A facile form of hydrothermal synthesis was utilized to obtain BaZrxTi1-xO3 (BZT) nanoparticles (NPs) with a wide range of Zr concentrations. The amorphous B-site precursor prepared by the hydrolysis of alkoxide using an ammonia solution, efficiently promotes the reaction with the A-site precursor solution. Subsequently, the BZT NPs were adopted for a flexible piezoelectric energy harvester (PEH) by employing a simple, low-cost, and scalable spin-casting method. The output performance achieved by the fabricated flexible PEH depends on the Zr/Ti ratio inside a piezoelectric nanocomposite. The BZT NP-embedded PEH with Zr concentration of 32 mol% generated a stable output voltage of similar to 20 V and a current signal of 400 nA without performance decay.