

Ask HN: How do I accurately measure daily peak electricity load of my home? - efremjw


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kwhitefoot
Just put a current transformer on the incoming cable and connect it to the
line in of your computer with a small resistor across it to convert the signal
into a voltage. Connect another current transformer to a meter so that you can
calibrate the card. Run an oscilloscope application so that you can check to
make sure that you aren't overdriving the input, adjust the value of the
resistor down if necessary. Now you need a simple program to periodically
sample and calculate the rms current. Save the samples to a log file. I do
this using an Efergy wireless monitor that came with three current
transformers but since I only have a single phase supply I only needed one so
I used one of the spares and a program written in VB6 on an old Win98SE
machine to monitor the current.

To get good accuracy you will need to do some careful calibration but it seems
to be pretty stable. Actually the newer Efergy monitors come with a computer
hardware interface of some kind and software. if you want to do this in *nix
there are plenty of programs and libraries for reading from the sound card and
then you can use all sorts of techniques including shell scripting to analyse
the results, sound alarms, etc.

~~~
_delirium
Can you really safely and/or legally install a current transformer on a
typical incoming residential electric cable?

~~~
kwhitefoot
Why not? It's just a simple clip on transformer, similar in design to a clamp
on AC ammeter. It is designed for it and legally sold in UK and Norway at
least. The current transformer is encased in insulating plastic, it does not
make an electrical connection to any wiring only an electromagnetic one. This
is what I have [http://efergy.com/eu/products/electricity-
monitors/elitev1-m...](http://efergy.com/eu/products/electricity-
monitors/elitev1-monitor-124#.Uu5EWoJ_X1U) bought from
[http://www.clasohlson.com/no](http://www.clasohlson.com/no) although Clas
Ohlson no longer sell exactly this model.

~~~
efremjw
Thanks. The efergy clips seem too good to be true! How do I know if they're
accurate?

~~~
kwhitefoot
If you can borrow a clip on ammeter from an electrician you can compare the
them. You can also just integrate the readings over a day and then compare
that to the electricity meter. The Efergy display units also do this using a
running average. This of course assumes that you have a reasonably stable
voltage. In most countries this should not be a problem. The Efergy meters
measure current and multiply by the voltage you set to get the power, and the
sound card solution will need to do the same, while the electricity meter
measures power directly.

