How do I find out how much
electricity something uses?
Later on we'll cover how to measure the exact
amount of electricity something uses.
But the shortcut is to
just look at the label! Nearly
everything you can plug into the wall has a label
that says how much electricity it uses. (It may be
printed directly into the plastic or metal.) You
may have to hunt for the label. It's often located
on the bottom or side of the device, or possibly
where the power cord enters the unit. If the device
is powered with an AC/DC adapter, the electrical
rating is usually listed on the adapter itself.
If the label only gives the number of amps
and not the number of watts, then just multiply
the amps by 120 to get the number of watts.
(Amps x Volts = Watts, and most
U.S. electricity is 120 volts. So a hot plate that
uses 6 amps uses 6 x 120 = 720 watts. Most other
countries use 240 volts instead of 120, so outside
of North America and Japan use 240 instead of 120
in your calculations.) Note that if a device
is powered by a transformer (one of those great big
plugs), then the transformer has converted the
electricity from AC to DC, so you need to multiply
by the DC voltage, not the AC voltage of 120. For
example, if the device says "INPUT 9V, 0.5A", then
that's 9 volts x 0.5 amps = 4.5 watts.
You may have noticed that appliances may be
labeled 110, 115, or 120 volts. Appliances are
actually designed to accept a range of voltages,
between 110-120 volts, and the exact voltage coming
out of your electrical socket can vary depending on
conditions at the power plant and in your own home.
Let's just agree that when we say 120 volts, we
understand that it's actually a range from 110-120.
And just use 120 for your calculations (unless
you're outside of North America or Japan, in which
case you probably have 240 volts).
Your device might actually list a huge
voltage range, like 100-240V. That just means
that it will work with any country's voltage. For
your calculations, use the voltage for the country
where you're plugging the device in.
Some important
caveats:
- The amount of electricity listed on the
label is the maximum amount that the
appliance will ever use. For example, a
300-watt refrigerator will only run at 300 watts
when the compressor's running (which is when it
makes that humming sound, indicating that it's
actually chilling the air inside). Most of the
time the fridge just sits there, using only 5
watts or so for its electronics. If the amount
of work done by a device varies up and down,
then so does its energy use. (e.g., a stereo
that can be turned up or down, an oven that can
be set at various temperatures, a fridge that
sometimes runs and sometimes doesn't, a computer
that sometimes spins its various drives and
sometimes has to use more of its brainpower,
etc.) The label on computers is particularly
useless; a computer labeled at 300 watts
probably uses only about 65. (More on computers'
electrical use.) In just a bit we'll cover
how to measure the actual amount of electricity
being used by a device.
- Many consumer items are advertised
according to their power output, not
input. That means the stereo that
says 30 watts on the box might actually require
50 watts to make 30 watts of sound (assuming the
volume was cranked), and your 900-watt microwave
oven might actually use 1400 watts (on its
highest setting). That's because all electrical
devices are inefficient -- they have to use some
extra energy to do what they do.
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Energy
Hogs
|
|
4400
watts
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Clothes dryer
(electric)
|
|
4400
watts
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Electric
oven
|
|
3800
watts
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Water heater
(electric)
|
|
3500
watts
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Central Air
Conditioner (2.5 tons)
|
|
1500
watts
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Microwave
oven
|
|
1500
watts
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Toaster
(four-slot)
|
|
900
watts
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Coffee
maker
|
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800
watts
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Range
burner
|
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500-1440
watts
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Window unit
air conditioner
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200-700
watts
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Refrigerator
|
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60-100 watts
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Light bulb (energy
hog because houses have lots of lights,
and it's easy to leave them on when
they're not being used)
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Fans
|
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100 watts
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Floor fan or box fan (high
speed)
|
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15-95 watts
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Ceiling fan (Bigger
fans and faster speeds use more energy.
My 2004 42" Hampton Bay uses 24/28/42
watts on low/med/high respectively,
according to the manual.
Progress
Energy says
on high speed fans use 55/75/95 watts
for 36"/48"/52" models
respectively.)
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Computers
(see
more about electrical
use of
computers)
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140-330 watts
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Desktop Computer & 17" CRT
monitor
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1-20 watts
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Desktop Computer & Monitor (in
sleep mode)
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120 watts
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17" CRT monitor
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40 watts
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17" LCD monitor
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45 watts
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Laptop computer
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Other
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60-100 watts
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Regular light bulb
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4-165 watts
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Video game (While
playing game, 30W for PS2, 70W for
XBox, and 165W for XBox 360. See full
report at DX
Gaming)
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55-90 watts
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19" television
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18 watts
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Compact fluorescent light bulb
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4 watts
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Clock radio
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3 watt-hours
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Total power stored by an alkaline AA
battery. This is to put batteries into
perspective. If you could power your
clock radio with a AA battery, it
wouldn't even last an hour. We have
more on batteries on our Guide
to Household Batteries.
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Knowing how much electricity a
device uses at a given moment doesn't tell you
how much it's using in a month, because it's
probably not running 24/7 (and if it
is running 24/7 like a fridge, it's
probably not using the maximum amount of
electricity, as we discussed earlier). To
measure how much electricity something uses for
a certain period of time (like a week or a
month), you can use a watt-meter.
- Some devices use a small amount of
electricity even when they're not on. For
example, VCR's and microwaves draw a small
amount to power the time display. This amount is
often 5 watts or less. Devices which run off
transformers
also draw a small amount of power.
