There are 5 fundamental sources of energy :
(i) Nuclear fusion in the Sun (solar energy),
(ii) Gravity generated by the Earth and Moon,
(iii) Nuclear fission reactions,
(iv) Energy in the interior of the Earth, and
(v) Energy stored in chemical bonds. Most of the energy we use today come from fossil fuels (stored solar energy). But fossils fuels have a disadvantage in that they are non-renewable on a human time scale, and causes other potentially harmful effects on the environment.
