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.
the mitochondria produces the energy
the sun produces thermal energy
Fusion produces the Sun's energy .
Salt produces lattice energy
energy
a big building that produces 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.
The mitochondria is what produces energy. It makes it ATP.
heat
the thing that produces energy is nutrients
One (of many) is animal respiration. But it also produces energy.
helium