Experimented using different numbers of prechargers, and controlled energy input using creative energy cells:
With 10,000 RF/t input to each precharger for setups with 1,2,3 & 4 prechargers; 4 prechargers completed most work cycles.
Next, testing a 4 precharger setup: With 5000 RF/t input to each precharger, the energy level could be maintained.
Additional RF/t did not increase the drill energy or speed.
Less than 5000 RF/t on any precharger (even with extra RF/t on the other 3 prechargers) resulted in drill energy decreasing and eventually a reduction in drill speed, suggesting each precharger is capped at 5000 RF/t transfer to the drill
Therefore the maximum input energy for a laser drill is 20,000 RF/t, supplied by 4 prechargers with 5000 RF/t inputs. (MFR V2.7.9)
Not sure if this is of any use, but I couldn't find the required energy input for prechargers anywhere. Even if the values aren't useful for you, the experiment method might be.