ERP is typically the magic number. However, many people think that if their license says 100 watts ERP, then they can buy a 100 watt repeater and run it at full power, and are good to go. This is not always the case. Your ERP is figured by all the losses, and gains in your system. Everything has to be caulculated, filtering loss, coax loss, and antenna gain. To put this into an example, Let's say you have a 450Mhz UHF license and the ERP for the repeater is 100 watts. You buy a 100 watt repeater, duplexer, coax cabling, and a 5 db gain omini fiberglass antenna. By the time you subtract the losses of the filtering, and coax, you have roughly 50 watts at the actual antenna, and a 100watts out of the actual repeater PA. So, with a 5 db gain antenna (system gain) the actual ERP of your repeater is 250 watts, ERP. Well over the 100 watts on your license. If you installed a unity gain antenna, you'd be at 50 watts ERP, 2 db gain antenna, you'd be at your correct ERP level of 100 watts. That's pretty much how ERP is configured. Many many many people just look at it and say, yep it says 100 watts.