This paper presents a modified design approach as well as a practical and effective neural network efficiency estimation procedure of a permanent capacitor single-phase induction motor (SPIM). The standard industrial motor frame sizes along with the current design trends of larger lengths and smaller diameters are taken into account, which are not likely presented in literature. In this context, a computer-aided design approach of a SPIM -based mainly on the classical output coefficient- is proposed first. Numerous simulations using Finite Elements Method (FEM) are conducted in order to verify the proposed procedure and investigations regarding the number of stator and rotor slots are performed. Secondly, based on the previous results, a neural network (NN) scheme is proposed in order to estimate the efficiency of SPIMs with various stator/rotor slots combinations and different output power. It is seen that the proposed methodology is verified satisfactorily and could be of great use as an aid tool to industrial SPIM designers.