My understanding is that an electron volt is the energy an electron needs to move against an electrical gradient of 1V (imagine like rowing against the current)

My understanding is that an electron volt is the energy an electron needs to move against an electrical gradient of 1V (imagine like rowing against the current). It is a unit of energy, so at first glance it doesn't really make much sense to use it a a mass unit. However, since e = mc2, you can calculate the mass that is equivalent to 1 eV (which, in kg, turns out to be about 10 to the power of -36). The electron volt is related to the electric charge of the electron, not to its mass. Similarly, a magnet weights more than its strength turned into mass.