On-site renewable energy generation systems are installed for buildings to compensate for their energy consumptions due to cooling and heating loads. Fluctuated energy load can significantly affect the decision on the selection of renewable energy systems. This study developed a new phase change foam concrete with low thermal conductivity and suitable phase change temperature to shave the temperature peak in summer and improve the economic feasibility of renewable energy systems. With the adsorption method, this study utilized the fumed silica to absorb paraffin for the composite phase change materials (PCM) formation. Through morphology and liquid leakage tests, this study found that the composite PCM with paraffin content of 45% (wt) had the best adsorption capacity and setting performance. According to the scanning electron microscopy (SEM), metallographic microscopy and X-ray powder diffraction (XRD) tests, the proposed composite PCM and phase change foam concrete blocks have stable morphological structures and physical properties. Also, the differential scanning calorimeter (DSC) showed the proposed composite PCM within the concrete have suitable phase change temperature (about 41 °C) and phase change latent heat (the endothermic process is 113.3 J/g and the exothermic process is −112 J/g) to avoid building overheating in summer. Finally, the thermal conductivity and heating experiments suggested the proposed phase change foam concrete blocks have low thermal conductivity and strong heat storage capacity.