Wat (means by the symbol W) is the unit of measurement of power in the International System of Units (SI). Power by definition is the ratio of work performed to the time for its implementation. As in physics work and energy are virtually identical, the power can be viewed as the rate of change of energy with time.
In practical calculations related to power is not always convenient to use unit watt. Basically watt is a small quantity. Sometimes when measured capacities are particularly large or particularly small (several times), more comfortable to have units formed with standard prefixes. For example, when designing an estimate of radar or radios are often used pW and nW (pikovati and nanovati), while medical devices (eg Electroencephalography and electrocardiography) consumed μW (mikrovati). In electricity production, but also in the design of railway locomotives and trains used MW (megawatts) and GW (gigavati). In the table below are given different values, formed by standard prefixes in SI.