That depends on your antenna. If your station is running a single-bay antena such as a Nicom BKG-77 or a Shively 6812b, your antenna will have a 3 dB loss. This means that in order to run 100 watts ERP, you will have to run more than 200 watts transmitter power output.
A two-bay arrnagement of those and other antennas will give you around a 0 dB gain. This means that you will have to run less power from the transmitter thus allowing you to run up to or close to 250 watts ERP from a 300 watt transmitter.
Your feedline and any equipment between the transmitter and the antenna such as lightning arresors will also contribute to your dB loss.
Updated 2021: We have gone into greater detail about this subject in REC LPFM Advisory Letter #11. Please read that document for a more expanded answer to this question:
https://recnet.com/node/3382
