Power Meter Exercise
By Jerry Loomer
The rate at which energy is consumed is known as power. The amount of energy
consumed per second often is recorded as Watts. One Watt is one Joule of energy used in
one second. One thousand Joules in one second would be a kilowatt. The greater the
wattage, the more it will cost to run a device for a given amount of time.
Electricity is charged based on the total amount of energy that is being consumed. When
a one thousand watt object (1 kW) is operated for one hour, the total amount of energy
being consumed is one kilowatt-hour. The one thousand watt object could be a 1000 W
electric hair dryer, ten-100 W light bulbs, or two-500 watt motors. The electrical meter
can add up the wattage and the time to determine the total energy consumption for a
billing cycle. Therefore, the electric bill can be reduced by either reducing the wattage of
things being used (like compact fluorescent light bulbs instead of regular incandescent
light bulbs) or by reducing the amount of time these appliances are being used (like
turning things off when they aren’t being actively used).
A wattmeter is a device that will tell the rate at which electricity is being consumed.
Different objects use electricity at different rates, and the rate at which they consume
energy can depend on it cycle.
PURPOSE:
The purpose of this exercise is to help students measure and appreciate the rate of
electrical consumption of various household appliances.
PROCEDURE:
Plug the Kill-A-Watt meter into a standard household outlet. Plug the appliance into the
meter. Record the name of the appliance being tested and record the reading on the
wattmeter when the appliance is in the different parts of its cycle (Press any key and then
press the Watt key in the center of the pushbuttons. The watts being used will show on
the display).
Appliance
Refrigerator
Television
Microwave Oven
Clock
Reading lamp
Cell phone charger
Computer
Computer printer
Iron
Hair dryer
Operating
Standby
Off
Other Appliances
Operating
Standby
Off
Some appliances, such as a clock, do not have a standby mode. Place an N.A. in the
block to indicate that for that appliance, the reading is not applicable. Also, watch the
meter while the appliance is operating and see if it fluctuates very much during your
timing period. What actions take the most power? Put a word or two in the block next to
the meter reading telling what was happening when the reading was taken.
QUESTION:
Does the power consumption of a microwave oven depend on how much is in the
microwave?
Place a cup of water in the microwave oven and record the maximum wattage the meter
reaches while the microwave is working (Press any key and then press the Watt key in
the center of the pushbuttons). Record the amount of water you have in the microwave
and the Watt reading the meter gives you. Repeat this for two, three, and four cups of
water.
Was the consumption of the microwave the same for all four tests? If there was a
difference, what caused the difference? Do you think the four cups of water would heat
the same number of degrees in the same amount of time as when only one cup of water is
in the microwave?
QUESTION:
Does the location of the item in the microwave affect the power consumption of the
microwave oven? At what location will the power consumption be the greatest?
Record the readings on the wattmeter when a cup of water is placed in the middle of the
oven (Press any key and then press the Watt key in the center of the pushbuttons).
Record the location of the water in the microwave and the Watt reading the meter gives
you. Then move the water to a corner, middle side, halfway between center and side,
halfway between center and corner, near the door, or other places. Record the Watt
readings for all of these locations.
Is the reading on the wattmeter the same or does it differ? Explain why the readings
confirm or contradict your original thoughts about power consumption of microwave
ovens.
QUESTION:
Does the power setting of the microwave affect the high reading of a wattmeter while the
microwave is operating?
Try setting the microwave oven for different power settings while you have the same
amount of water in the same location inside the microwave oven. Record the readings of
the wattmeter for many seconds. Does the wattmeter reading stay steady? Does it vary?
Does it vary on a regular basis? Can you determine a relationship between the readings
of the wattmeter and the power setting?
QUESTION:
Does an electric iron (electric hairdryer, electric curling iron, toaster, or other heating
device) maintain a constant rate of power consumption or does it vary?
Plug a heat-generating device into the wattmeter. Record the value given on the
wattmeter for several minutes at regular intervals (say five second intervals).
Appliance
0
Watts
65
Watts
5
10
15
20
25
30
35
40
45
50
55
60
70
75
80
85
90
95
100
105
110
115
120
125
Does the power stay the same?
Graph the power consumption value for the different times. Why does the value change
or why does it not change?
QUESTION:
Does your refrigerator consume electricity even when nobody is using it, like during the
night? How much does it use?
When you have collected data for the various appliances, zero-out the time and kilowatthours values. (zero the time and kilowatt-hours registers by unplugging the meter
momentarily). Plug the meter into an outlet.
Then plug the refrigerator into the wattmeter and let it go overnight. In the morning,
record the total number of hours and the total energy consumption that your refrigerator
registered overnight while nobody was using it. (Press the purple button on the far right
side. It will give the kilowatt hours and when pressed again it will give the hours it has
been collecting data.) Record these values.
Divide the total kilowatt-hours by the number of hours to get the average kilowatt rate of
energy consumption. Multiply it by 1000 to get the average number of watts per hour.
How does this value compare with the operating rate from the previous experiment?
Why are the values different? Why isn’t the vale equal to zero, nobody was putting
anything in the refrigerator to make it cool down?