And of course, electricity consumption of a
device varies from brand to brand, and condition to
condition.
Exercise:
The power adapter on your laptop computer says
its output is 24V and 1.875 amps. The input
specs aren't listed for some reason. What's the
maximum number of watts your computer could ever
use? (see
answer)
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24V x 1.875a = 45 watts
Your adapter could draw a little
more than this, because it's a little
inefficient at converting the 120V to
24V, but that doesn't really matter
because your computer will rarely need
the full 45 watts anyway (which would
require all the drives to be spinning
and the processor to be fully
busy).
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Electrical usage of
household items
Naturally, electrical usage will
vary from model to model, so remember that the
table at right are just examples. But
now that you know how to find out how much
electricity things use (from the previous
section), it's best to make your own table.
Remember that the maximum
amount of electricity your appliance uses will
be printed on the appliance! Just
remember the caveats above.
Appliances that create or remove heat use
the most electricity. In the table,
appliances that make things hot are
listed in
red and ones that make things cold
are listed in
blue. As you can see, together they
dwarf everything else on the list.
Don't like my table? Here's another
table from GeneratorSales.com.
Also, More
Energy Savings is trying to compile a list
of energy usage for hundreds of specific TV's,
refrigerators, and other devices, though as of
3/08 it's a new site and it runs rather
slowly.
Exercise:
Pretend the price of fossil fuel surges and they
start rationing electricity. (You may not have
known it, but a main way that power plants make
electricity is by burning fossil fuels.) You're
given a bonus of 1000 watt-hours of electricity
to use because you won a special drawing. If
you're choosing between using your stereo or
your central air conditioner, how long could you
run each? Assume that your stereo uses 30 watts
and your AC uses 3500 watts. (see
answer)
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1000 Wh will
run a 30-watt stero for 1000 / 30 = 33
hours.
1000 Wh will
run a 3500-watt AC for 1000 / 3500 =
0.29 hours (17 minutes)
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Estimating use per
month
See my electrical
use calculator above to calculate how
much a given appliance is costing you per month or
per year.
Of course, knowing that your refrigerator uses,
say, 500 watts when the compressor's on doesn't
tell you how much energy it uses in a month,
because the compressor's not on 24/7. Here are some
websites that give sample costs for various
household items considering how much those items
are used:
Daniel Peterson has an electrical
usage calculator, which will show you how much
an appliance costs you for a day, month, or year,
and what percentage of your electric bill it
comprises.
But even that's just an estimate. Our next
section explains how
to MEASURE the amount of electricity
something uses (at a given moment or
for a whole month).
U.S. household
use of electricity, 2001
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Which devices use how
much?
The chart at right shows how the average home
used electricity in 2001. (Source:
Dept.
of Energy; see also
Energy
Info. Administration
& Energy.gov)
Of course, air conditioning uses a bigger chunk
of the pie in the summer. According to Austin
Energy, AC accounts for 60-70% of the average
home's summertime power bill.
Here's how much various strategies can save
you.
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Easy
Strategies
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Strategy
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Up front cost
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Savings per year
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(1) Use space
heaters to heat only the rooms you're
in, rather than a central system that
heats the whole house, and turning off the
heat when you're not home.
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$80
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$1286
|
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(2) Use ceiling
fans instead of the air
conditioner
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$100
if you don't already have
ceiling fans
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$665
|
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(3) Wash laundry
in cold water instead of hot or warm
|
none
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$167
|
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(4) Use a clothesline or a laundry
rack instead of a dryer
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$20
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$141
|
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(5) Turn off a single 100-watt light
bulb, from running constantly
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$0
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$96
|
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(6) Replace regular light bulbs with
compact
fluorescents
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$32
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$90
|
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(7) Sleep your
computer when you're not using it
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$0
|
$73
|
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Total
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$232 once
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$2518 every
year
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Aggressive
Strategies
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(8) Replace top-loading washer with
front-loading
washer
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$500
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$90
|
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(9) Replace 1990 fridge
with 2004 model
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$300
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$45
|
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(10) Replace a CRT computer
monitor in a home office with an LCD
display
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$200
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$21
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Total
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$1000 once
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$156 every
year
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Assumptions:
National
average electrical rate of
11¢/kWh.
(1) One 5000-watt central system running
24/7 for four months, vs. two 1500-watt
heaters running 8 hours a day for four
months.
(2) Stop running a 3500-watt AC 12 hours a
day for five months, use two large ceiling
fans instead, 12 hours/day.
(3) 1/3 of loads originally on the
Hot/Warm setting and 2/3 on Warm/Warm
setting; electric water heater; 8
loads/week.
(4) 36¢/load as per the clothes
dryers page, 8 loads a week. (Gas
dryer isn't much better @
34¢/load.)
(6) Ten 15-watt fluorescent bulbs vs.
60-watt incandescent bulbs, each burning
5.5 hours a day.
(7) Computer system sleeps for 21 hrs/day
@ 5 watts, vs. on for 24 hrs/day @ 100
watts
(8) All loads washed on Warm/Warm setting.
8 loads/week. Water heated electrically.
Includes water costs.
(9) Replacing a 900 kWh/year top-freezer
model with a 450 kWh/year top-freezer
model
(10) Used ten hours a day, five days a
week. 120 watts vs. 40 watts
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