Raspberry pi 3 access point - broadcast power calculation / measurement?
The specifications to look at are the dBm transmit numbers. dBs are always an expression of a power ratio. In this case it is the logarithmic ratio the AP output power compared to a 1 mW transmitter.

To convert dBm back to power output in watts, divide the dBm figure by 10, then raise 10 to this power and finally multiply by 0.001 watt. For example, to convert the 20 dBm figure to watts, take the 20 dBm divided 10 to get 2. Then raise 10 to this power (102) to get 100. Then multiply 100 times 0.001 watts to find that the output power is 0.1 watts.

Here is the same thing in its full formula:

Watts = 10(dBm/10)*0.001

Of course power output is not the only variable affecting range or effectiveness. The gain of the transmit antenna, the gain of the receive antenna (often shared with the transmit antenna), the sensitivity and selectivity of the receiver, interfering signals, and the distance between APs all play a role.