Which is the best number of junctions for solar cells under ever-changing terrestrial spectrum?

Monolithic multi-junction solar cells are constrained in that the current output of the stack is limited to the current being generated in the worst performing cell. This becomes a problem when the spectral content of the radiation incident on the solar cell is different to the ´designed´ spectrum. The terrestrial spectrum always changes by different sun height or air mass as well as fluctuation of aerosol density. The annual power generation is expected to be smaller than the designed one, due to the spectrum mismatching. This influence may be emphasized by the increase of the number of junctions, while the efficiency under the designed standard spectrum will be increased with the number of junctions. The annual efficiency of monolithic multi-junction solar cells of more than three junctions were calculated and compared. No distinct advantage with more than 5 junctions in view of annual and daily power generation. The practical efficiency limit in real terrestrial sunshine is a little above 50%.