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Reading an electricity meter... - brad's life [entries|archive|friends|userinfo]
Brad Fitzpatrick

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Reading an electricity meter... [May. 21st, 2007|03:17 pm]
Brad Fitzpatrick
How do I read this?



I'm trying to measure power savings with various things,

-- making my Mac mini auto-sleep when idle,
-- using a CONFIG_NO_HZ kernel on my server,
-- powering down my DVD storage disks when idle,

But I'd like some better units to express this all in, rather than "seconds per rotation of that disc in the middle", which is all I can measure.

For what it's worth, my house spins that thing once per 34.4 seconds normally, or once every 38.5 seconds when the Mac mini is sleeping. But what does it all mean?
LinkReply

Comments:
[User Picture]From: scsi
2007-05-21 10:29 pm (UTC)
But what does it all mean?
Too much time on your hands.. :)
You can buy wall-plugin analzers (ask Dan, he had one I think) that will measure power consupmption on a per-outlet basis (it just fits between the wall and what you plug into it).
There are too many variables (water pumps, etc) going on in your house to make reading the box itself worth anything. Its like measuring output down to a teaspoon accuracy out of a firehose. Buy one of those per-outlet voltage meters before you go crazy. :)
Oh, turn your A/C and Dryer on at the same time and watch that sucker spin..
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[User Picture]From: brad
2007-05-21 11:14 pm (UTC)
There are too many variables (water pumps, etc) going on in your house to make reading the box itself worth anything.

I turned the heat/air/fan off. No washer/drying running. No running running, so septic pumps aren't running.

What else would there be that isn't a constant?

And I do want to see how much the washer/dryer cost, to see if me outsourcing my laundry to alabastercleaners.com actually makes sense or not. (not that I'd stop doing it... it's too convenient. :P)
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[User Picture]From: scsi
2007-05-21 11:18 pm (UTC)
Fridge/Freezer/Water Heater (if its electric).
If the outsourcing laundry place folds your undies and matches your socks its totally worth it.
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From: kevinspencer
2007-05-22 07:54 pm (UTC)
Outsourcing your laundry? Ha.

Mental note to self. Going to my local deli to buy sandwiches for lunch instead of making them in my own kitchen is now known as 'outsourcing my catering' ;-)
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[User Picture]From: magicpacket
2007-05-21 10:29 pm (UTC)
Your overall house usage could vary based on entirely unrelated things. Don't use this meter to measure anything, go buy one of these.
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[User Picture]From: boggyb
2007-05-21 10:30 pm (UTC)
The dials usually give you electricity usage in kilowatt-hours, and you read them most significant digit at left by noting down the number the needle has just passed. Note that some of the dials turn counter-clockwise. On the picture, the reading is something like 30460.

The spinning disc at the bottom will be x watt-hours per revolution. Looking at wikipedia a common value is 7.2, but it may say on the meter itself.

Or you can buy a watt-meter (marketed as "Kill-a-watt" or similar) and measure stuff that way.
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[User Picture]From: dan_lane
2007-05-21 10:36 pm (UTC)
damn beaten to it!

Add another vote here for a pass-through meter (such as the kill-a-watt) as you can plug specific devices into it and see which is pulling the most power and how much you can drop.

You can actually get meters now that have a separate handheld terminal that shows you your energy usage in various user-friendly formats. Mine has an ethernet cable that's patched through to some equipment elsewhere in the building that uploads my usage to the utility company for billing and diagnostics.
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[User Picture]From: bitwise
2007-05-21 10:55 pm (UTC)
When comparing devices you might have to watch out for measuring watts vs volt-amps, which is a nasty can of worms. Note also that some handheld power meters don't calculate power very accurately when the current draw is nonlinear -- i.e. switching supplies.
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[User Picture]From: brad
2007-05-21 11:17 pm (UTC)
Yes, if you click that picture in my post, you can see the number 7.2 in the bottom right corner. So after 139 rotations (or 1.3 hours, in my case), the rightmost kwh dial will advance to the next number...

thanks!
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[User Picture]From: bandicoot
2007-05-21 10:36 pm (UTC)
From the left, each dial reads a significant digit. When it's between two numbers, use the lowest. So the first digit is 3, then next is 0, etc, until you have 30460 Kilowatt hours.

If you go back to the leftmost digit, you'll see it reads just a shade over 3, which would be 30000 kwh plus a bit. The next digit is just over 0, so you're still at 30000. The next one is just over 4, which increases the accuracy to 30400. The next one is just about on 6, so you have 30460 kwh. The trick when the dial points close to 0 is to see if it's just over or just under. If it's just under, the next digit should be 8 or 9, if it's just over, the next digit will be 1 or 2.
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[User Picture]From: pyesetz
2007-05-21 10:42 pm (UTC)
Write down the numbers on the dials, then check again in 24 hours. Has the rightmost dial moved about 2½? If so, the spinning disk makes one turn every watt-hour while the rightmost dial moves one number every kilowatt-hour. If you use a watt-hour every 34.4 seconds, that's 2.54 kilowatt-hours per day. Multiply by the $/KWH figure from your electrical bill to find out how many dollars per day that is.

If the disk spins more slowly when the Mac mini is sleeping vs. when it is turned off, something has gone wrong with your test. Maybe your fridge turned itself on during the test and screwed up the numbers?

If the disk spins 11.9% faster when the Mac mini is active vs. when it is sleeping, then the Mac is using 0.30 KWH/day, which means it is using about 12 Watts continuously. That sounds awfully low for an active computer.

