A method is proposed using mean-field theories to help solve a long standing problem in the study of ionically solid electrolytes. While it has been long known that the ionic conductivity in solid electrolytes is comprised of two terms, the mobility and number of mobile charge carriers, there has been no accurate method developed to determine these two quantities independently. In this paper, we apply a mean-field method based upon low frequency a.c. impedance measurements of the limiting low frequency space charge polarization capacitance that develops as a result of the mobile carrier population diffusing to blocking electrodes. The space charge capacitance that develops is shown to be a simple function of the number of charge carriers and is found to be independent of the d.c. conductivity, but strongly dependent upon temperature. Measurements on two simple but well studied ion conducting glasses, LiPO3 and NaPO3, suggest that the carrier population is thermally activated where only a small fraction of the cations are mobile in the glass. The activation energy for carrier creation in LiPO3 is larger (49 kJ/mol) than that for NaPO3 glass (44 kJ/mol) and is in agreement with models of activation energies in ion conducting glasses that associate the creation energy with the cation charge density.