Electrical Energy Consumption: Power and Energy

1
Electrical Energy Consumption Power and
Energy
12.7

• Power
• Electrical power is the rate at which
  electrical energy is produced or consumed during
  a given period of time.
• Power is measured in watts (W).
• Energy
• Energy is measured in joules (J).
• Since the joule is a small unit of energy,
  electrical energy usage is usually measured in
  kilowatthours (kWh).

Electrical power kilowatthour
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Calculating Power

• P E / t
• Power (P) is measured in watts (W)
• Energy (E) is measured in joules (J)
• Time (t) is measured in seconds (s)
• Example What is an iPods power in watts if it
  was on for 1.30 h and used 210 000 J of
  electrical energy?
• Given t 1.30 h x 3600
• t 4700s
• E 210 000 J

3

• Required P ?
• Analysis P E / t
• Solution P 210 000 J / 4700 s
• P 45 W
• Statement Therefore, the iPods power is 45
  watts.

4
Electrical Energy Consumption Efficiency
12.7

• Some electrical devices use less electrical
  energy to do a task than other devices.
• The efficiency of a device is a measure of how
  much useful energy the device produces compared
  with the amount of energy that the device uses.
• The more efficient the device, the less
  electrical energy it requires to do a particular
  task.

efficiency
5
Electrical Energy Consumption Efficiency
12.7

• Many major appliances have energy usage
  stickers (EnerGuide labels) that indicate how
  much electrical energy the device uses during one
  year. The kilowatthour ratings on EnerGuide
  labels are based on the average Canadian
  household use.
• Energy Star appliances are the most
  energy-efficient.

6
Electrical Energy Consumption Energy
Conservation
12.7

• We all have the responsibility to use less
  energy.
• There are many ways to conserve energy.
• Use more efficient devices. For example, use the
  EnerGuide and Energy Star labels on major
  appliances to make an informed decision when
  making a purchase.
• Only buy and use devices that are necessary. For
  example, an electric kettle, breadmaker, and
  rice cooker are appliances that are not
  necessary if you have an energy-efficient stove
  and oven.
• Maximize the use of an appliance when you use
  it. For example, wash only full loads of clothes
  in a washing machine.

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Calculating the Efficiency of a Device

• You can calculate the percent efficiency of a
  device using the equation
• percent efficiency energy out x 100
• energy in
• efficiency E out x 100
• E in

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SAMPLE PROBLEM 1

• A light bulb uses 100 J of electrical energy and
  produces 35 J of light energy. Calculate the
  percent efficiency of the light bulb.
• Given E out 35 J
• E in 100 J
• Required percent efficiency ( efficiency)

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• Analysis efficiency E out x 100
• E in
• Solution efficiency 35 J x 100
• 100J
• efficiency 0.35 x 100
• efficiency 40
• Statement The efficiency of the light bulb is
  40.

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SAMPLE PROBLEM 2

• A toaster oven uses 1200 J of energy to produce
  850 J of thermal energy. Calculate the percent
  efficiency of the toaster oven.
• Given
• E in 1200 J
• E out 850 J
• Required
• efficiency ?

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• Analysis
• efficiency E out x 100
• E in
• Solution
• efficiency 850 J x 100
• 1200 J
• efficiency 70.8 or 71
• Statement
• The efficiency of the toaster oven is 71.

12
Calculating the Cost of Operating anElectrical
Device

• The cost of operating an electrical device can be
  calculated using the equation
• cost to operate power used x time x cost of
  electricity

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SAMPLE PROBLEM 1

• A laptop computer uses a 75 W adapter when it is
• plugged in. Electricity costs 5.6 /kW.h.
  Calculate
• how much it would cost to operate the laptop for
  1 year for 24 hours per day.
• Given power 75 W (converted to kW 75 W x 1
  kW 0.075 kW)
• 1000 W
• time 24 hours per day for 365 days 8760
  hours
• cost of electricity 5.6 /kW.h
• Required cost to operate
• Analysis cost to operate power used x time
  x cost of electricity

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• Solution
• Cost to operate 0.075 kW x 8760 h x 5.6
• kW.h
• Cost to operate 3679 or 3700
• Statement
• It would cost 3700 (or 37.00) to operate a
  laptop computer for 24 hours per day for 365
  days.

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SAMPLE PROBLEM 2

• Calculate the cost of operating a 1500 W hair
• dryer to dry your hair for 6 minutes per day for
  3
• days. The cost of electricity is 5.6 /kW.h.
• Given
• power 1500 W (converted to kW 1500 W x 1 kW
  1.5 kW)
• 1000 W
• time 6 minutes per day for 3 days
• 6 min x 1 h x 3 days
• day 60 min
• 0.3 hours
• cost of electricity 5.6 /kW.h

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• Required
• cost to operate
• Analysis
• cost to operate power used x time x cost
  of electricity
• Solution
• Cost to operate 1.5 kW x 0.3 h x 5.6
• kW.h
• Cost to operate 2.52 or 3
• Statement
• It would cost 3 to operate your hair dryer
  for 6 minutes per day for 3 days.

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• HOMEWORK pg 535 1 - 7