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When we think about the costs associated with various home conservation strategies, such as ditching our incandescent bulbs for LEDs, turning off lights in unoccupied rooms, and turning down the temperature on our water heaters, it’s usually in terms of annual savings. Instead of an aggregated estimate, what about the cost per use? With your electric bill in hand, appliance model number, and a calculator, you can figure out how much it costs to run most of the major appliances in your home on a per use basis. We begin by calculating the cost per kilowatt hour (kWh). You’ll need your electric bill for this one. First, note the total electricity usage in kilowatthours and the total cost for a given month. Second, we do the following calculation: (1) Total cost ÷ Total kilowatthours = Cost/kWh
My total electric usage for the month of March was 1584 kWh, which cost $160.78. Therefore, my cost per kilowatthour is: (2) $160.78 ÷ 1584 kWh = $0.10/kWh
Now that we have the cost per kilowatt hour, we can calculate the cost per use of almost any home appliance. We’ll use the average energy usage of an electric dryer as an example. First, we’ll need the wattage. You can find this number on a label somewhere on the back or on the underside of most appliances or by searching for the make and model of an appliance on the manufacturer’s website. Let’s assume that the average energy usage of an electric dryer is 3400 watts. To convert the wattage of the dryer to kilowatt hours for one hour, we do the following:
(3) (Wattage x Number of Hours) ÷ 1000 = kWh
(4) (3400 watts x 1 hour) ÷ 1000 = 3.4 kWh
We need to determine the usage factor by dividing the amount of time per use in minutes by 60 minutes, following the number of hours chosen in equation 4. We’ll assume that the amount of time per cycle of an electric dryer is 45 minutes. Therefore, we calculate the usage factor as follows:
(5) 45 minutes ÷ 60 minutes = 0.75
Next, we multiple the usage factor by the kilowatthour of the appliance calculated in step 4. (6) 0.75 x 3.4 kWh ≅ 2.55 kWh
Finally, we multiply the kilowatthour of a single use by the cost per kilowatthour calculated in step 1.
(7) 2.55 kWh x $0.10 ≅ $0.25
Each dryer cycle costs approximately 25 cents. If you’re looking for average wattages of home appliances, head over to the Estimating Appliance and Home Electronic Energy Use page on the U.S. Department of Energy’s website at: http://energy.gov/energysaver/articles/estimatingapplianceandhomeelectronicenergyuse.
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The DENR Sustainability Team is composed of representatives from 27 DENR divisions, offices and programs. Its mission is "to inspire a culture of sustainability within DENR by promoting environmentally responsible actions." More information on DENR Sustainability is available at portal.ncdenr.org/web/sustainability/home
Information on sustainability in buildings and lands in North Carolina is available on the state sustainability team NC Project Green website, www.ncprojectgreen.org. 