In this work, we analysed the cavity length dependence of the internal, differential and power conversion efficiencies in AlGaAs/GaAs multiquantum well (MQW) lasers. We developed a theoretical model to calculate the threshold current and from it, the corresponding efficiencies. In this model we take into account a radiative process, and nonradiative mechanisms such as recombination from X and L valleys of the conduction band, Auger recombination, interface recombination and the carrier leakage over the heterostructure. Our modelling results for the threshold current are compared with the experimental data reported in the literature for different structures, showing a good agreement. We show that the internal and differential quantum efficiencies are greater in MQW lasers for all cavity lengths. In the case of the power conversion efficiency, it is higher only in MQW lasers than in their single quantum well counterparts for small values of cavity lengths.