^What Christopher says. Also, in particle physics and astrophysics, the units commonly used are electron volts (eV) for energy and eV/c² for mass -- the latter usually being abbreviated to eV if natural units are used which set c=1.
One volt (V) is defined as being the potential difference when one coulomb (C) of charge crosses that potential resulting in a change of one joule (J) of energy. One electron volt is defined as the magnitude of the energy change of an electron moving through an potential difference of one volt. So 1 eV = 1.602x10^−19 J given that the magnitude of the charge on the electron is 1.602×10^−19 C (the charge is actually a negative quantity for electrons and positive for positrons, of course).