Power is an indication of the amount of work to do in a specific amount of time, ie, an index of performance of work. Since work is measured in joules, and time in seconds, the power is measured in joules per second. The unit of measurement of electrical power is the watt, which is equal to 1 joule / second. In equation form:


imagen1 articulo2

The measurement unit, the watt, is derived from the name of James Watt, who participated in the establishment of standards for measuring power. He presented the horsepower (hp) as a measure of the average power of a strong draft horse for a full day of work. It is about 50% more than you would expect from an average horse. Horsepower and watts are related as follows:

1 horsepower = 746 watts  

The power delivered or absorbed by a system or an electrical device can be determined in terms of the current and voltage.


but       imagen3 articulo2 so that    imagen4 articulo2    (watss)

The power supplied by a power source is given by

imagen5 copia articulo2    (Watts ), where E is the electromotive force of the source and I the current drawn from the source.


(Watts ), where E is the electromotive force of the source and I the current drawn from the source.


                    imagen6 articulo2

or                imagen7 articulo2   and     imagen8 articulo2



Example 1 . Determine the power supplied to the motor of Figure 1.


                p= VI = 120 (5) = 600 watts =0.6 kw


Example 2 . What is the power dissipated in a resistor of 5 ohms if the current through it is 4 amperes ?


                 p = 12R = (4)2 . 5 = 80 watts


Example 3. The characteristics J - V of a bulb is shown in Figure 2. Note the non-linearity of the curve, indicating a range of resistances of the bulb, according to the applied voltages. If the rated voltage is 120 v, find the power rating of the bulb.

Similarly, calculate the resistance of the bulb under the given conditions and determine if the resistance of the bulb for a lower applied voltage is higher or lower than those values.


potencia1                       Figure 1.


potencia2   Figure 2.



A 120 v, I = 0.375 amp y
             p = VI = 120 (0.375) = 45 watts
A 120 v,
R = 120    = 320 Ohms
       I       0.375
Since the curve approaches the axis of the voltage, the resistance is higher for lower applied voltages.
Font: Robert L. Boylestad, Análisis introductorio de circuitos, Pág. 78-80.