Power Ranger: Monitoring Electrical Appliances

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Kerry Thompson
7 years ago
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1 Power Ranger: Monitoring Electrical Appliances Outcomes: 1. Understand how to use a power monitoring meter to measure line voltage, current, and power consumption. 2. Understand the difference between real power and apparent power. 3. Know how to calculate the number of hours in which an electrical appliance will consume a specific amount of energy, such as 1 kilowatt-hour. Materials: Source of AC power (electrical outlet) At least one AC electrical appliance (as many as you can get) Power consumption meter 1 P a g e

2 What to do: 1. Plug the power consumption meter into an AC electrical outlet. 2. Plug a chosen electrical appliance into the power consumption meter. 3. On the meter, select and record in the data worksheet the voltage (Volt), current (Amp), real power (Watt), and AC frequency (Hz). 4. For each measured appliance: a. Convert the power consumption from Watts to kilowatts by dividing Watts by b. Calculate the number of hours that the appliance will consume 1 kilowatt-hour of energy by dividing 1 kilowatt-hour by the power consumption (in kilowatts). 5. For appliances that have some sort of sleep or standby mode (like computers in power saving mode, VCRs with clocks, etc.), make sure to record the readings when in this mode as well. 6. For each measured appliance, check to see if power is being consumed even when off. If so, record the readings for that appliance both when on and off. 2 P a g e

3 Power Ranger Data Worksheet Appliance Volts Amps AC Frequency (Hz) Real Power (Watts) Real Power (kilowatts) = W / 1000 Hours per kw-h = 1 / (real power kw) 3 P a g e

4 Questions: 1. Which appliance used the greatest amount of power? 2. Which appliance used the least amount of power? 3. How many of the appliances that you tested were Energy Vampires appliances that draw power when they are plugged in, but turned off? 4. How would you defeat any Energy Vampires that you found? What s Going On? Electrical voltage is always present at a standard electrical outlet, whether or not an appliance is plugged it. You can think of this voltage as a source of stored electrical energy, just as the water stored in a toilet tank is a source of stored water pressure even when not flowing into the bowl. An electrical appliance draws power from the voltage to drive an electrical current through its circuitry to do some useful work: drive a motor, light a light, heat a heating element, run a radio or television. Some electrical devices will draw power even when they are off. Some appliances require a small amount of power to do things like run a clock that displays even when the appliance is off, or maintain a memory of device settings, or listen for a signal from a remote control telling it to power up. The question you have to ask is: is the energy used to keep a device in its off, sleep, or standby mode worth what it does for you? Extra: Real Power and Apparent Power If you understand that power is calculated by multiplying electrical voltage by electrical current: Power = Volts x Amps you may have noticed that the power you measured isn t always equal to the product of the voltage and current you measured. Sometimes yes, sometimes no. For example, I measured an incandescent bulb with these results: Volts, 0.83 Amps, and 101 Watts 4 P a g e

5 Multiplying Volts by Amps to get the calculated power gives: x 0.83 = which is very close to the measured value. No problem here. But when I measured a CFL bulb, I got: Multiplying Volts by amps gives: Volts, 0.13 Amps, and 9.1 Watts x 0.13 = Why the difference? The long answer is a bit complicated, but the short answer goes like this: Not all electrical devices actually consume the amount of power that they appear to based on the voltage and current applied. Simple devices that only have electrical resistance, like an incandescent light bulb filament, consume the amount of power you would expect. But other types of devices, ones that contain components that have electrical inductance and electrical capacitance (which we will not go into here) may actually consume only some of the apparent applied power, returning the unused power to the power source. In essence, some of the electrical current is consumed to do real work, and some remains in the power system as potential. Your power consumption meter will show you both of these quantities. Watts shows the real power (actual power consumption), while VA shows you the apparent power the product of V times A. For purposes of calculating energy usage and cost, it is the real power that you want to work with. 5 P a g e

6 Further Investigation: How Much Energy to Boil Water? Question: Is there a difference in the power needed to boil water with an electric stove and a microwave oven? Experiment: 1. Plug an electric stove into the power consumption meter and place a pot with 1 cup of water in it on the burner. Turner the burner to High. 2. Record the Real Power (Watts) that the stove consumes. 3. Using a clock, measure how many minutes it takes to bring the water to a full boil. 4. Calculate the total energy used to bring the water to boil using the equation: ENERGY = POWER x TIME, with power in Watts and time in hours. This will give you energy in Watt-hours, of course. You can also read the energy consumed directly from the meter by pressing the KWH button. For comparison, you can record both the calculated and measured energy consumption. 5. Next, do the same experiment using a microwave oven to heat up the cup of water. Questions: 1. Are the results you got what you expected? 2. If one of the appliances used less energy, what percentage of energy did it use compared to the appliance that used more? The more efficient appliance used % the energy that the less efficient appliance did. Worksheet: Electric Stove: Real Power Watts Time to boil 1 cup of water minutes Energy Consumed = (Real Power / 1000) x (Time / 60): kwh KWH (measured by meter) = kwh Microwave Oven: Real Power Watts Time to boil 1 cup of water minutes Energy Consumed = (Real Power / 1000) x (Time / 60): kwh KWH (measured by meter) = kwh 6 P a g e

A. Title of Lesson: Quantification of Appliance Electricity Consumption at Home. D. Timeframe for entire lesson: 3 classroom sessions of 55 minutes

A. Title of Lesson: Quantification of Appliance Electricity Consumption at Home B. Author: Geoffrey Kavulya C. Audience: 5th -8th grade science students D. Timeframe for entire lesson: 3 classroom sessions