This paper presents a nonlinear observer for DC-DC power converters based on an averaged Port- Hamiltonian model. The averaged model of the standard port control Hamiltonian is used to describe the switching behavior and to design a nonlinear observer. Alike a state-space averaged model of the kind of systems, properties of port-Hamiltonian modeling and Lyapunov theory allows to overcome an observability issue encounters in the observer design for switched systems and to guarantee that the convergence of the observer error is independent of the cycle duty. An application to a Single-Ended Primary Inductor Converter (SEPIC) is presented in simulation. An extended observer is proposed to estimate the load resistor. Experimental results on a real process conrm the performances of the proposed observer.