Mathematics for Studying Radio
Watts

To understand what a watt is, one must first consider energy. One definition of energy is the ability to do work. There are many forms of energy, including electrical energy, chemical energy, thermal energy, gravitational potential energy, kinetic energy, and acoustic energy. The metric unit for measuring energy is the Joule. Energy can be thought of as an amount.

A watt is the basic unit of power, and power is related to energy. However, power is a rate, and energy is a quantity. The formula for power is

P = DE / Dt
  • DE is the amount of energy transferred
  • Dt is the time interval over which that energy is transferred.

If one Joule of energy is transferred in one second, this is one watt (W) of power. Figure shows the amount of power associated with some common functions. A watt is defined as one ampere (A) of current at one volt (V).

The United States Federal Communications Commission allows a maximum of four watts of power to be emitted in point-to-multipoint WLAN transmissions in the unlicensed 2.4-GHz band. In WLANs, power levels as low as one milliwatt (mW), or one one-thousandth (1/1000th) of a watt, can be used for a small area. Power levels on a single WLAN segment are rarely higher than 100 mW, enough to communicate for up to three-fourths of a kilometer or one-half of a mile under optimum conditions. Access points generally have the ability to radiate from 30 to100 mW, depending on the manufacturer. Outdoor building-to-building applications are the only ones that use power levels over 100 mW.