A change in nuclei
Fusion rearranges nuclei, unlike chemical burning, which mainly changes electronic bonding. A reaction may produce a heavier nucleus along with other particles. Whether it releases energy depends on the complete initial and final states, not merely on the statement that two nuclei have combined.
Binding and released energy
In an energy-releasing reaction, products have lower total rest energy, with the difference carried by motion and radiation. Binding energy per nucleon helps explain favourable light-nucleus reactions. Mass–energy accounting must still include the actual products and cannot determine reaction probabilities by itself.
Reaction networks in stars
The Sun converts hydrogen into helium through a sequence of reactions that also produces particles such as neutrinos. Other stellar stages involve further reactions. Temperature, density and composition affect rates, so a description of stellar fusion is not an engineering recipe transferable unchanged to terrestrial equipment.
From a reaction to an energy system
Terrestrial research studies how to maintain conditions and recover released energy. A favourable energy ratio within one experimental subsystem does not automatically establish net electricity after heating, cooling, power supply and conversion losses. Reaction physics, experiment performance and power-station engineering require distinct but connected measures.
References: [2]