Question

What is the rest energy of an electron?

Answer 1

mass of electron is resy.

Answer 2

The rest energy of electron is 0.511 MeV.

To calculate the rest energy of any substance, Einstein's mass energy equivalence relation is always used which is E=MC^2

The rest energy of an electron doesn't literally means that electron is at rest because that never happens.

Electron is always spinning, by saying rest energy of electron it means that the centre of mass of electron is not moving.

Although by referring to spinning of an electron what we actually mean is the wave nature of the electron. The electron has an intrinsic spin just like a cyclone or tornado. If that intrinsic spin is removed theywon't anymore be called as what they are, and so is the case with an electron.