12.7: Electrical Power and Efficiency
pg. 530
Key Concepts:
1. Electrical energy is produced from renewable and non-renewable
resources.
2. Electrical energy consumption should be reduced.
Electrical Power
Electrical Power: the rate at which electrical energy is produced or used.
- The unit of measure for electrical power is the Watt (W).
- 1 watt is equal to 1 joule per second (J/s)
- Power rating value indicates the amount of energy used or produced, and is
known as wattage.
- Consider and compare a 60 W incandescent light bulb to a 15 W compact
fluorescent bulb.
- The incandescent light bulb uses more energy then the fluorescent bulb.
- Both produce about the same amount of light. The extra energy from the
incandescent light bulb is lost in the form of thermal energy (heat).
Measuring Electrical energy Usage
Kilowatt-hour (kW.h): the SI unit for measuring electrical energy usage; the
use of one kilowatt of power for one hour.
- Generating Stations generate electricity measured in megawatts (MW), or
a million watts.
- A gigawatt (GW), represents a billion watts.
- When measuring large amounts of energy used it is measure in watt .hours
(W.h), kilowatt.hours (kW.h), or gigawatt.hours (GW.h).
- A watt.hour is equal to 3600 joules. (60 seconds X 60 minutes)
- A kilowatt.hour is a Standard International unit of measure (SI) and is 1000
greater then a watt.hour.
- The electrical meter is a measuring device which measures the electricity
used by houses, schools, and industries.
Efficient Devices
Efficiency: comparison of the energy output of a device with energy
supplied.
- Efficiency is a comparison between how much energy is used by a device
and the amount of useful energy that is produced by that device.
- Example: An old dryer uses 800 kW.h per year. A new dryer uses 300
kW.h per year. The newer model is 60% more efficient then the older model.
Evaluating Efficiency in Devices
- Light bulbs illuminate our world. Some light bulbs are more energy
efficient then others.
- Incandescent lights heat a filament to generate light (energy
transformation).
- The efficiency of incandescent light, electrical energy is 90% converted to
heat and 10% is light.
- The incandescent light is more efficient in producing heat then light.
- Example: 100 W incandescent light bulb; uses 40 kW.h of electrical
energy in 400 h.
- Example: 25 W compact fluorescent light bulb; (CFL), uses only 10 kW.h
of electrical energy in 400 h.
- Both light bulbs emit the same amount of energy; therefore the CFL is
more efficient.
- Toxic concerns about CFLs, which contain small amounts of mercury gas,
is that they are safe as long as they are properly disposed.
- Light Emitting Diodes (LEDs) is a new technology which uses less
electrical energy to produce light and has a long life span.
EnerGuide and Energy Star Labels
- When buying electrical appliances and technology, it is important to
consider prices and energy usage.
- A less expensive device may cost more to run because it uses more
electricity to function, while a more costly appliance may use fur less energy
to function.
- EnerGuide labels indicate the energy efficiency of the appliance. The guide
indicates energy usage per year in kW.h.
- This information is used to purchase an appliance wisely.
- Energy Star appliances meet the minimal level of efficiency.
Calculating the Efficiency of a Device
- The greater the percentage the more efficient the appliance.
- Energy Out is the measure of how much useful energy (J) released to do
the task.
- Energy In is the measure of energy (J) required to complete the task.
- Formula used to determine efficiency:
percent efficiency = energy out X 100%
energy in
% efficiency = Eout X 100%
Ein
Sample Problem: Calculating the Efficiency of a Light Bulb
A light bulb uses 100 J of electrical energy and produces 35 J of light.
Calculate the percent efficiency of the light bulb. (GRASP)
Given:
Required:
Analysis:
Solution:
Paraphrase:
Timely Use of Electrical Energy
- The price of electricity fluctuates through out the day. There are low points
and high points during the day.
- Between 9 a.m. to 5 p.m. has the highest demand, therefore has the highest
cost.
- When using large quantities of electricity at home (washing clothes, dishes,
etc.) it is best to do it during off peak hours to lower cost.
- Smart meters of been installed in most homes in Ontario. They monitor
both the amount of electricity used and the time period when it was used to
determine billing charges.
- Common sense is the best way to save electricity, turn off your devices
when not in use.
Cost of Electricity
- Current cost of electricity is 5.6 ¢/kW.h in Ontario.
- In winter cost does not change for the first 1000 kW.h, it then increase to
6.5 ¢/kW.h.
- In the summer the cost does not change for the first 600 kW.h, it then
increases to 6.5 ¢/kW.h.
- To calculate the cost of using your electrical device, you need to know its
power rating in kilowatts and the length of time operated in hours.
- Electronic devices still use electricity when they are in standby mode.
- You also billed for extra charges, such as; electrical distribution and
transmission.
- Use of electricity also negatively impacts the environment, pollution.
- Formula to calculate:
Cost to operate = power used X time X cost of electricity
Sample Problem: Calculating the Cost of Operating a Laptop Computer
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. (GRASP)
Given:
Require:
Analysis:
Solution:
Paraphrase:
Evidence of Learning …. Students can
- evaluate efficiency in electrical devices.
- calculate the efficiency of an electrical device.
- calculate the cost of electricity for different electrical devices.
Check Your Learning
Questions 1 – 7, page 535
Summary:
- Electrical power is the rate at which electrical energy produced or used.
- Electrical energy should be conserved and used wisely.
- Purchasing electrical energy-efficient devices saves money and is better for
the environment.
- Percent efficiency can be calculated using the equation
% efficiency = Eout X 100%
E
in
- When purchasing an electrical device, you should also consider the cost of
operating it.
- The time of day affects the costs of producing and purchasing the electrical
energy that you use.
- The cost of operating an electrical device can be calculated using the
equation:
cost to operate = power used x time x cost of electricity