In this paper, building upon Bussgang's theorem, we present a new expression for the total degradation (TD) in OFDM systems. This novel expression is applicable to any high power amplifier (HPA) model. In case of ideal clipping HPA model and for particular realizations of Rapp and Saleh HPA models, it and can be expressed in closed form. It enables a significant speed-up in computing TD, because it avoids end-to-end system simulations. As a consequence, it simplifies the evaluation of TD as a function of the back-off, yielding a more accurate evaluation of the optimum HPA back-off (i.e., the back-off value that minimizes TD). Numerical results show a perfect agreement between the proposed analytical framework and the Monte Carlo simulation results.Copyright (c) 2014 John Wiley & Sons, Ltd.