Supporting document to the parameter list of rf power source for the high beta elliptical cavities for ESS and prototypes. These calculations are based on the ESS high level parameter table 15 April 2011 and the Accelerator Science and Lattice parameter table 13 May 2011. I.e, the beam pulse length is 2.9 ms and the power coupler peak power, to beam is 0.9 MW. The long pulses in combination with a high peak voltage is challenging for the klystron modulators. In order to avoid unnecessary safety margins that will worsen for the klystron modulators, careful analysis is needed to find out the requirements on the rf power source.