Transmitter power output (TPO) is the actual amount of power (in watts) of RF energy that a transmitter produces at its output. This is not the amount of power that a broadcast station reports as its ERP, such as "we're 100,000 watts of rock 'n' roll", but is in fact many times less for the high-power VHF and UHF stations. The radio antenna's design "magnifies" the signal toward the horizon, creating gain. There is also some loss (negative gain) from the feedline, which reduces some of the TPO to the antenna by both resistance and by radiating a small part of the signal.
The basic equation is:
TPO × lossfeedline × gainantenna = ERP