The paper proposes a study of the phenomenology driving the ageing mechanisms of electrochemical double layer supercapacitors (SCs) during their most common operating conditions. In particular, the paper focuses on the so-called life endurance (LE) and power cycling (PC) stresses. The paper first provides a description of the variables governing the SC behaviour at the microscopic scale (i.e., porosity of the electrode surface, diffusion and conductivity of the electrolyte). Then, the paper discusses the link between these variables and the ones describing the SC behaviour at the macroscopic scale (i.e., temperature of the electrolyte and delivered current). This analysis is applied to both LE and PC stresses and as well as to their combination. The discussions make reference to experimental results obtained at the Author’s laboratory by means of a dedicated test bench.