Mass deficit
The mass/energy relationship, E = Dm.c2 , allows the energy released as heat in a nuclear fission or fusion reaction to be found from the mass deficit, Dm.

The fusion reaction of hydrogen to helium proceeds by two body interactions, but the net result is the creation of one 4He nucleus from four 1H nuclei.

The atomic weights are ...

.1 H = 1.673x10-27 kg
4He = 6.645x10-27 kg

The mass deficit, Dm = [4x1.673 - 6.645]x10-27

= 0.047x10-27 kg per 4He fusion

Energy released = 0.047x10-27 x (3.0x108)2

= 0.42x10-11 Joules per 4He fusion

= 0.42x10-11 / (1.6x10-19) eV per 4He fusion

= 26 MeV per 4He fusion