Cool Your School – Electricity Lab
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
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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 1000.
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.
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Power Ranger Data Worksheet
Appliance
Volts
Amps
AC
Frequency
(Hz)
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Real Power
(Watts)
Real Power
(kilowatts)
Hours per
kW-h
= W / 1000
= 1 / (real
power kW)
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Cool Your School – Electricity Lab
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:
122.1 Volts, 0.83 Amps, and 101 Watts
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Multiplying Volts by Amps to get the calculated power gives:
122.1 x 0.83 = 101.3
which is very close to the measured value. No problem here.
But when I measured a CFL bulb, I got:
122.3 Volts, 0.13 Amps, and 9.1 Watts
Multiplying Volts by amps gives:
122.3 x 0.13 = 15.90
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.
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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
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