This paper presents a systematic design approach for output consensus in a class of heterogeneous uncertain nonlinear multi-agent systems (MASs). By employing a feedback linearization technique, the output consensus problem of heterogeneous nonlinear MASs can be recast as a state consensus problem for homogeneous linear MASs. Consequently, a sub-optimal consensus design for linear MASs is presented to provide a constructive method for solving the initial output consensus problem in heterogeneous nonlinear MASs. Finally, the proposed approach is verified through a secondary voltage control problem in smart grids revealing its effectiveness.