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.
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Heating
|
|
26,500
watts
|
Elec. furnace,
2000sf, cold climate
|
|
7941
watts
|
Elec. furnace,
1000sf, warm climate
|
|
1440
watts
|
Electric space
heater (high)
|
|
900
watts
|
Electric space
heater (medium)
|
|
600
watts
|
Electric space
heater (low)
|
|
750
watts
|
Gas furnace
(for the blower)
|
|
1100
watts
|
Waterbed
heater
|
|
450
watts
|
Waterbed
heater (avg. 10 hrs./day)
|
|
Cooling
|
|
3500
watts
|
Central Air
Conditioner (2.5 tons)
|
|
1440
watts
|
Window unit
AC, huge
|
|
900
watts
|
Window unit
AC, medium
|
|
500
watts
|
Tiny-ass
window unit AC
|
|
750
watts
|
Central AC fan
(no cooling)
|
|
More
efficient cooling
|
|
400
watts
|
Evaporative
cooler
|
|
350
watts
|
Whole-house
fan
|
|
100
watts
|
Floor or box
fan (high speed)
|
|
90
watts
|
52" ceiling
fan (high speed)
|
|
75
watts
|
48" ceiling
fan (high speed)
|
|
55
watts
|
36" ceiling
fan (high speed)
|
|
24
watts
|
42" ceiling
fan (low speed)
|
|
Major
appliances
|
|
4400
watts
|
Clothes dryer
(electric)
|
|
see
sep. page
|
Washing machine
|
|
4400
watts
|
Electric
oven
|
|
3800
watts
|
Water heater
(electric)
|
|
200-700
watts
|
Refrigerator
(compressor)
|
|
57-160
watts
|
Refrigerator
(average)
|
|
3600
watts
|
Dishwasher
(washer heats water)
|
|
1200 watts
|
Dishwasher (washer doesn't heat
water)
|
|
Lighting
|
|
60 watts
|
60-watt light bulb
(incandescent)
|
|
18 watts
|
CFL light bulb (60-watt
equivalent)
|
|
5
|
Night light
|
|
0.5
|
LED night light
|
|
Computers
(see
more about electrical
use of
computers)
|
|
150-340 watts
|
Desktop Computer & 17" CRT
monitor
|
|
1-20 watts
|
Desktop Computer & Monitor (in
sleep mode)
|
|
90 watts
|
17" CRT monitor
|
|
40 watts
|
17" LCD monitor
|
|
45 watts
|
Laptop computer
|
|
Televisions
& Videogames
|
|
340 watts
|
50-56" Plasma television
|
|
260 watts
|
50-56" LCD television
|
|
170
watts
|
50-56" DLP
television
|
|
270 watts
|
42" Plasma television
|
|
210 watts
|
42" LCD television
|
|
125
watts
|
32" LCD
television
|
|
55-90
watts
|
19" CRT
television
|
|
45 watts
|
HD cable box
|
|
194 watts
|
PS3
|
|
185 watts
|
Xbox 360
|
|
70 watts
|
Xbox
|
|
30 watts
|
PS2
|
|
18 watts
|
Nintendo Wii
(source)
|
|
Other
|
|
1440
watts
|
Microwave oven
or 4-slot Toaster
|
|
900
watts
|
Coffee
maker
|
|
800
watts
|
Range
burner
|
|
4 watts
|
Clock radio
|
|
3 watt-hours
|
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.
|
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.
- 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)
|
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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U.S.
household energy
|
Electrical usage of
household items
Of course, knowing that your
refrigerator uses, say, 350 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. The chart at right shows how the
average home used energy (not just electricity)
in 2007. (Source:
Dept.
of Energy) 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 are some websites that give sample costs
for various household items considering how much
those items are used:
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.
Remember that electrical usage varies from
model to model, so remember that the tables on
this site are just examples. My page
on measuring electrical
use shows you how to measure the exact
amount of electricity used by your particular
device.
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)
|
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)
|
Energy-saving
strategies
Here's how much various strategies can save
you.
|
Easy
Strategies
|
|
Strategy
|
Up front cost
|
Savings per year
|
|
(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.
|
$80
|
$1286
|
|
(2) Use ceiling
fans instead of the air
conditioner
|
$100
if you don't already have
ceiling fans
|
$665
|
|
(3) Wash laundry
in cold water instead of hot or warm
|
none
|
$167
|
|
(4) Use a clothesline or a laundry
rack instead of a dryer
|
$20
|
$141
|
|
(5) Turn off a single 100-watt light
bulb, from running constantly
|
$0
|
$96
|
|
(6) Replace regular light bulbs with
compact
fluorescents
|
$32
|
$90
|
|
(7) Sleep your
computer when you're not using it
|
$0
|
$73
|
|
Total
|
$232 once
|
$2518 every
year
|
|
Aggressive
Strategies
|
|
(8) Replace top-loading washer with
front-loading
washer
|
$500
|
$90
|
|
(9) Replace 1990 fridge
with 2004 model
|
$300
|
$45
|
|
(10) Replace a CRT computer
monitor in a home office with an LCD
display
|
$200
|
$21
|
|
Total
|
$1000 once
|
$156 every
year
|
|
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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