Using a walkie-talkie, you can watch the meter while someone turns on a 60-watt lightbulb. Such a bulb uses 60 watt-hours in a hour, or 1 additional turn of the watt-hour spinner every minute.
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[User Picture]From: nick
2007-05-21 10:58 pm (UTC)
Your mac-mini accounts for 11% of your total "average" power consumption?... wow that seems ridiculously high.
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[User Picture]From: brad
2007-05-21 11:18 pm (UTC)
Not average. But perhaps 11% of my constant regular background power consumption (servers/appliances which are always on). I currently have all my lights, washer/dryer, etc all off.
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[User Picture]From: valiskeogh
2007-05-21 11:15 pm (UTC)
http://www.hsvutil.org/customer/meters.shtml

The Dial Meter.

On a dial meter there are five dials, numbered 0 through 9, with the 0 at the top. You'll notice that the numbers go around the face clockwise on some dials, but counterclockwise on every other dial.

Read the dials from left to right. When the pointer lies between two numbers, record the smaller number. If the pointer is directly on a number, look at the dial to its immediate right. If that pointer has not passed zero, write down the smaller number.

Just subtract the previous reading from the new reading, and you will know how many kilowatt hours you have used.



This meter is reading 81384.

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[User Picture]From: valiskeogh
2007-05-21 11:18 pm (UTC)
ALSO, i bought one of these recently and found it to be very useful:


Kill-A-Watt http://www.thinkgeek.com/gadgets/travelpower/7657/
The Kill-A-Watt allows you to connect your appliances and assess how efficient they are. A large LCD display counts consumption by the Kilowatt-hour, just like utility companies. You can figure out your electrical expenses by the hour, day, week, month, even an entire year. Monitor the quality of your power by displaying Voltage, Line Frequency, and Power Factor.



i've used it to measure all the computer systems i have, and calculate how much money they cost me a month, it's very enlightening!




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[User Picture]From: jahiel
2007-05-22 01:40 am (UTC)
We have one of these available for general use in the Cernio colo cabinets. Brad, I'll send you the info you need to borrow it from the colo.
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[User Picture]From: robbat2
2007-05-22 12:02 am (UTC)
Buy the pro version of the Kill-A-Watt, has a nice RS232 interface.
I wrote some scripts for handling the data a while ago, as the company has the RS232 protocol docs on their site (the version in the docs is slightly outdated, but the only added datafield in mine was Hz).

We used it when we we suspected a PSU of doing weird things over a long time period but couldn't prove it. (Logging interval of 1 second, with reports of how many times in that second power cut out, and the min/max power factor during that second, and more).
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[User Picture]From: way2tired
2007-05-22 12:34 am (UTC)
I don't know if its negligible or not, but depending on the distance of wire between your meter and your mac, you could burn more electricity in one spot in your home vs another (loss to resistance).

I remember enough that it can be a problem, not enough to remember if its a _real_ problem.
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[User Picture]From: dormando
2007-05-22 12:47 am (UTC)
Lucky chump with a house meter to measure :P
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[User Picture]From: jahiel
2007-05-22 01:51 am (UTC)
Excellent resource for such things: Home Power Magazine. It's flippin' sweet.
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[User Picture]From: avatraxiom
2007-05-22 03:03 am (UTC)
Perhaps you want something like PowerTOP instead? :-)

Well, at least for the computers.

-Max
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[User Picture]From: brad
2007-05-22 04:26 am (UTC)
Yes, been playing with that since it was released. Upgraded my laptop VMs to a dyntick CONFIG_NO_HZ kernel to save battery life.
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[User Picture]From: avatraxiom
2007-05-23 12:56 am (UTC)
Cool. Yeah, I'm looking forward to running it on my laptop, see how things go. Hopefully Linux will start to even surpass the other two major OSes in power savings, now that we can really see where the drain comes from.

-Max
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[User Picture]From: sillyape
2007-05-22 03:31 am (UTC)
Just start burning stray cats to power your house.
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[User Picture]From: drbrain
2007-05-22 05:29 am (UTC)
Puget Sound Energy has radio-read meters and a website that gives me daily gas and electricity usage. Maybe yours has the same?

You'd have a different-looking meter, though.
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[User Picture]From: scosol
2007-05-22 06:35 am (UTC)
my preferred calculation is:

"how much do i make per hour?"
"how much time am i willing to spend to see if i could be saving $50/month?"

that calculation always ends in "FUCK IT"
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[User Picture]From: anilnaik
2007-05-22 06:38 am (UTC)
It means you should start hanging your clothes out to dry.
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[User Picture]From: dorkmatt
2007-05-22 09:29 am (UTC)

Leave it to the FreeBSD dorks..

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[User Picture]From: blueroo
2007-05-22 11:47 am (UTC)
If you want to measure house usage, you could also install one of these:

http://www.theenergydetective.com/what/overview.html

http://www.energymonitor.com/
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[User Picture]From: ydna
2007-05-22 03:02 pm (UTC)
I always wanted to hook up a photocell or something to my meter to detect the dark patch on the dial as it comes triggering a timestamp in a log. The other route (and way more fun) would be to get dozens of inductive current pickup coils to put around each circuit feed line in the breaker box to log current usage on each circuit in the house. That would require some kind of dedicated data logger to integrate the current used per sampling period.

Bah, too much work. Fuck it. :)
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[User Picture]From: hachi
2007-05-25 06:43 am (UTC)
I actually wanted to do this 3 times on 3 different houses so far. I haven't done so yet, but I believe taking readings from hall effect sensors would be pretty easy using a PIC as an ADC and serial bridge to pull it off.

Someday... someday
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[User Picture]From: meowpurrr
2007-05-30 12:09 pm (UTC)
wow, and i thought my (also not got round to yet) plan of just mounting a webcam in front of the meter to image-recognition the digits was extravagant.
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