What is the difference between transmitter power and ERP power?

Transmitter power is the power the transmitter box develops, but is not necessarily the power the radiates from the antenna. The power that comes from the antenna depends on the quality and length of the antenna cable and the antenna system.

Here are three examples:

If you use a 500W transmitter with 25 meters of RG213 cable and a single dipole antenna you will get 500W ERP.

If you use a 500W transmitter with 25 meters of RG213 cable and 4 dipoles carefully stacked and phased you will get 2000W ERP or 2KW ERP.

If you use a 200W transmitter with 20 meters of foam LMR400 cable and a 5/8 antenna you will get 500W ERP.

ERP means Effective Radiated Power, this figure is more important than transmitter power as ERP indicates the power coming from the antenna. A 50W transmitter with a very good antenna and cable will give a similar signal to a 500W transmitter on a poor antenna and cable.