Polymer nanocomposites with ferroelectric fillers are promising materials for modern power electronics that include energy storage devices. Ferroelectric filler, Ba0.85Ca0.15Zr0.1Ti0.9O3 (BCZT) nanopowder, was synthesized by sol-gel method. X-ray diffraction (XRD) studies confirmed the phase purity and the particle size distribution was determined by transmission electron microscopy (TEM). Extended aromatic ligand in the form of naphthyl phosphate (NPh) was chosen for surface passivation of BCZT nanoparticles. Surface functionalization was validated by thermogravimetric analysis (TGA), X-ray photoelectron spectroscopy (XPS), and impedance spectroscopy using slurry technique. The dielectric constant of surface-passivated BCZT nanopowder was similar to 155, whereas pristine BCZT nanopowder dielectric constant could not be assessed due to high innate surface conductivity. Furthermore, BCZT-epoxy nanocomposite films were prepared and analyzed by differential scanning calorimetry (DSC), dielectric spectroscopy, dielectric breakdown strength (DBS), and scanning electron microscopy (SEM). Owning to stronger polymerparticle interface, dielectric measurements of 5 vol.% NPh surface functionalized BCZT-epoxy nanocomposites indicated improved DBS and glass transition temperature (T-g), reduced dielectric loss, and enhanced energy storage density compared to untreated BCZT-epoxy composites and pure epoxy. The energy storage density of 30 vol.% NPh surface functionalized BCZT-epoxy nanocomposite of 20 mu m film thickness was almost three times that of pure epoxy polymer of identical film thickness.