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5.73: Electric Power and Electrical Energy Use

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Miranda has to use a hair dryer in order to dry her long hair quickly enough to get to school on time. She has a powerful hair dryer that dries her hair faster than a less powerful hair dryer would. That’s because a more powerful hair dryer changes electric current to thermal energy more quickly.

Electric Power

The rate at which a device changes electric current to another form of energy is called electric power . The SI unit for power—including electric power—is the watt. A watt equals 1 joule of energy per second. High wattages are often expressed in kilowatts, where 1 kilowatt equals 1000 watts. The power of an electric device, such as a hair dryer, can be calculated if you know the voltage of the circuit and how much current the device receives. The following equation is used:

Power (watts) = Current (amps) x Voltage (volts)

Assume that Miranda’s hair dryer is the only electric device in a 120-volt circuit that carries 15 amps of current. Then the power of her hair dryer in kilowatts is:

Power = 15 amps x 120 volts = 1800 watts, or 1.8 kilowatts

Q: If a different hair dryer is plugged into a 120-volt circuit that carries 10 amps of current. What is the power of the other hair dryer?

A: Substitute these values in the power equation:

Power = 10 amps x 120 volts = 1200 watts, or 1.2 kilowatts

Electrical Energy Use

Did you ever wonder how much electrical energy it takes to use an appliance such as a hair dryer? Electrical energy use depends on the power of the appliance and how long it is used. It can be calculated with this equation:

Electrical Energy = Power x Time

If Miranda uses her 1.8-kilowatt hair dryer for 0.2 hours, how much electrical energy does she use?

Electrical Energy = 1.8 kilowatts x 0.2 hours = 0.36 kilowatt-hours

Electrical energy use is typically expressed in kilowatt-hours, as in this example. How much energy is this? One kilowatt-hour equals 3.6 million joules of energy.

Q: Suppose Miranda were to use a 1.2-kilowatt hair dryer for 0.2 hours. How much electrical energy would she use then?

A: She would use:

Electrical Energy = 1.2 kilowatts x 0.2 hour = 0.24 kilowatt-hours

You can learn much more about electrical energy use—including your own—at the URL below. Use the tables and other information, along with the handy electrical energy calculator, to estimate how much electrical energy you use. http://michaelbluejay.com/electricity/howmuch.html


  • Electric power is the rate at which a device changes electric current to another form of energy. The SI unit of power is the watt. Electric power can be calculated as current times voltage.
  • Electrical energy use equals the power of the appliance multiplied by the amount of time the appliance is used.


  • electric power : Rate at which a device changes electric current to another form of energy.


At the following URL, review how to calculate electric power and then solve problems 1-7. http://webs.rps205.com/curriculum/science/files/A3874CAB2CB74F2B9EEEF914DB22E2EA.pdf


  1. Define electric power. What is the SI unit for power?
  2. A microwave is the only electric device in a 120-volt circuit. If the circuit has 7.5 amps of current, what is the electric power of the microwave?
  3. Calculate the electrical energy use of the microwave in question 2 if it used for 0.05 hours.

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Difficulty Level:

At Grade


7 , 8

Date Created:

Nov 01, 2012

Last Modified:

Sep 05, 2014

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