dBi is a reference unit. Watts is a unit of power, and in context refers to antenna and/or amplifier gain. It is directly convertible with dBm. I can't answer the last part of your question. Look up ...

When you think of power, you think of watts. It’s a different story with RF PAs, though. Not only is the gain of the amplifier given in dB, but the power output also is normally quoted in dBm. Get ...