The type of source that gives you the most energy per bit of ‘stuff’ is nuclear energy … either fission or fusion. We can show this using the formula
E=m times c squared.
That means that the energy (E) given out when mass (m) is converted to energy, is equal to the mass (m) times the speed of light (c) squared (c times c). The speed of light is a HUGE number – 300,000 kilometers per second …
So a little bit of mass gives you loads of energy.
As Laurence said in a previous answer
“Destroying a single gram of plutonium gives out as much energy as burning 2 tonnes of oil”
The SUN has incredible energy. It burns at over 5000°C, and its power is about 386 billion billion mega Watts, produced by nuclear fusion. We have a power source which sustains life on earth and which has the potential to supply all our energy needs.