Polymer dielectrics have extensive applications as capacitors in modern electronic and electrical systems due to their distinct merits, such as low cost, good processability, and high breakdown resistance. In the last decade, incorporating ultralow loading of nanoparticles into polymers has realized increases in dielectric constant as well as breakdown strength, thus leading to excellent capacitive performance. However, there are still a series of tough issues to be dealt with, such as the use of organic solvents, which face enormous challenges in scalable preparation. Here, a new strategy of scalable dual in situ synthesis is proposed, namely polymerization of polyethylene terephthalate (PET) synchronizes with growth of calcium borate nanoparticles, making polyester nanocomposites from monomers directly. Importantly, this route is free of organic solvents, surface modification of nanoparticles, and by-products, which is readily accessible to large-scale preparation of polyester nanocomposites. Meanwhile, uniform dispersion of as ultralow as 0.1 wt% nanoparticles and intense bonding at interfaces have been observed. Furthermore, the PET-based nanocomposite displays obvious increases in both dielectric constant and breakdown strength as compared to the neat PET. Its maximum discharged energy density reaches 15 J cm−3 at 690 MV m-1 and power density attains 218 MW cm-3 under 150 Ω resistance at 300 MV m-1, which is far superior to the current dielectric polymers that can be produced at large scales. This work presents a scalable, safe, low-cost, and environment-friendly route toward polymer nanocomposites with extremely low filling, which carves a path to explore capacitive energy storage and other applications of polyester nanocomposites.