The term "alternative" presupposes a set of undesirable energy technologies against which "alternative energies" are contrasted. The nature of what was regarded alternative energy sources has changed considerably over time:
- Energy fuelled in ways that do not use up natural resources or harm the environment.
- Energy derived from sources that do not use up natural resources or harm the environment
- Energy derived from nontraditional sources (e.g., compressed natural gas, solar, hydroelectric, wind).
- Energy that is not popularly used and is usually environmentally sound, such as solar or wind energy (as opposed to fossil fuels).
- Fuel sources that are other than those derived from fossil fuels. Typically used interchangeably for renewable energy. Examples A** include: wind, solar, biomass, wave and tidal energy.
- Energy generated from alternatives to fossil fuel. Need not be renewable.
In a general sense in contemporary society, alternative energy is that which is produced without the undesirable consequences of the burning of fossil fuels, such as high carbon dioxide emissions, which is considered to be the major contributing factor of global warming. Some interpretations exclude nuclear energy.
Renewable energy is generated from natural resources—such as sunlight, wind, rain, tides and geothermal heat—which are renewable (naturally replenished). When comparing the processes for producing energy, there remain several fundamental differences between renewable energy and fossil fuels. The process of producing oil, coal, or natural gas fuel is a difficult and demanding process that requires a great deal of complex equipment, physical and chemical processes. On the other hand, alternative energy can be widely produced with basic equipment and naturally basic processes. Wood, the most renewable and available alternative energy, burns the same amount of carbon it would emit if it degraded naturally.
Renewable energy sources such as biomass are sometimes regarded as an alternative to ecologically harmful fossil fuels. Renewables are not inherently alternative energies for this purpose. For example, the Netherlands, once leader in use of palm oil as a biofuel, has suspended all subsidies for palm oil due to the scientific evidence that their use "may sometimes create more environmental harm than fossil fuels".[