This paper reports the results of an experimental investigation on the thermal performance of a phase change material based cuboidal heat sinks with 72 pin fins on the inside to act as thermal conductivity enhancers. The heat sink base is heated with four equal sized heaters. The total power of the heaters is fixed, while the individual wattage can vary. First, in house experiments are conducted for different combinations of power. Based on the experimental results, correlations are then developed for the characteristic time for phase change with natural convection as a function of the individual heat input values. Additional experiments are conducted to validate the developed correlation and a good agreement is found. The correlations developed are useful for future optimization studies.