The impact of natural convection on melting in high temperature flat plate latent heat thermal energy storage systems is studied with an experimentally validated numerical model in a parameter study with various widths and heights of enclosure dimensions. The storage material is the eutectic mixture of sodium nitrate and potassium nitrate (KNO3-NaNO3). The investigated half widths of the rectangular enclosures between two heated vertical flat plates are 5, 10 and 25 mm; their heights are 25, 50, 100, 200, 500 and 1000 mm. These parameters result in low to very high aspect ratios between 0.5 and 40 and Rayleigh numbers between 1.2.10(4) and 1.6 .10(6). The results are evaluated by dimensional analysis to find general dependencies between enclosure dimensions and natural convection occurrence and strength. To assess the influence of natural convection on the heat transfer enhancement, the convective enhancement factor is introduced. This non-dimensional number is defined as the ratio of actual heat flux by natural convection to a hypothetical heat flux by conduction only. The central findings of the present work are correlations for the mean convective enhancement factor and the critical liquid phase fraction for natural convection onset that are valid for a wide parameter range. The results indicate that heat transfer enhancement due to natural convection increases with greater widths and smaller heights of storage material enclosures. Hence, the vertical segmentation of high enclosures into smaller ones should be considered to enhance heat transfer during charging. (C) 2016 Elsevier Ltd. All rights reserved.