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.

http://www.csgnetwork.com/antennaecalc.html

The information was largely sourced from the Radio Spectrum Management website of the New Zealand Government.  The website stated that the information was last updated on 23 July 2009.  It is reproduced here for the convenience of readers only and is incomplete.  We recommend readers to go to the RSM website for up to date information.

http://www.rsm.govt.nz/cms/licensing/types-of-licence/general-user-licences/short-range-devices/information-on-the-operation-of-wireless-lan-and-related-systems-in-the-2-ghz-and-5-ghz-bands

Under the provisions of General User Licences, various uses of radio spectrum are exempt from individual licensing fees. General User Licence bands are commonly known as "spectrum public parks", "unlicensed bands", or "class licence bands". In many instances the nature of the technologies employed, or the short operating ranges involved, limits the potential for interference between different application sharing the same spectrum.

2.4 - 2.4835 GHz Frequency Range, 1 watt peak eirp Power, any modulation
  
2.4 - 2.4835 GHz, 4 watt peak eirp, frequency hopping or digital modulation only
 
5.15 - 5.25 GHz, 200 mW peak eirp, any modulation, indoor W-LAN only
 
5.25 - 5.35 GHz, 200 mW peak, any modulation, W-LAN only
 
5.725 - 5.875 GHz, 1 watt peak eirp, any modulation
  
5.725 - 5.875 GHz, 4 watt peak, frequency hopping or digital modulation only
 
5.725 - 5.825 GHz, 200 watt peak eirp with a max 1 watt peak transmitter power, digital modulation only, Fixed Radio Link Devices (FRL) only

The information was largely sourced from the Radio Spectrum Management website of the New Zealand Government.  The website stated that the information was last updated on 23 July 2009.  It is reproduced here for the convenience of readers only and is incomplete.  We recommend readers to go to the RSM website for up to date information.

http://www.rsm.govt.nz/cms/licensing/types-of-licence/general-user-licences/short-range-devices/information-on-the-operation-of-wireless-lan-and-related-systems-in-the-2-ghz-and-5-ghz-bands

Under the provisions of General User Licences, various uses of radio spectrum are exempt from individual licensing fees. General User Licence bands are commonly known as "spectrum public parks", "unlicensed bands", or "class licence bands". In many instances the nature of the technologies employed, or the short operating ranges involved, limits the potential for interference between different application sharing the same spectrum.

2.4 - 2.4835 GHz Frequency Range, 1 watt peak eirp Power, any modulation
  
2.4 - 2.4835 GHz, 4 watt peak eirp, frequency hopping or digital modulation only
 
5.15 - 5.25 GHz, 200 mW peak eirp, any modulation, indoor W-LAN only
 
5.25 - 5.35 GHz, 200 mW peak, any modulation, W-LAN only
 
5.725 - 5.875 GHz, 1 watt peak eirp, any modulation
  
5.725 - 5.875 GHz, 4 watt peak, frequency hopping or digital modulation only
 
5.725 - 5.825 GHz, 200 watt peak eirp with a max 1 watt peak transmitter power, digital modulation only, Fixed Radio Link Devices (FRL) only

The information was largely sourced from the Radio Spectrum Management website of the New Zealand Government.  The website stated that the information was last updated on 23 July 2009.  It is reproduced here for the convenience of readers only and is incomplete.  We recommend readers to go to the RSM website for up to date information.

http://www.rsm.govt.nz/cms/licensing/types-of-licence/general-user-licences/short-range-devices/information-on-the-operation-of-wireless-lan-and-related-systems-in-the-2-ghz-and-5-ghz-bands

Under the provisions of General User Licences, various uses of radio spectrum are exempt from individual licensing fees. General User Licence bands are commonly known as "spectrum public parks", "unlicensed bands", or "class licence bands". In many instances the nature of the technologies employed, or the short operating ranges involved, limits the potential for interference between different application sharing the same spectrum.

2.4 - 2.4835 GHz Frequency Range, 1 watt peak eirp Power, any modulation
  
2.4 - 2.4835 GHz, 4 watt peak eirp, frequency hopping or digital modulation only
 
5.15 - 5.25 GHz, 200 mW peak eirp, any modulation, indoor W-LAN only
 
5.25 - 5.35 GHz, 200 mW peak, any modulation, W-LAN only
 
5.725 - 5.875 GHz, 1 watt peak eirp, any modulation
  
5.725 - 5.875 GHz, 4 watt peak, frequency hopping or digital modulation only
 
5.725 - 5.825 GHz, 200 watt peak eirp with a max 1 watt peak transmitter power, digital modulation only, Fixed Radio Link Devices (FRL) only