Modern Castle: MAYBE they get the electricity they need just like any other house...
Medieval Castle: The don't, they used to hang torches on the walls for light.
Fusion produces the Sun's energy .
Salt produces lattice energy
produces energy for maintenance and repair of cells and tissues; produces energy for muscular activities and movement; prodices energy for nerve impulse conduction in animals and people; produces energy for light bioluminescent organisms.
A device that produces energy is often referred to as a generator. Generators convert mechanical energy into electrical energy.
The mitochondria is what produces energy. It makes it ATP.
heat
One (of many) is animal respiration. But it also produces energy.
Helium
Which internal energy source produces heat by converting gravitational potential energy into thermal energy?
Thermal energy to electrical energy
electric energy
Friction.