In electromagnetic transmission systems, particularly those dealing with telecommunications, effective radiated power or equivalent radiated power (ERP) is a standardized theoretical measurement of radio frequency (RF) power using the SI unit watt, and is determined by subtracting system losses and adding system gains. ERP takes into consideration transmitter power output (TPO), transmission line attenuation (electrical resistance and RF radiation), RF connector insertion losses and antenna gain (the combination of antenna efficiency and antenna directivity), but not height above average terrain (HAAT). ERP is typically applied to antenna systems.