Throughout this study, a systematic investigation was carried out on heating performances of phase change materials doped by graphene nanoplatelets (GNP) in an energy storage unit. The composite samples were prepared by dispersing GNP into organic PCM via melting temperatures between 61 and 66 A degrees C and at various mass fractions that included 3, 5 and 7%. A linear increase in thermal conductivity of the GNP/PCM composites was observed as the GNP mass fraction increased. With respect to PCM, thermal conductivity of GNP/PCM composites, mixed with GNP at 3, 5 and 7% mass fractions, increased by 105, 181 and 253%, respectively, at 10 A degrees C. On the other hand, a decrease in latent heat values occurred in the composites, by 2.2, 8.6 and 15.6%, respectively. Due to the increase in the doped GNP mass fraction, the temperature difference between the closest and farthest points to the heat source in the energy storage unit reduced significantly when compared to that of the PCM. When delaying durations of the closest point to the heat source were compared, due to the doped GNP fraction, it was determined that the 7% GNP/PCM composite extended the effective use of energy storage unit by 32 min compared to the PCM. Finally, after 50 heating/cooling cycles it also retained stability of GNP nanoparticles in the composite.