Transient heat transfer during melting of graphene-based composite PCMs heated from below was investigated experimentally. Composite PCMs filled with graphene nanoplatelets (GNPs) were prepared at various loadings up to 3% by weight. The thermal conductivity, dynamic viscosity and latent heat of fusion of the composite PCMs were measured. A variety of boundary temperatures were also adopted to vary the intensity of natural convection. It was shown that when the boundary temperature is 55 degrees C, melting is accelerated by 8% at the highest loading of 3 wt.% GNPs due to the doubled thermal conductivity of the composite PCM as compared to that of the base PCM. Increasing the boundary temperature leads to more intensive natural convection that in turn slows down melting because the contribution by natural convection is significantly suppressed by the dramatically grown dynamic viscosity, e.g., 10-fold increase at the loading of 3 wt.%. The melting rate is determined by competition between the enhanced heat conduction and deteriorated natural convection. In addition, both the melt fraction and heat transfer were correlated to dimensionless groupings that govern this problem. Universal correlations that are valid for the entire ranges of the parameters investigated were proposed with an uncertainty below 20%. (C) 2014 Elsevier Ltd. All rights reserved.