The rest energy of an electron is

Rest energy of an electron $=m_e c^2$

Here $m_e=9.1 \times 10^{-31}$ kg and c = velocity of light

∴  Rest energy $=9.1 \times 10^{-31} \times\left(3 \times 10^8\right)^2$ joule

$=\frac{9.1 \times 10^{-31} \times\left(3 \times 10^8\right)^2}{1.6 \times 10^{-19}}$ eV = 510 keV