This paper presents results for the eciency of a stratiยฎed chilled-water storage tank with one inlet and one outlet. Numerical solutions for the two-dimensional, unsteady, laminar ยฏow during stably stratiยฎed tank ยฎlling are compared with a one-dimensional model involving only conductive heat transfer across the thermocline separating the entering cold water and the exiting warm water. This one-dimensional model represents the minimum level of thermal mixing. The dierence between the one-dimensional and twodimensional models are revealed by a horizontal average of the governing equation for the two-dimensional model. Comparison reveals that for inlet Reynolds numbers of approximately 100, the eciency of the actual, two-dimensional ยฎlling is less than 10% below the optimal eciency of the model. Examination of an eective diusivity, which can be associated with the mixing ignored in the one-dimensional model, reveals that the early and late stages of the tank-ยฎlling process are responsible for most of the deviation between the actual and ideal performances. For the present Reynolds number range, the two-dimensional predictions for eective diusivity agree well with values derived from published experimental data.