From its natural form to its final form, energy undergoes several changes before use.

Primary Energy

Available in nature and may come from renewable sources such as the sun or wind, or non-renewable sources such as natural gas or oil.

Final Energy

Available to users, such as the sun, biomass, electricity and fuels.

Useful Energy

Energy that is effectively used to satisfy needs, be it for heating, air conditioning, lighting, mobility, production, etc.


Their use does not compromise future generations since they exist in nature, they are capable of regenerating and, therefore, they are virtually inexhaustible and non-polluting, resulting in a rational and intelligent use of natural resources. Sources are the sun, water, wind, biomass and geothermal (earth heat). Commonly, people refer not to the source but to the associated energy:

Solar radiation can be considered the main source of energy since all others depend on it. It is a source of natural light; heat, through solar thermal collectors for heating sanitary water; and of electricity, through photovoltaic panels, which make it possible to supply remote locations in a decentralized manner.


The displacement of water produces electricity, whether through large or small hydro dams, or through waves and tides; it is the traditional form of renewable production in Portugal, although it only guarantees around 13.7 % of the electricity mix.


The kinetic energy contained in a moving air mass (wind) can be converted into electrical energy through wind turbines (wind turbine).


The energy contained in residues from forests, the agricultural and food industries, and effluent treatment, among others. It can be transformed into biofuel used in transportation or burned to produce heat and electricity.


The energy (heat) within the Earth. It is widely exploited in areas with volcanic activity for heating purposes (air conditioning of buildings or industrial purposes) or for electricity generation.


Found in nature in finite quantities, so that they cannot be regenerated or reused over a time scale that can sustain their rate of consumption. Their conversion into final forms of energy impacts the environment in different ways due to GHG emissions.

Extracted from mines and the cheapest and most used of the non-renewable fuels. It is estimated to have the largest reserves and has the greatest environmental impact. Among other things, it is used for electricity production in thermal power plants.


Results from the decomposition of organic matter over millions of years and is contained in deposits better known as oil wells. Its extraction and refining originate several fuels: petrol oils, gasolines, and propane and butane gases, among others. These are used in for electricity production in thermal power plants, but also directly for air conditioning, mobility, etc.

Natural Gas

Found underground, most often associated with oil fields and results from the degradation of organic matter buried at great depths. Its characteristics make it a less polluting source of energy than coal or oil. It is used to produce electricity in thermal power plants and directly as fuel, both in industry and in our homes.

Uranium or other radioactive elements

The energy concentrated in the nucleus of the atom, binding protons and neutrons together, is released through fission of the nucleus of radioactive elements such as uranium. It is a finite source of energy used for electricity production in nuclear power plants, and carries serious risks related with radioactivity.