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.