The finite size of atomistic models of interfaces forces us to use slabs introducing a second interface. This is in particular problematic for the modelling of the electric double layers that form at the interface of a charged surface with an ionic solution (the electrolyte). For an insulating slab one now has to choice between surfaces with charge of the same sign, or with opposite sign. The second option of opposite charges is generally regarded as incompatible with periodic boundary conditions because of the net cell dipole moment due to aligned double layers dipoles. We will argue that this is not the case. We will show that the familiar dipole correction applied to cancel the effect of the cell dipole can be regarded as a periodic cell under zero dielectric displacement (D=0) boundary conditions in the direction perpendicular to the slab. Omitting this correction gives a system under zero electric field (E=0). The double layers on either side are now no longer compensated but acquire a net finite charge of opposite sign. Charge compensation can be restored by application of a finite macroscopic electric field E. This is demonstrated for a classical force field model (SPC) consisting of a NaCl solution confined by hard walls carrying opposite charge. We will then show how the application of finite fields can be used to obtain an estimate of the capacitance of the double layers.