A watt is the unit of measure used to express electrical power, generation and use. We choose light bulbs based on wattage. The greater the wattage, the higher the amount of power consumed to produce light.
In a single-phase AC circuit like the one in our homes, the relationship between wattage, voltage, and current is known as Watt's Law, and is stated as :
Watt's Law: W = V x A x p.f.
wattage equals voltage times current times power factor.
W = wattage (watts)
V = voltage (volts)
A = current (amps)
p.f. = power factor
For inductive/Capacitive Loads: p.f. < 1.0
For purely resistive loads, such as heaters, or light bulbs, the power factor equals 1.0. For inductive or capacitive loads like motors, the power factor is less than 1.0 and must be determined by actually measuring it or from the nameplate of the device.
Knowing Watt's Law is very helpful when troubleshooting electrical systems where circuit breakers are tripping or fuses are blowing. For instance, if you were trying to run a microwave, coffee maker, toaster, and blender all at the same time and kept tripping the circuit breaker, how could you figure the total load on this circuit?
By adding the wattage of each of the loads and dividing by the circuit voltage, an approximation of the current flowing through the circuit could be found. Comparing this value to the rating on the circuit breaker would determine whether the circuit breaker is functioning properly. If the calculated current was less than the circuit breaker rating, then there is a problem in that circuit.