The heat in the sun is from fusion. Nuclear fission has a by-product of radioactive waste that will last for 500,000 years. In other words, if Neanderthal Man had used fission, we would be burdened with his radioactive waste even today. It is everywhere in great abundance. Since the beginning of time, nature has been producing and reproducing energy in quantities so astronomical that we could never begin to use it all, not even with the most advanced technology.
Most energy sources are ongoing or renewable while others are finite.
Ironically, we have chosen to produce appreciable energy from only one source, fossil fuels. A small fraction of one percent of the sun's energy that the Earth absorbs is converted into plant tissue, and it is this tiny fraction that has produced all fossil fuels. Fossil fuels are usually environmentally destructive in their harvesting and in their burning to produce usable energy. They are nonrenewable. When they are used up, they are gone forever.