|What is ERP or EIRP?|
ERP stands for Effective or Equivalent Radiated Power that is directed in a given direction. It is the power supplied to an antenna multiplied by the antenna gain in a given direction.
EIRP stands for Effective or Equivalent Isotropic Radiated Power. It is the apparent power transmitted towards the receiver, if it is assumed that the signal is radiated equally in all directions, such as a spherical wave emanating from a point source.
The above definitions were obtained from the website below and simplified for the convenience and ease of reference. The same website provides a calculator whereby the entry of transmitter power and antenna gain will produce an ERP and an EIRP.