Mr. Electricity is your guide to saving energy in your home.
We're recommended by the government of Berks County, PA.
Related sites:
Watt Watt. News about efficiency and conservation, written by readers of the site.
Home Power Magazine. All about renewable energy for the home.
Thin House. Blog about a family committed to cutting its energy use by 80%.
No-Impact Man. Blog about a family striving to have no net impact. (i.e., What little they use, they offset.) Inspirational.
Off-Grid. News and resources about living without being connected to a utility company.
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Battery
Guide
Which battery is best? We cover
rechargeable and alkaline batteries to show you what's hot,
what's not, and the best way to charge them. (visit
now)
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How do I
measure the amount of electricity something
uses?
Remember
that almost every device you can plug into the wall has a
label or engraving that tells you how much energy it uses.
(More on figuring energy
use without directly measuring it.)
But if you want to measure the energy use
yourself, you can definitely do that too.
There are three ways to measure how much electricity
something uses:
- Buy a cheap, simple watt-hour meter like the
$24 Kill-A-Watt
- Buy & install a wired-in watt-hour
meter
- Look at the existing electric meter on the side of
your house and use a simple calculation.
Let's look at each option in turn.
(1) Buy a cheap, simple
watt-hour meter
A watt-hour meter is a little device that tells
you how much electricity something uses, either at a
given moment or over an extended period of time. Just
plug the device into the meter, plug the meter into the
wall, and read the display. I like the Kill-A-Watt
(pictured at left) sold for $24 through SmartHome. Just
plug it in and see how many watts a device is drawing at
any given moment, or how many kWh you've used since you
turned it on. This is especially useful for finding the
amount of kWh used in a month for devices that don't run
constantly, like refrigerators and window unit air
conditioners.
I couldn't find a 240V watt-hour meter that can
measure large appliances running on U.S. current, such as
electric clothes dryers. Promolife sells a 240V
version of the Watts Up? meter for $131 but it's
designed for European electrical systems, not U.S., and
they're not compatible (we have split-phase and Europe
has straight 240V). If you need to measure 240V then you
can go with a whole-house meter like the EUM-2000
described below.
(2) Buy & install a
whole-house meter.
A whole-house meter tells you how much energy
your whole house is using at any given moment, and how
much you've used so far for the month -- as well as how
much it's costing you. These meters cost around $150 and
need to be installed by an electrician. (It's an easy,
15-minute job.) Two whole-house meters you might like are
The Energy
Detective, and the EUM-2000.
(3) Look at the electric meter
on the side of your house.
This method is cumbersome, but it's free.
First, make sure the device you want to measure is
turned off. Also, turn your air conditioner, heating
system, and refrigerator; if they kick in while you're
making your measurements, that will change the results.
Once you've down that, go outside and figure out how many
kW your house is using, as described below. Then go back
inside, turn on the device you want to measure, go back
outside, and clock your meter again. The difference is
how watts your device is using.
The way you clock your electric meter depends on
whether it's digital or analog.
 Digital
Meters
If your meter is digital, look for three little
arrows or blocks on the bottom-left or bottom-right of
the display. The marks keep changing as electricity is
used. Use a wristwatch and time how long it takes one of
the marks to change. (That's any changes of the
marks, not a complete cycle of changes. In the picture at
right, the marks change once per second. The pattern of
changes on your meter might be different, but it doesn't
matter. We're just looking for any change in the
marks at all.) Each change is one watt-hour of
electricity. You can now calculate the electricity being
used with this formula:
3600 ÷ number of seconds = watts
For example, let's say you want to measure the energy
used by your computer system. Turn off the computer, as
well as anything that could suddenly turn on by itself
and screw up your results (like refrigerators, AC's, and
heaters). You check your meter, you see a mark change,
and then it takes 10 seconds until the next change. Using
our formula, that's 3600 ÷ 10 = 360 watts, which is
how much everything in your home is using at that moment.
Then you fire up your computer system and check again.
This time the marks change every 6 seconds. That's 3600
÷ 6 = 600 watts. So your computer system uses 600
watts - 360 watts = 240 watts.
If whatever you're measuring uses more than 3.6 kWh,
then the above formula won't work because your marks will
be changing faster than once per second. In that case,
count how many times the marks change in 10 seconds and
use this formula:
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3600 x [number of
changes]
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watts =
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10
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You can also use this second formula to measure
devices more accurately, especially low-power devices.
Count the number of changes in, say, 60 seconds, and then
use 60 in the formula instead of 10. The longer you count
the marks, the greater the accuracy.
All of the above works for residential meters. If your
location is commercial, look for the "kH" factor on the
meter. If the kH is anything other than 1.0, then
multiply your result by the kH factor that is
listed.
Many thanks to Steve, who worked in the meter shop of
a utility company, for the above tip.
Analog Meters
(1) Use a wristwatch and measure how many
seconds it takes for the disc to spin around one time.
(2) Look on the meter for something that says "Kh
X.X", where "X.X" is some number, often 7.2.
(3) Plug your numbers into the following formula:
3600 x Kh factor
----------------- = watts
number of seconds
For example, let's say that you're measuring your
fridge, your Kh factor is 7.2, it took 60 seconds for the
disc to spin once the first time you checked, and 40
seconds the second time you checked. Your first check
(fridge off) showed you were using (3600 x 7.2)/60 = 432
watts. Your second check (fridge on, compressor running)
showed you were using (3.6 x 7.2)/40 = 648 watts. The
difference between 648 watts and 432 watts is 216 watts,
which is how much electricity your fridge uses when the
compressor is running.
Now that you know how to
measure electric usage and you understand terms like
Kilowatt, let's find out How
to Save on Cooling.
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