Measuring Power Usage?

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Mar 1, 2001, 10:04:31 PM3/1/01
I'd like to monitor the power usage (kwh) in my house. I want to be able to
keep track of it and turn off heavy users (water heater, A/C units, etc.)
if the usage gets too high. For example, I want to limit it to, say, a max
of 10 kwh. I used to have a load controller that did this but it died on me
and I haven't been able to find a reasonable replacement. It used a couple
toroidal transformers...the legs of the mains (transformer primary) each go
thru a toroid (transformer secondary) and I assume the voltage generated is
a function of the current flow. I'd imagine this would work like those
snap-on ammeters electricians use where the loop snaps around a wire and it
reads current. I should be able to monitor that with a computer and
calculate the power usage over time and use that to turn stuff off with X-
10 devices or something similar. I can probably design an A-to-D interface
to measure the voltage with a computer and I'm sure I can write the
software (I'm a software engineer) but I don't know what the algorithm
should be. So, any of you EE types have any suggestions? Pointers to
articles, books, web sites, etc. will be appreciated.



James Waid

Mar 1, 2001, 10:17:08 PM3/1/01
I'd recommend the Power Sentry (if it ever comes back in stock) Power Sentry

M.M. <> wrote in message

Dave Houston

Mar 1, 2001, 11:30:37 PM3/1/01
Because the current (I) and voltage (E) can be out of phase, you need to
make many simultaneous measurements of both.

There are several approaches that are fairly accurate although most of them
also lose accuracy at very low loads. Someone else has already suggested the
PowerSentry which has the advantage of measuring the same thing that the
utilty uses to compute your bill.

There is also this...

If you are into DIY, check out the CS5640A from Cirrus Logic.

There is also a similar chip from South Africa that Marc Hult has been
experimenting with. I believe it is SAMES SA2002E.

And there was an article in the August 1999 Nuts & Volts that used a
different approach to simultaneously measure I & E. I wanted to investigate
this one but I've been unable to get much of a response from the author. (M.M.) wrote:

BX24-AHT All Housecode Transceiver

Edward Cheung

Mar 3, 2001, 9:06:55 AM3/3/01
"M.M." wrote:

> I'd like to monitor the power usage (kwh) in my house. I want to be able to
> keep track of it and turn off heavy users (water heater, A/C units, etc.)
> if the usage gets too high.

Please go to the link below for my home built unit.

Harvey Jeane

Mar 3, 2001, 7:49:38 PM3/3/01

KWH is not a measurement of power - it is a measurement of energy (i.e.,
Power x Time). KWH is a cumulative measurement that is power integrated over
time (i.e., the total area under a power -vs- time plot). KW is the
measurement of power; it is the complex product of voltage and current
(i.e., V x A) and is representative of your instantaneous energy usage.
Maybe a small example would explain the difference...

If you operate a 100W light bulb for 100 hours, you have used 10 KWH of

If you operate the same 100W light bulb for 1 hour, you have used 0.1 KWH of

But, if you measure your power usage while the light bulb is on in both of
the above examples, your power usage will be identical (i.e., 100W)

Are you interested in measuring and controlling power usage or energy usage?
In one scenario (the KW scenario), you would be attempting to limit
instantaneous power usage to some value (i.e., 10 KW). In the other scenario
(the KWH scenario), you would be attempting to limit the amount of energy
you use over some period of time (i.e., between monthly electric bills).
Fortunately, if you know your instantaneous power usage, you can compute
your cumulative energy usage by doing a simple numerical integration. You
can calculate instantaneous power usage by measuring the instantaneous
current and voltage and multiplying these two quantities together. This is
not entirely accurate because of a phenomena called "power factor" which is
the cosine of the phase angle between the current and voltage waveforms
(i.e. both voltage and current are complex numbers containing a real and
imaginary component). If your house only contained resistive loads (i.e., no
motors), the power factor would be "1" (i.e., the voltage and current
waveforms are in phase with each other) and the cosine of zero is 1). This
is usually not too important in residential situations because most large
motors are capacitive corrected for power factor to simulate a resistive
load. In industrial situations, the power company installs not only a KWH
meter (like the one on the outside of your house), but a power factor meter
to develop an accurate charge for energy usage. As you mentioned, you can
accurately measure current using some type of inductive device (or a Hall
effect device if you have access to one). You can also measure voltage. You
can simplify things by assuming a constant voltage (i.e. 120 VAC RMS) and
just measure current.

Anyway, I can help you with the math - I'm an EE. But, by the time you
design and build something, there may be commercially available devices that
will do the same things for less cost when you consider your time. I was
most recently president and CEO of a software company, and I know the hourly
rate for good programmers. ;)


"M.M." <> wrote in message

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