Electron Volts (eV) are units of energy that are convenient for use in describing energy scales of particles. One electron Volt is equal to 1.6 x 10^-19 Joules. Therefore, ten electron Volts are equal to 1.6 x 10^-18 Joules. In order to produce 20 Joules of energy from 10 eV photons, we would need 20 / (1.6 x 10^-18) = 1.25 x 10^19 photons.