There are four aspects which influence the overall transmission power of a WiFi radio. The first in the chain is what’s being transmitted from the chipset or SoC natively. This is typically around 20 dBm or 100 mW and is often expressed in the operating system as txpower.
Next is any given amplifier which will boost the source signal before it reaches the antenna. This additional element to the chain is not necessarily integrated with the SoC, and thus may not reflect the actual txpower determined by the operating system.
The final part of the chain is the antenna, which offer the gain as rated in dBi. Additionally, higher gain antennas may be equipped, with 9 dBi being a common size for a standard omnidirectional antenna.
The total output power of this chain is expressed as EIRP, or equivalent isotropically radiated power. The EIRP is calculated by adding the output power of the radio (plus any amplification) in dBm with the gain of the antenna in dBi. For example a 24 dBm (250 mW) radio with a 5 dBi antenna will have a total output power of 29 dBm (800 mW).
Local regulations will determine the maximum transmission power of any WiFi equipment. For example in the United States the FCC states that a 2.4 GHz point-to-multipoint system may have a maximum of 36 dBm EIRP (4 watts) while point-to-point systems may achieve much higher EIRP.