The downshifting from Ce3+ blue emission to Yb3+ near-infrared emission has been studied in the garnet host Ca2.8-2xCe0.1YbxNa0.1+xSc2Si3O12 (x = 0-0.36). The downshifting does not involve quantum cutting, but one incident blue photon is transferred from Ce3+ to Yb3+ with an energy transfer efficiency up to 90% when x = 0.36 for the Yb3+ dopant ion. For x <= 0.15, a multiphonon-assisted electric dipole electric quadrupole mechanism of energy transfer dominates, while for the highest concentration of Yb3+ employed, the electron transfer mechanism is confirmed. A temperature-dependent increase of the Ce3+ -> Yb3+ energy transfer rate does not exclusively indicate the electron transfer mechanism. The application of the material to solar energy conversion is indicated.