Therefore, an ideal candidate for dielectric energy storage requires a high permittivity and the ability to withstand high electric fields, i.e. a high breakdown strength Eb. The commercially available organic material of choice is biaxially oriented polypropylene (BOPP), which registers an Eb ~ 700 MV•m -1 , however, its energy density is ~ 4 J•cm -3 due to its low permittivity (εr = 2.2), which is common for linear dielectrics. Alternatives include organic and inorganic non-linear dielectrics, such as ferroelectrics or relaxor ferroelectrics, which show higher permittivities and thus high macroscopic polarization, P. This leads to enhanced UE as P ~ D in these materials where εr >> ε0.