We demonstrate that lightweight, highly electrically conductive, and three-dimensional (3D) carbon aerogels (CAs) can be produced via a hydrothermal carbonization and post pyrolysis process using various melons as raw materials. This two-step process is a totally green synthetic method with cheap and ubiquitous biomass as the only raw material. These black-colored, highly electrically conductive and 3D structured CAs are ideal materials for energy conversion and storage. Paraffin wax was impregnated into the CA scaffold by vacuum infusion. The obtained CA–wax composites show excellent form-stable phase change behavior, with a high melting enthalpy of 115.2 J g⁻¹. The CA–wax composites exhibit very high solar radiation absorption over the whole UV-vis-NIR range, and 96% of light can be absorbed by the phase-change composite and stored as thermal energy. With an electrical conductivity of 3.4 S m⁻¹, the CA–wax composite can be triggered by low electric potential to perform energy storage and release, with an estimated electric–heat conversion efficiency of 71.4%. Furthermore, the CA–wax composites have excellent thermal stability with stable melting–freezing enthalpy and excellent reversibility. With a combination of low-cost biomass as the raw materials, a green preparation process, low density, and excellent electrical conductivity, the 3D CAs are believed to have promising potential applications in many energy-related devices.