How does the brightness of a bulb tell you about the energy of electrons passing through it?

The brightness of the bulb doesn't tell you anything about the energy of the electrons passing through it.It tells you how much total energy is lost by the total flow of electrons through the bulb.  You can't tell what part of that is due to a large or small number of electrons, or what part of it is due to each electron losing a large or small amount of energy on the way through.