The rest energy of the electron is E=m(0)*c^2=0.511MeV, where the rest mass m(0)=9.109x10^(-31)Kg multiplies c squared. This is the energy of the electron in its own frame, i.e. at rest. When we say an electron gains (or has, I'm quite confused about this point too ) 1eV of energy when accelerated through a 1Volt potential, to which frame do we refer to? Also, when we speak of low energy electrons, say one with energy of a few eV, we must mean in our frame (or the "laboratory frame"), as their energy in their own frame is bounded below by their rest energy of 0.511MeV. Correct? Thanks!