ERP is mainly determed by antenna current and efffective height. If this
is known then it is very simple to estimate radiated power. Certanly this
is very aproximative estimation. But something is better then nothing :-)
be careful with that. I speak from my own experience :
Antenna = 14m heigh inverted-L with elevated loading coil and 4 x 23m
topload wire, calculated and simulated Rrad = +/- 0.06 Ohm
Antenna current = 1.8A
Theory : radiated power = 0.06*1.8^2 = 194mW EMRP (= 583mW EIRP or 355mW ERP)
Reality : about 35mW ERP (based on field strength measurement)
Difference : -10dB !
Reason : antenna surrounded by many trees
Maybe most other hams have more luck (less trees), but take into account
that real ERP is 2 to 6dB below the calculated ERP.
73, Rik ON7YD