Clean energy includes natural energetic processes that can be harnessed with little pollution. Anaerobic digestion, geothermal power, wind power, small-scale hydropower, solar power, biomass power, tidal power and wave power fall under such a category. Some versions may also include power derived from the incineration of waste.

More controversial is nuclear energy's claim to be green. It is not sustainable, nor renewable. However nuclear waste is a pollutant if released into the biosphere, and there are carbon dioxide and other greenhouse gases generated by the large amount of energy required by transport, mining and pre- and post-production. Hence, nuclear power is rarely included in official green energy schemes. Proponents of nuclear power tend to argue that the nuclear waste is not released into the biosphere during normal operation and that the carbon dioxide emissions per kilowatt hour of generated electricity is comparable to those of other green energy sources, such as wind power.

Likewise, medium or large-scale hydroelectric power or sources of air pollution such as burning biomatter or petroleum, consume water and are often excluded from the label 'green energy'.

Renewable energy certificates (green certificates, or green tags) are currently the most convenient way for consumers and businesses to support "green power". Over 35 million homes in Europe, and 1 million in the United States, are purchasing such certificates.

No power source is entirely impact-free. All energy sources require energy and gives rise to some degree of pollution from manufacture of the technology.