You can't just measure amps and volts to derive power because it does not
take into account power factor. By far the cheapest way to measure
total power is with the common utility-type power meter. Call your
local utility, ask to speak to someone in engineering and ask them for
a reference to a supplier. Some utilities will sell you a meter directly
if you convince them you're not going to use it to rip them off. Cost
is typically under $100.
I have just that kind of meter. It is mounted in a small round meter base
which in turn is mounted in a wooden box. The glass cover is removed
so access can be gained to the hands. Mine is the dial type rather than
the mechanical digital type. I can thus easily set the dials back to
zero by simply turning them.
These meters are incredibly accurate and hold their calibration over
time very well.
If you want to see the duty cycle of your refrigerator or other appliance,
Ampclamp makes a nice recording clamp-on ammeter that consists of
an amp-clamp feeding a small stripchart recorder. Retail is about $400
for the setup. I found one new but surplus and paid $50 for it.
Very good investment.
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>You can't just measure amps and volts to derive power because it does not
>take into account power factor. By far the cheapest way to measure
>total power is with the common utility-type power meter. Call your
>local utility, ask to speak to someone in engineering and ask them for
>a reference to a supplier. Some utilities will sell you a meter directly
>if you convince them you're not going to use it to rip them off. Cost
>is typically under $100.
There was a very good tip I looked up in sci.electronics reciently that
is relevant. In the 1/7/93 Issue of EDN in the `Circuit Ideas' section
there is a very nifty circuit for measuring real average power. The
beauty of this circuit is its simplicity. One IC and a handful of resistors
is all that is needed (total cost under $5). Given the lengths I went
to to design a similiar circuit, I am really impressed with this one's
elegance and low cost.
The circuit is designed to output 200mv full scale (to be hooked up to
a digital volt meter) hand has two ranges 0-200W and 0-2000W (you could
easily add more ranges.
Now this does not really solve your problem as you wanted something that
will record this information over time, but it IS a big step in the
right direction (either have a PC or a strip chart recorder do the
For your original example of the refridgerator, you can certainly measure
the ON power consumption, and measure the duty cycle some other way
(do your homework in the kitchen and listen carefully for a couple hours?).
Note that if you do go the strip chart or PC data collection route, please
note that this circuit's output is NOT isolated from the AC lines. Thus
either your inputs to the recorder should be, or you should make sure
the inputs can take the common mode voltage.
I have not built one myself yet, but I have the only part I dont have
>In the 1/7/93 Issue of EDN in the `Circuit Ideas' section there is a very
>nifty circuit for measuring real average power. One IC and a handful of
>resistors is all that is needed (total cost under $5).
I looked up this article and found that resistor R2 was unlabeled on the
schematic. In the March 4 issue, page 136, there is a correction that says
R2 is 1M, 1/4 W.
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How can I prove I am not crazy to people who are?
Find your local building dismanteler (house demolisher) and buy a 'panel'
from them. Wire on a plug and socket. The panel is the thing on the
back of your house filled with breakers and a big glass thing that has
your watt-hr meter in it. This should be relatively cheap and easy.
Get a small panel if possible... This is where I got one once...
(I find it hard to believe that you can't get one of these new from
and electrical supply house ... maybe I'll check on my way home ...
You should be able to get one from the folks who sell Solar Power systems,
since even if you are 'off the grid' you might like to know how much
power you are using...)
E. Michael Smith e...@apple.COM
'Whatever you can do, or dream you can, begin it. Boldness has
genius, power and magic in it.' - Goethe
I am not responsible nor is anyone else. Everything is disclaimed.
I have gone through the Real Goods catalog without any luck. The folks at
Real Goods are the ones to go to for living off the grid. They have lots
of monitors for battery systems, but nothing to measure current output over time.
Until I find a more graceful solution, I am now taking manual periodic
readings from the dials on the electric meter in my house. I am noting readings
about two or three times a day. This might help as a rough idea of consumption
Places like Princess Auto, Edmond Scientific, and so on carry hour
meters. Many of these are just an electric clock and a vibration
detector. Stick it on a motor, and whenever the motor turns, so does
the hour meter.
Something like a refrigerator is going to have a pretty constant demand
when running. So if you can determine the power when it is running, add
a small fudge factor for startup, and read the hour meter now and then.
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=> University of Alberta Lab Manager, Space Physics Group <=
=> tel:403 492-3713 fax: 403 492-4256 <=
The last few issues of Home Power Magazine has had ads for just that
device. Priced not too bad either, under $200 I think.
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