A solar cell is horribly inefficient. Had the sunlight not been free, no one would care about an “engine” that wastes two-thirds of the input energy at the cell level and five-sixths of the incident energy at the farm level. A bifacial tandem cell promises to return much of the wasted energy, especially in the high-albedo sandy deserts of Middle East where soiling resistant vertical bifacial farms can radiatively cool through the cloudless skies. In this talk, I will discuss the ultimate performance limits and global energy yield of this technology. I will also illustrate the design challenges to achieve the limiting performance with a Perovskite-Silicon bifacial tandem solar cells.