The objective of this study is to develop a novel phase change nanocomposite for efficient electromagnetic and solar energy conversion and storage. The multifunctional nanocomposites are formulated by using PEG/SiO2 as form-stable phase change material and well-dispersed Fe3O4-functionalised graphene nanosheets (Fe3O4-GNS) as energy converter. The magnetothermal effect of Fe3O4 nanoparticles and light harvesting properties of graphene in Fe3O4-GNS, enabled the efficient magnetic- or light-to-heat conversions under an alternating magnetic field or solar illumination. The heat generated during the energy conversion process is stored in the form-stable phase change material due to the melting phase transition of PEG. Moreover, the phase change material nanocomposites exhibit high energy storage capacity (exceeding up to 100 J/g), excellent thermal stability and reversibility. This novel energy conversion and storage system will provide new insights into the field of energy management for electromagnetic or solar driven devices.