Power

The unit of measure for power is the watt, named for James Watt who invented improvements that made steam engines practical around the time of the American revolution.

A mass market WiFi receiver can detect a signal with a power level as low as one 100 billionth of a watt. Here are some other power levels for comparison:

In 1946, Arthur C. Clarke estimated that a broadcast satellite in geostationary orbit would require a 50 watt transmitter if signals were to be received on the surface of the earth.
The light bulbs in your home are around 75 watts.
One horsepower is 745.7 watts.
Wolfman Jack reached most of the southwestern US by broadcasting 250,000 watt radio signals from Mexico.

Power is the rate at which energy is used in generating radio waves, light, heat mechanical work, etc. For example:

an auto engine converts energy into mechanical work and heat
an electric motor converts energy into mechanical work and heat
a sprinter converts energy into mechanical work and heat
a light bulb converts energy into heat and light (high frequency electromagnetic radiation)
a radio transmitter converts energy to heat and radio frequency electromagnetic radiation

Power is a rate, and the definition of a watt is one joule of energy per second.

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