To calculate ERP, Take the transmitter power output, subtract the losses from the duplexers, subtract any measurable feedline loss and add the antenna gain.
Example: (2 dBd combined duplexer and feedline loss) + (5 dBd antenna gain)= 3 dBd gain. Use antenna gain in dBd not dBi.
100 watts at transmitter with 3 dbd gain = 200 watts ERP.
Many web sites are available to make this calculation, here is a link to one example: ERP Calculator