How is the energy emitted by a quasar measured?
The energy emitted by a quasar is estimated using the red shift the distance of the quasar and the measure of its luminosities.
The further away an object is the dimmer the object is according the the inverse square law. ( that is an object 2x the distance away has a luminosity of 1/4 and an object 100 x the distance away 1/10000)
The red shift of the light emitted by the quasar indicates the vast distances away from the earthly observer that the quasars have.
Using this distance and the measured luminosities calculation of the electro magnetic light energy at the source of the quasar can be made.
The calculations using the inverse square law give such huge values that it was years before scientists were willing to accept the results of the calculations.