The claim did not hold up
Bifacial solar panels have solar cells on both the front and the back, so they turn some of the light that bounces off the ground below into extra electricity too. A widely repeated claim says putting these panels over a farm field raises that extra electricity by up to 15%. A global, peer reviewed study built to test exactly this question found something smaller. At a ground reflectivity typical of ordinary crops and soil, the fraction of sunlight the ground bounces back up, the study modeled a worldwide average bifacial gain of under 10%. The study checked its own model against real panels at test sites run by the National Renewable Energy Laboratory in Colorado and Sandia National Laboratories in New Mexico, and the real numbers landed close to that average, 8.3% in Colorado and 19% in New Mexico.
The global model behind these tests averaged under 10% worldwide for this type of ground, so even these real sites include some above average results
Show the numbers
| Golden, Colorado | 8.3 |
| Albuquerque, New Mexico | 19 |
| Hokkaido, Japan | 23.3 |
A more reflective ground or a different angle raises the gain
The same study measured what happens when the ground below reflects more light back, or when the panel is turned to face a different way. At a New Mexico test site, raising the ground reflectivity from 0.25 to 0.55 raised the measured gain from 19% to 32.5%, and turning the panel to face west instead of south raised it again, to 39%. At the same site, with the ground left at its ordinary crop like reflectivity of 0.25, turning the panel to stand vertical and split its two faces east and west raised the gain to 30.5% too. Both changes, a more reflective ground cover and a different mounting design, cost more to build than laying an ordinary panel flat and low, which is why a farm field, left as crops or bare soil, starts out as a below average setup for this extra electricity, not the better than average one the 15% figure implied.
Rows 1 and 2 are the same New Mexico site as chart 1 with the ground reflectivity raised from 0.25 to 0.55. Row 3 is the same test program with the ground left at its ordinary 0.25 reflectivity but the panel turned vertical and split to face east and west instead of tilted south
Show the numbers
| Ground reflectivity 0.55, tilted south | 32.5 |
| Ground reflectivity 0.55, tilted west | 39 |
| Ground reflectivity 0.25, panel turned vertical | 30.5 |
What a taller mount costs
Real agricultural solar arrays are typically built 4 to 7 meters above the crop, high enough for machinery and workers to pass underneath, according to a review of existing installations. One engineering study estimates that building a raised, two sided farm this way, rather than a standard ground mounted solar farm, costs about 20% more for every unit of electricity produced, a figure the study attributes to the National Renewable Energy Laboratory. A real raised farm built in Germany came in higher still, at 38% more. In that project, the study says the extra cost was covered by growing a high value crop underneath, since potato revenue stayed high enough to make up the difference. That crop side of the story belongs to a separate article.
The one real farm
That German project, at Heggelbach farm in Baden Wurttemberg, is the only physically built, metered example in this research. Bifacial panels stand 5 meters above the crop, in rows 9.5 meters apart. In its first 12 months, the farm produced 1,266 kilowatt hours of electricity for every kilowatt of installed capacity, compared with 950 for an average German solar installation the same guideline cites. The guideline itself says that extra output comes from 2 things together, above average sunshine in that part of Germany, and whatever the bifacial panels themselves add, and it does not say how much either one contributes. So this article will not split that number either.