This paper presents three physical sources of the electric field in dielectrics: excess free volume charges with the distribution qv(x,y,z), free surface charges with the distribution qs(x,y,z) and frozen polarization state in the dielectric. They have a deciding influence on the parameters of the electret, in particular they determine the total lifetime of the electret and technical components made of it. The indeterminacy related to the mutual proportions of the spatial and surface charges was discussed: one can find an infinite number of distributions of surface qse(x,y,z) and spatial qve(x,y,z) charges leading to the same distribution of the electric field E(x,y,z). A general case of electret was considered, where a coexistence of relaxation decay of frozen polarization and Maxwellian relaxation dependent on volume conductivity of the dielectric is assumed. An attempt to interpret the charge lifetime in real electrets was made.