Comment by spullara

If you look at the mass before of their fuel, 1 deuterium atom + 1 tritium atom:

    2.01410177811 u + 3.01604928 u = 5.03015105811 u

vs the mass of the fusion products of 1 helium atom and 1 neutron:

    4.002602 u + 1.008 u = 5.010602 u

You'll notice that even though we started with 5 neutrons and 2 protons and ended up with the same number there was some additional binding energy that is unaccounted for in the new configuration. This is the energy released by the fusion reaction via E = mc^2. Here we see the mass difference is:

    5.03015105811 u - 5.010602 u = 0.01954905811 u

Converting that to energy you find that is 17.6 MeV. As you go up the periodic table fusing nuclei you will get less and less marginal energy until you get to iron where at that point fusion become net negative and fission is then takes over where breaking nuclei apart gains energy, marginally more as you go up the periodic table. That's why you want to fuse light particles and fission very heavy particles. It is also why there is so much iron as it is kind of the base state of both of these reactions.