Tech Directions - August 2007 - (Page 25) POWER PLAY Calculating Home Electricity Consumption By Harry T. Roman htroman49@aol.com ITH both energy usage and energy costs rising rapidly, all of us can benefit from paying closer attention to our consumption of energy. Students can gain greater awareness of their personal use of electricity and get some excellent experience with the practical application of mathematics by studying their families’ consumption of electricity. Here’s how to go about it. W Some Energy Basics The fundamental unit in electric energy consumption is the watt (W), which is used to rate the power of electric appliances. One kilowatt (kW) equals 1000 W. A kilowatthour (kWh) is 1 kW of power consumed for exactly 1 hour—or 2 kW consumed in 0.5 hours; 0.5 kW consumed in 2 hours; or 250 kW consumed in 0.004 hours. (Get the idea?) Keep in mind that power and energy are different. Power is energy consumed per unit of time, while energy is a total consumption over time. To get kilowatt-hours of energy consumed, you must multiply the power (kilowatts) or rate of consumption of the appliance by the total time it consumes that power. Utility companies bill their customers for the kWhs, or energy, consumed on a monthly basis. This is the number a meter reader records when visiting your home every month. If the average residential Harry T. Roman is an electrical engineer, inventor, writer, and technology education supporter, East Orange, NJ. customer consumes 6,650 kWhs per year, and the average cost per kWh is approximately $0.10, then a home’s yearly electricity costs amount to $665. The actual month-to-month bill for the home will vary because customers tend to use energy differently at each home (reflecting their lifestyles), and they also tend to use more electricity in the summer than the winter. But looking at the total consumption and using an average cost per kWh does provide an overall energy picture of the home. Doing the Math To make a rough estimate of the cost of operating an electric appliance, use this simple formula: Power draw of appliance (in W) × Time of operation (in hours) × Cost of electricity ($/kWh) 1,000 = Estimated operating cost (Note that you must divide by 1,000 to convert watts to kilowatts.) As an approximation, suppose you have your color television on for 6 hours a day for 1 month (30 days), which would add up to 180 hours, and that the power draw of the TV is 200 W. Assume the cost per kWh is $0.10. The cost to operate the TV for this period would be: 200 × 180 × 0.10 = $3.60 1,000 The list in Table 1 (page 26) shows the power draw and time of use for some typical home appliances. Using these fig- www.techdirections.com POWER & ENERGY 25 http://www.techdirections.com
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