The introduction of polymer nanocomposites (PNCs) has brought about a transformative shift in the field of energy storage. PNCs integrate one or more materials (nanofillers) into a polymer matrix, with at least one dimension smaller than 100 nm in the distributed form. This enhances electrical conductivity, mechanical strength, and overall performance. The present article offers an overview of PNCs intended for energy storage applications. These materials present a distinct blend of characteristics that surpass the constraints of conventional energy storage technologies by augmenting their efficiency and functionality. This review delves into the basics of polymers and polymer composites, followed by introducing polymer nanocomposites and their classification based on various criteria such as types of nanofillers, their structure, polymer matrix, and processing methods. This manuscript examines the influence of PNCs in various energy storage applications, encompassing batteries, fuel cells, supercapacitors, solar cells, and thermoelectric devices. PNCs exhibit optimization of energy density, charge/discharge rates, cycling stability, adaptability and adjustability, making them ideal for the upcoming generation of energy storage devices. In summary, this review emphasizes the potential of PNCs in transforming sustainable energy storage and addresses associated challenges and opportunities.