Green power is a subset of renewable energy. It represents those renewable energy resources and technologies that provide the greatest environmental benefit. Green power is defined as electricity produced from solar, wind, geothermal, biogas, eligible biomass, and low-impact small hydroelectric sources. To qualify as green power, this renewable electricity must also go above and beyond what is otherwise required by mandate or requirement. In other words, green power is voluntary, or surplus to regulation. Customers often buy green power for its zero-emissions profile and carbon footprint reduction benefits.