The efficiency of an on-body wireless power transfer system for implant powering is defined by how the electromagnetic energy interacts with the lossy, heterogeneous, and dispersive body tissues. The objective of this study is to discuss the methodology and evaluate the theoretical bounds for the frequency-dependent electromagnetic energy transfer efficiency. We propose a simplified model that uses a finite tissue-equivalent phantom enclosing an implantable receiver surrounded by a medium that represents a transmitter matched to the wave impedance of the body. This model is used to study different cases and evaluate the wireless power transfer efficiency as a function of the operating frequency and implantation depth. The obtained results can be used as a guideline to choose the design parameters and constraints of the on-body power source and gauge its performance against the predicted maximum achievable efficiency.