Currently, nanoscience (NS) and nanotechnology (NT) are the world’s fastest growing areas of science. Nanomaterials (NMs) are the foundation of NS and NT. They have large surface-to-volume ratio, high electronic properties, and enhanced optical properties. Magnetic properties, mechanical properties, and thermal properties are other important characteristics of NMs. Thus NMs are widely used in different areas of science and engineering including in renewable energy. However, only a specific NM with certain characteristics can be used to achieve an expected property. The characteristics of NMs are highly dependent on their size. This chapter discusses recent examples of the NM characterizations using UV-Visible spectroscopy and their applications in renewable energy.