In this paper an analytical parametric formulation of total degradation in Orthogonal Frequency Division Multiplexing (OFDM) systems including High Power Amplifiers (HPA) is presented. Two classes of non-linear devices are considered: Traveling Wave Tube Amplifiers (TWTA) and Solid State Power Amplifiers (SSPA). In the results the accuracy of the proposed method is checked and the impact of coding to mitigate non-linear distortion is easily assessed.