This paper presents a method to model an energy harvesting circuit and a propagation channel as parts of a wireless power transfer (WPT) system. Previously , an original solution was proposed by the authors to maximize the collected DC power level, implying to model the different parts of the system by using state space representation and using convex optimization algorithms to calculate multi carrier signal weights. Firstly, a state-space model of the radio channel is obtained by the use of multivariable output-error state space (MOESP) algorithm. Then, by the vector fitting method a state-space representation is obtained in order to model the linear part of the energy harvesting circuit. At last, by the polynomial-nonlinear state-space (PNLSS) method, the non-linear part of the rectifier is also modeled.