This article proposes a model-based prognosis approach to proton exchange membrane water electrolysis system (PEM-WES) for the unknown degradation of various components. PEM-WES is an emerging technology to produce the green hydrogen from the intermittent renewable energy sources. The key components of PEM-WES are the membrane, catalyst layers, gas diffusion layers, bipolar plate with flow channels, gasket, seals, end plates, which undergo different nature of degradations due to ageing and dynamical operational behaviour. Most of the existing approaches focus on modelling and performance analysis of the PEM-WES. But, degradation detection and remaining useful life (RUL) prediction of PEM-WES is a challenging task under unknown degradation of the components due to involvement of multi-energy coupled phenomena. Thus, real-time prognosis of such system is very important from the viewpoint of safety, reliability and maintainability during operation. In this article, a bond graph (BG) model-based health indicator is generated for the degradation detection and RUL prediction of the PEM-WES under multiple degradations. The developed approach is demonstrated through simulation study on PEM-WES.