Watt is a unit of power. In cars, it measures the amount of power that an amplifier or radio can produce or the powered speaker is capable of handling.
A wattmeter is an instrument that is used for measuring the electric active power or the average of the rate of flow of electrical energy in watts of any given circuit. Electromagnetic wattmeters are used for measuring utility frequency and audio power frequency, other types are required for radio frequency measurements.
In terms of electromagnetism, one watt is the rate at which electrical work is performed when a current of a one ampere flows across an electrical potential difference of one volt which means that the watt is equivalent to the volt-ampere