Yes, you are right, you can destroy an electron with a positron, but then you destroy the positron too, so you have no information about "the energy of the elec

Yes, you are right, you can destroy an electron with a positron, but then you destroy the positron too, so you have no information about "the energy of the electron". To be more concrete, bmacho equation, E = mc^2 + q*6*10^23 J/e holds for every experiment, every theory (standard model, GR). It states that energy is equivalent to mass and charge.