Nature provides two major categories of energy sources: non-renewable and renewable energies.

Non-renewable energies are mainly fossil fuels (oil, natural gas, coal), which are forms of chemical energy from biomass stored in past millennia, as well as the nuclear fission of uranium. As their name suggests, reserves for these energy sources are limited and do not replenish themselves, at least not in human time scales.

Their capital has already been largely tapped, so the baton must be taken up by other sources in the medium term.

Renewable energies were the first used by man, and largely continue to be used in traditional societies. This term covers a wide range of energy forms such as geothermal, solar, wind, hydro, and tidal, or energy of draft animals. Being in the form of flow instead of stock, their development has advantages but also poses specific problems that will be discussed later. However, they are practically inexhaustible.