WiFi
WiFi uses low power radio signals to connect a laptop, tablet computer, smartphone or other device to a computer network. Government regulations limit the strength of the radio signal produced by a WiFi router (sometimes called an access point). Typically the maximum transmitting power is one tenth of a watt, which is similar to a cellphone operating at or close to its maximum power.
The average transmitting power, however, is much lower than this because the router only transmits when sending data to a device connected to it. The rest of the time it is silent, apart from brief signals to alert nearby devices that it is available.
These measurements were made on a home router (Linkysys WRTG54). In order to ensure that the router was active, a downloading speed test was run on a device connected to the router while a measurement was made.