Efficient green and red upconversion emission in Y2O3:Yb3+, Er3+ nanorods under 978nm radiation excitation is achieved. Experimental effective lifetimes, luminescent emissions, and nanorod sizes depend strongly on the solvent ratios used during the synthesis. A microscopic nonradiative energy transfer model is used to approach the dynamics of the green, red, and infrared emissions. The excellent agreement between simulated and experimental decay suggests that the energy transfer mechanisms responsible of the visible emission depend on the solvent ratio.