QRP is a measure of the transmitter output . . . on a practical basis how would an operator accurately measure the ERP if you were using ERP (effective radiated power) as a QRP benchmark . . . you would need instrumentation to measure the feedline losses and sophisticated instrumentation to measure the gain of an antenna . . . most of that type of instrumentation is beyond the average amateur radio operator . . . . but most do have access or use of a watt meter . . . . thus the simple way of calculating QRP . . . ERP is used primarily in satellite operations because it affects the transponders (not FM repeaters)
I have had several private emails in response to my comments about QRP . . . I have seen folks operate the CQ WW CW contest in the QRP category with stacked beams or monoband beams . . . . I have never seen a definition of QRP that went into ERP . . . . they just use the transmitter output with all of the ambiguities that brings . . . have fun
Jim W9VNE