The utilization of renewable energy sources has become essential in improving the energy efficiency of buildings. In this study, n-octadecane (nOD) was integrated with fly ash (FA) as low-cost industrial waste to produce the shape-stabilized composite PCM (SSC-PCM) for thermal energy storage in buildings. However, this combination resulted in a low-thermal conductivity SSC-PCM. In this regard, to enhance the thermal conductivity and enlarge the TES employment potential of the developed FA/n-OD (30 wt %) composite, it was doped separately with three different kinds of carbon-based materials, multiwalled carbon nanotubes (CNTs), carbon nanofiber (CNF), and graphene nanoplatelet (G). The effect of the amount (2, 4, 6, and 8 wt %) of the doping materials on the thermal conductivity, TES properties, thermal degradation stability, cycling reliability, and heat charging/discharging times of FA-based SSC-PCMs were systematically investigated. Chemical and crystalline structure, surface morphology, latent heat storage properties, and thermogravimetric characteristics were examined by FTIR, XRD, DSC, and TG analyses, respectively. DSC findings indicated that the SSC-PCMs have appropriate phase-change temperatures (25.01-26.43 degrees C) and reasonable latent heat storage capacities (60.50-64.47 J/g) for passive solar TES operations in building applications. The enhancements in the thermal conductivity of the SSC-PCMs were 187.09, 135.48, and 203.22% with the addition of 8 wt % CNTs, CNFs, and G, respectively. The influence of carbon nanoadditives on the thermal conductivity of the FA/nOD composite was evaluated by considering their heat storage/release performances. Consequently, the properties of the carbon nanomaterial-doped composites make them promising SSC-PCMs for thermal management of buildings.