|
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 for whatever reason 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. You can save
$40 and use your electric meter to find out how
much electricity something's using at a given
moment, as long as you don't have a modern digital
meter. Unfortunately, this won't tell you how much
it's using over a longer term, but hey, this method
is free.
First, make sure the device you want to
measure is turned off. Also, turn your air
conditioner off and unplug your refrigerator; if
they kick in while you're making your measurements,
that will change the results. Go outside with a
stopwatch and measure how many seconds it takes for
the disc to spin around one time. Go back inside
and turn on the device you want to measure (or plug
in the fridge if it's the fridge you want to
measure. And if you're measuring the fridge, wait
until the compressor kicks in -- i.e., it starts
making noise). Don't change anything else at
all -- turning even one light on or off will
significantly change your results. Go back outside
and count how many seconds it takes for the disc to
go around once now.
Now you're ready to calculate. Look on the meter
for something that says "Kh X.X", where "X.X" is
some number (often 7.2). Plug your numbers into the
following formula:
3.6 x Kh factor
----------------- = kW
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 (3.6 x 7.2)/60 = 0.432 kW, or 432 watts.
Your second check (fridge on) showed you were using
(3.6 x 7.2)/40 = 0.648 kW, or 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.
The meters with the disc that spins around
are slowly being replaced by utility companies with
digital meters that automatically radio in to
the utility company how much you're using, so they
don't have to pay someone to come manually read
your meter. These meters also have a battery backup
so if your power goes out, your meter can
automatically radio that in to the utility company
too. Unfortunately, with many of these digital
meters you can no longer see how much energy
something's using, unless you want to stand outside
for a long time. The typical resolution is 0.1 kWh,
so if you're measuring a 60-watt appliance, you'd
be monitoring the meter for a couple of hours
before you could get an idea of how much energy it
was using.
These things will be standard equipment before
long. For example, the utility in Austin, Texas
expects to be all-digital by 2010.
Now
that you know how to measure electric usage and you
understand terms like Kilowatt, let's find out
How
to Save on Cooling.
|
