Mo. Generators!

In matter-antimatter collisions resulting in photon emission, the entire rest mass of the particles is converted to kinetic energy.

Furthermore, mixing matter and antimatter can lead to the annihilation of both, in the same way that mixing antiparticles and particles does, thus giving rise to high-energy photons (gamma rays) or other particle–antiparticle pairs. The result of antimatter meeting matter is an explosion.

Not all of that energy can be utilized by any realistic propulsion technology because, while electron-positron reactions result in gamma ray photons, in reactions between protons and antiprotons, their energy is converted into relativistic neutral and charged pions. While the neutral pions decay into high-energy photons, the charged pions decay into a combination of neutrinos (carrying about 22% of the energy of the charged pions) and unstable charged muons (carrying about 78% of the charged pion energy), with the muons then decaying into a combination of electrons, positrons and neutrinos (cf. muon decay; the neutrinos from this decay carry about 2/3 of the energy of the muons, meaning that from the original charged pions, the total fraction of their energy converted to neutrinos by one route or another would be about 0.22 + (2/3)*0.78 = 0.74). Gamma radiation can be largely absorbed and converted into heat energy, although some is bound to be lost. Neutrinos very rarely interact with any form of matter, so for all intents and purposes, the energy converted into neutrinos can be considered to be lost.

particle and its antimatter particle annihilate when they meet: they disappear and their kinetic plus rest-mass energy is converted into other particles (E = mc2). For example, when an electron and a positron annihilate at rest, two gamma rays, each with energy 511 keV, are produced.