A bifacial solar panel has solar cells that can absorb light on both sides. The front faces the sun as usual, while the rear, usually covered by glass or a transparent backsheet instead of an opaque one, collects light reflected from the ground, from nearby surfaces and from the diffuse sky.
How much extra energy
Two numbers describe the effect:
- Bifaciality is the rear side’s efficiency as a share of the front side’s. The International Energy Agency’s PV research programme (IEA PVPS) reports bifacial cells ranging from above 60% to over 90%, with some designs reaching about 95%.
- Bifacial gain is the extra energy a bifacial system produces compared with an identical one-sided (monofacial) system in the same position.
The gain depends mostly on how reflective the surface under the panels is (its albedo) and how high the panels sit above it. According to IEA PVPS, bifacial panels on single-axis trackers over ordinary ground, with an albedo of roughly 0.2 to 0.3, generally gain less than 10%. Snow, pale gravel or white membranes increase it. The same report notes that different models can disagree about the expected gain by as much as a factor of two, so manufacturers’ headline figures need site-specific checking.
| Setting | Rear-side light | Typical benefit |
|---|---|---|
| Ground-mounted, raised, light-coloured ground | Good | Highest gains |
| Ground-mounted over grass | Moderate | Modest gains |
| Flush on a pitched roof | Little or none | Close to a standard panel |
Where it makes sense
Bifacial panels suit ground-mounted solar farms, carports, flat roofs with frames that lift the panels, and vertical fences. On a typical UK house roof the back of the panel sits a few centimetres above the tiles, so very little light reaches it and the extra cost buys little. Panel ratings in kilowatt-peak usually refer to the front side alone, which is worth remembering when comparing quotes.
Read more: Solar energy: how solar power works and what it means for the UK.