Using first-principles calculations, the phase diagrams of polyvinylidene fluoride (PVDF) and its copolymers under an applied electric field are studied and phase transitions between their nonpolar alpha and polar beta phases are discussed. The results show that the degree of copolymerization is a crucial parameter controlling the structural phase transition. In particular, for tetrafluoroethylene (TeFE) concentration above 12%, PVDF-TeFE is stabilized in the beta phase, whereas the alpha phase is stable for lower concentrations. As larger electric fields are applied, domains with smaller concentrations (< or = 12%) undergo a transition from the alpha to the beta phase until a breakdown field of approximately 600 MV m(-1) is reached. These structural phase transitions can be exploited for efficient storage of electrical energy.