In terms of Electromagnetism, one Watt is the rate at which work is done when one ampere (A) of current flows through an electrical potential difference of one volt (V).
In terms of Classical mechanics, one Watt is the rate at which work is done when an object's velocity is held constant at one meter per second against constant opposing force of one newton.
The wattmeter is an instrument for measuring the electric power (or the supply rate of electrical energy) in watts of any given circuit.
A modern digital electronic wattmeter/energy meter samples the voltage and current thousands of times a second. The average of the instantaneous voltage multiplied by the current is the true power. The true power divided by the apparent volt-amperes (VA) is the power factor. A computer circuit uses the sampled values to calculate RMS voltage, RMS current, VA, power (watts), power factor, and kilowatt-hours. The simple models display that information on LCD. More sophisticated models retain the information over an extended period of time, and can transmit it to field equipment or a central location.
The kilowatt is equal to one thousand watts. This unit is typically used to express the output power of engines and the power consumption of tools and machines. It is also a common unit used to express the electromagnetic power output of radio transmitters.
One kilowatt of power is approximately equal to 1.34 horsepower. A small electric heater with one heating element can use 1.0 kilowatt. The average annual electrical energy consumption of a household in the United States is about 8,900 kilowatt-hours, equivalent to a steady power consumption of about 1 kW, for an entire year.