This work is an experimental investigation of the relative contributions of conduction and convection to the melting rate for a phase change material. Experiments were conducted in a rectangular enclosure where heat was transferred from a vertical heated wall to the PCM (Polyethylene glycol (PEG 1500)). PCM is a high energy density material that can be used to reduce the size of thermal energy storage system (TES) significantly and store the energy at constant temperature. Six different enclosures were used for the study of increasing aspect ratio to show how the melting process is influenced by PCM thickness. Performance was recorded using both thermocouples and digital photos to record the melt temperatures and progression of the melt interface with time. In practice it is difficult to measure directly the heat transferred from the heated wall. Therefore, for a given time period the percentage of PCM melted was measured and this was used to calculate the heat transfer rate for the given time period. A sandwich type energy balance was then conducted across the full section of the block to confirm the calculated rate was correct. The results show that natural convection dominates the heat transfer from the hot wall for aspect ratios less than 6.2 shortly after heating was initiated. The results also show that conduction heat transfer from the hot wall becomes the dominant mode during the whole melting process when the aspect ratio rises above 6.2 (width =25 mm).