The urgent requirement to combat climate change and maintain energy security has led to an increased focus on green energy technology in the global quest of sustainable energy solutions. The development of innovative materials with improved performance and environmental benefits is of significant importance due to the urgent demand for sustainable energy solutions. In this field, nanomaterials and nanocomposites have become important players with hitherto unheard-of potential for applications in green energy. This chapter examines the synthesis, properties, and applications of nanoparticles and nanocomposites made for sustainable energy technology. Nanomaterials are exceptional due to their unique size-dependent properties, which have the potential to revolutionize energy storage and conversion technologies. Numerous nanomaterials have been extensively researched for their potential in catalysis, fuel cells, solar cells, and batteries. These materials include metal nanoparticles (gold, silver, and platinum), metal oxides (titanium dioxide, zinc oxide), and carbon-based materials (graphene, carbon nanotubes (CNTs), and nano-diamonds). Carbon-based nanomaterials, metal nanoparticles, and metal oxides have garnered a lot of attention due to their potential to totally revolutionize solar cells, fuel cells, batteries, and catalytic processes—all crucial components of the infrastructure supporting renewable energy. Creating nanocomposites by combining many nanomaterials offers a synergistic approach that optimizes each one’s special benefits and reduces its drawbacks. Significant obstacles in green energy technologies are surmounted by enhancing charge transfer, surface area, and stability through engineered interfaces and structures inside nanocomposites. Recent advancements in the use of nanoparticles and nanocomposites for renewable energy projects are highlighted in this chapter, with particular attention paid to their use in energy storage, solar photovoltaics, hydrogen generation, and environmental remediation. It also looks at how important nanotechnology is to grid optimization, sustainable development, and the integration of renewable energy. A more sustainable and clean energy landscape may be achieved by utilizing nanoparticles and nanocomposites. The development of effective, financially feasible, and environmentally friendly energy solutions will be fueled by interdisciplinary cooperation and technical innovation. These solutions are essential for halting climate change and guaranteeing energy security in the quest for a greener future. Solar energy is one of the most promising renewable energy sources, but before it can be extensively utilized, several challenges must be addressed, including sporadic power supplies and a small amount of energy storage capacity. As a result, nanotechnology has become a game-changer, providing creative ways to improve the sustainability, cost, and efficiency of green energy applications. Because of their special qualities at the nanoscale, nanomaterials are essential to the revolutionary advancement of solar energy conversion technology. High-performance photovoltaic (PV) device improvements are fueled by the customized optical and electrical features of nanowires (NWs), nanoparticles made of perovskite, and other nanostructures. Simultaneously, nanocomposites provide prospects for enhancing energy storage systems, catalytic processes, and energy-efficient construction technologies by using nanoscale components inside traditional materials. An overview of the usage of nanoparticles and nanocomposites in green energy is given in this abstract, with an emphasis on solar energy conversion, energy storage, catalysis, and energy-efficient building technologies. Nanotechnology provides scalable and sustainable routes toward a cleaner and more resilient energy future through multidisciplinary cooperation and technical innovation. However, achieving the full potential of nanotechnology in creating green energy requires resolving issues including scalability, stability, and environmental effect. Research, development, and implementation of nanotechnology-enabled solutions should be given top priority in order to expedite the shift to a more sustainable energy paradigm and preserve the environment for future generations.