Lightbulbs work on this principal.
Chem/Physics 101
Everyone know about the solar system model of the atom right? If you don't, it's pretty much the nucleus of an atom is like the sun, and the electrons are the plantets (they orbit the nucleus)
When an atom recieve energy from any type of source, light, heat, electricity...etc. the electrons become excited and jump from a low level to a higher level by aborbing this energy. (farther out from the nucleus. it's a bit more complicated than this, but i'm trying to keep it short and simple). But nature likes to keep things with the lowest amount of energy possible, so the electrons "want" to return to the lowest energy level they possibly can, ie. they want to get back as close to the nucleus as they can. The only way this is possible is for them to release their newly absorbed energy. They do this and then go back to where they were before they became energized.
The light bulb works on the same principal. The electricity (energy) runs through the fuse and excites the electrons, they jump but want to go back to a low level energy state so they release their energy in the easiest way possible, light and heat....this happens a gazillion times a second...and that is how the light and heat is produced from a light bulb.
Fusion is like nuclear stuff. Its when a neutron is taken and sent at a very high velocity to hit and split apart an atom, releasing a huge amount of energy. IE Atom Bomb
I think you misinterpreted what Morpheus meant.
"...25000 btu's of body heat. Combined with a form of fusion, the machines had found all the energy they would ever need."
I think that they found a way to convert bio electricty and body heat into something they could use....either that or they use the bio electrity and body heat to power nuclear power plants.
But the Light Bulb DOES NOT use fusion