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| What Does it Cost? |
How do you figure out the cost of energy over time?
Safety Check
- Do not play with electricity.
- Do not place objects on top of electrical cords or wires becasue the wires may become damaged.
- When using electrical devices, follow all instructions.
- When you remove a plug from a wall outlet, use the plug; do not pull on the cord.
Did You Know?
A watt (W) is a unit of measurement that tells how much electrical power has been used. A kilowatt (kW) equals 1,000 watts. To change watts to kilowatts, divide the number of watts by 1,000 or multiply the number of watts by 0.001.The more electricity you use, the more it costs. You can save money by conserving electricity. Since different electrical devices in your home use different amounts of electricity, knowing how much it costs to run each of them can help you decide where you need to conserve more.
The amount of energy, or power, it takes to run an electrical device is usually printed somewhere on the device itself. Power is measured in watts (W) or kilowatts (kW). For example, P.U.'s lamp uses 125 watts of power. We often call this the "wattage" used by the device.
Once we know how much power an electrical device uses, we need to figure out how much time it is used. The time an electrical device is used can be estimated and is usually referred to by the total hours used. P.U. estimates that he uses his lamp an average of 3 hours a day.
The only other thing we need to know, is how much the utility company charges. It looks like P.U.'s electric company charges him 7 cents per kilowatt-hour.
Now we're ready to figure out how much it costs to run the lamp! To make things easier, let's put all of our information in a chart.
ELECTRICAL USAGE RECORD Appliance Wattage Hours Used Watts x Hours = Wh Wh ÷ 1000 = kWh Rate Cost = kWh x Rate Lamp 125 W 3 hrs. 125 W x 3 hrs. = 375 Wh 375 Wh ÷ 1000 = 0.375 kWh $0.07/kWh 0.375 kWh x $0.07 = $0.03 Appliance - In this box, we put the name of the appliance we are looking at. In this case, we're trying to figure out how much it costs to run P.U.'s lamp, so we write "Lamp" in the box.
Wattage - In this box, we put the amount of power, or wattage, used by the appliance. P.U. said his lamp uses 125 watts, so we write "125W" in the box.
Hours Used - In this box, we put the amount of time the appliance is used per day. P.U. said he uses his lamp an average of 3 hours a day, so we write "3 hrs." in the box.
Watts x Hours - To figure out the cost, we'll need to know the number of watt-hours used by the appliance. To find this out, we just multiply the wattage by the hours used. So that means 125W x 3hrs., which equals 375Wh.
Wh ÷ 1000 = kWh - We figured out how many watt-hours were used, but the electric company charges P.U. for every kilowatt-hour. That means we need to convert the watt-hours (375Wh) to kilowatt-hours. There are 1000 kilowatts in a watt, we can figure out the number of kilowatt-hours by dividing the number of watt-hours by 1000. That's 375 ÷ 1000, which equals 0.375. So now we know the lamp used up 0.375 kWh.
Rate - In this box, we're just going to put the amount of money the electric company is charging P.U. per kilowatt-hour. He says they charge him 7 cents per kilowatt-hour, so we write $0.07/kWh in this box.
Cost = kWh x Rate - The last step is where we write how much it costs to operate the lamp each day. To figure out the cost, we just multiply the number of kilowatt-hours by the rate (0.375 x 0.07). In this case, it equals 0.02625. We need to round this answer to the nearest hundredths place. The number 2 is in the hundredths place. If the number to its right is 5 or more, add one more. If it's 4 or less, let it rest. So, we round 0.02625 to 0.03, which means it cost P.U. 3 cents a day to run his lamp.
Practice Time!
Now it's your turn! Read the information below fir each of the three appliances to see how much power they use and how often they are used, then fill in the missing data on the chart below.
TV - This 110 W television is used for 5 hours a day.
Hair Dryer - This 1,875 W hair dryer is used an average of 2 hours a day.
Lava Lamp - This lava lamp that needs 100 W of power is on 4 hours a day. The rate the utility company charges is $0.07 a kWh. Round the cost of running the electrical device to the nearest hundredths place.