The storage of thermal energy in phase change materials (PCMs) has found wide applications that enable energy conservation and management. Paraffin is a major PCM with its low cost, wide availability, and relatively high latent heat, yet its low thermal conductivity may become a drawback in high-power applications. In this study, composites of paraffin were prepared with multiwalled carbon nanotubes and activated carbon by a dispersion technique to overcome these drawbacks. Thermal, chemical, and physical influences of incorporating carbon additives with varying structures in paraffin composites on thermal storage capacity were determined. Results indicated that the thermal conductivities of paraffin-activated carbon composites (PACC) and paraffin multiwalled carbon nanotube composites (PCNC) were improved by a factor of 39.1 and 34.1%, respectively, compared with the conductivity of pure paraffin. As a bonus, the thermal energy storage capacities of PCNCs were enhanced by 9.6%, whereas this remained unchanged for PACCs. Copyright (C) 2015 John Wiley & Sons, Ltd.