The talk will present a universal approach for calculating the power transfer efficiency (PTE), maximum power transfer efficiency (MPTE), and conditions for achieving MPTE for arbitrary multiple input (or transmitters) multiple output (or receivers) wireless power transfer (MIMO-WPT) systems. The PTE, also known as the RF efficiency, between M transmitters and N receivers, is precisely formulated using the equivalent M+N port S-parameters circuit network as the Rayleigh quotient. Accordingly, the MPTE is achieved by calculating the eigenvalue of the generalized Rayleigh problem that has been formulated. The effectiveness and universality of the proposed method are demonstrated through highlighting the MPTEs of single transmitter single receiver (SISO) and multiple transmitters single receiver (MISO) among many applications. Several potential applications of the proposed approaches are introduced based on the MPTE performance of the MISO system.