facing east and west | Each face receives direct sun at a different time of day | Designed around bifaciality, with morning and afternoon peaks |
For elevated ground mounts over reflective surfaces, design estimates of rear gain commonly range from a few per cent up to the low teens, depending on height, spacing and albedo. Flush rooftop installations can expect much less. Any bifacial gain shown in a proposal is an estimate, and it should be tied to the actual surface and mounting at your site.
Even where rear gain is small, bifacial panels are usually glass-glass, which brings other traits: a stiffer laminate, good moisture resistance and sometimes a longer product warranty. Those can be sound reasons to choose them for a roof, provided the rear gain is not oversold. Warranty structures are compared in solar panel warranty types explained.
How designers account for rear-side output
Yield modelling for bifacial arrays adds a rear irradiance estimate to the usual front-side calculation. A careful design report states the albedo assumed, the mounting height and the bifaciality factor used, so the estimate can be checked.
There is an electrical consequence too. Extra light on the rear raises the current each string delivers, so inverter inputs, fuses and cabling must be sized with that contribution in mind. Accredited designers check these limits against the inverter's datasheet, allowing for conditions such as bright, cool days when output runs higher than usual.
Next steps
Bifacial panels have the most to offer where there is space for an open, elevated array. Ground-Mounted Solar (from $11,900 for a 6.6 kW array, installed) and a Solar Feasibility Report (from $149 per property) are listed in the energy market. Prices are indicative and confirmed after a site assessment. To find out whether bifacial panels suit your roof, yard or surface, request a free assessment from Blue Energy Solar.
Frequently asked questions
Do bifacial panels cost more than standard panels?
They can carry a small premium, although many current panel ranges use glass-glass bifacial construction as standard, so the difference is often modest. The fairer comparison is the cost per kilowatt-hour produced over the system's life at your site. On a flush roof, where rear gain is small, judge a bifacial panel mostly on its front-side performance, durability and warranty terms.
Would painting the surface under panels white increase output?
Only where panels sit well above the surface, such as tilt frames on a flat roof or an elevated ground mount. On a flush-mounted pitched roof the rear of the panel barely sees the roof, so a lighter colour adds little. Any coating should be applied before installation by suitable trades, never by working on or around an existing array.
Do bifacial panels need cleaning on both sides?
The rear face collects far less dust than the front because it faces downwards or away from falling dirt, so it rarely needs attention. Spider webs, nests or leaf litter caught behind the panel can shade the rear, though. On ground mounts and carports, a periodic visual check from a safe position is usually enough, with any cleaning done by professionals.
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