The wall-plug efficiency of a vertical-cavity surface-emitting laser is investigated, The optimum current which produces a maximum wall-plug efficiency is derived when the mirror reflectivity is fixed and the series resistance and parasitic leakage current are taken as variable parameters, The results are compared with experimental data, The optimum reflectivity in terms of both output power and wail-plug efficiency is determined for both unstrained and strained quantum lasers with fixed supply current and series resistance as a parameter, Temperature is also considered as a variable.