Due to the rapid depletion and significant environmental impact of conventional energy sources, research is being conducted worldwide to find out alternative energy sources. The use of renewable energy technologies is being explored around the world as an alternative to conventional sources. These sources are non-depletable and environment-friendly. They use natural resources like solar and wind to generate the required energy. They can be used as single sources or as a combination of more than one type of renewable energy source along with storage devices like battery or fuel cells. Renewable energy systems (RES) can be used as grid-connected or standalone systems. However, the unpredictable nature of these natural resources reduces their reliability to a great extent. Hence, proper selection and sizing of the sources are necessary to avoid excessive establishment and maintenance costs or compromised power reliability. This chapter discusses different methodologies available in the literature on optimum sizing of the renewable energy systems, including wind power plants.