How photoelectron energy was measured in first experiment?

One of the astounding results of the photoelectric effect experiment was the indepedence of photon electron energy from light intensity. The photoelectron energy rather depends on light frequency.

But, in the first experiment how photoelectron energy was measured?

When the observer found that the energy of the ejected electron depends upon the frequency of the light instead of the intensity, he measured the potential required for no emission of electron and this was named as "stopping potential". The energy required to stop the moving electron is the work done by the external cell, i.e. $e\Delta V$. This is how the photo electron's energy was determined.