The general theory of Ragone plots for energy storage devices (ESD) is discussed. Ragone plots provide the available energy of an ESD for constant active power request. The qualitative form of Ragone plots strongly depends on the type of storage (battery, capacitor, SMES, flywheel, etc.). For example, the energy decreases as a function of power for capacitive ESD, but increases for inductive ESD. Analytical results for a representative set of ideal ESD (battery, capacitor, and SMES) are compared. Furthermore, the effect of leakage and of the specific loss type (Coulomb, Stokes, and Newton friction) is discussed for inductive ESD. Finally, we address the problem of how composite ESD should be treated, and illustrate it for a battery with inductance.