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#1
11-16-2008, 09:37 AM
 Unregistered Guest Posts: n/a
Home power meter; How much is 1 rev?

On the outside of my house is the electric power meter with some counter numbers to keep track of how much electricity I have used in kilowatt-hours. Also, there is a small wheel with one small black section that goes around and around and is geared to the counter numbers.
My question is, What does one revolution of this small wheel mean?

My guess is that it is one watt-hour but I have never had the patience to stand there and count 1000 revolutions to find out.
Am I right?
Is there a such standard for all US residential power meters?

What I would like do is time the wheel with a stop watch to figure power consumption at a given time.
Don
#2
11-16-2008, 04:01 PM
 JohnS Moderator Join Date: Dec 2007 Location: SE Michigan, USA Posts: 9,538 Rep Power: 19
Re: Home power meter; How much is 1 rev?

Quote:
 Originally Posted by Unregistered On the outside of my house is the electric power meter with some counter numbers to keep track of how much electricity I have used in kilowatt-hours. Also, there is a small wheel with one small black section that goes around and around and is geared to the counter numbers. My question is, What does one revolution of this small wheel mean? My guess is that it is one watt-hour but I have never had the patience to stand there and count 1000 revolutions to find out. Am I right? Is there a such standard for all US residential power meters? What I would like do is time the wheel with a stop watch to figure power consumption at a given time. Thank you for your help, Don
This Wikipedia article explains the math,
http://en.wikipedia.org/wiki/Image:M...r_1965_(1).jpg

EDIT: Oops. The link above is the photo I mentioned. This is the article. http://en.wikipedia.org/wiki/Electricity_meter

However, there doesn't seem to be a standard. You would have to find the Kh factor on the meter, in units of watt*hours/ rev. Note the adjacent picture shows a meter labelled with the inverse, 250 rev/kWh, which would work out to 4 Wh/rev

Last edited by JohnS; 11-18-2008 at 04:57 AM.
#3
11-18-2008, 05:34 AM
 JohnS Moderator Join Date: Dec 2007 Location: SE Michigan, USA Posts: 9,538 Rep Power: 19
Re: Home power meter; How much is 1 rev?

This is a really interesting concept. I'm glad you asked the question as it forced me to do a little digging.

There is no standard, but the meter should be labeled with a meter factor. Commerical meters (very large power uses) may use a current transformer so the whole load current doesn't go through the meter. That includes extra factors not considered here. Most residential meters are direct reading in kWh, and the full load current is measured (and voltage).

The meter factor may be labeled as Kh, without any units. The various articles on the web imply that Kh = 7.2 is a "quasi-standard," but my house has a meter with Kh = 6. The units of Kh are watt*hours/revoultion. The meter may also be labeled as in the Wiki article above with the number of revolutions per kilowatthour (may be labeled Rr).

The formulas are a little confusing when units aren't included, so I will show the math WITH units. To measure power, time revolutions of the rotating disk with a stopwatch. Assume you get N revolutions in T seconds. You can use N=1, or if the wheel is too fast use a few revolutions, but count them. If the wheel is too slow, you may be able to use the subdivisions on the wheel to time a fraction of a revolution. The math will depend on whether you have Kh or Rr for your meter.

With Kh:
P w = (N revs/T s) *(3600 s/h) * (Kh Wh/rev)
Note all other units cancel out, and only watts are left.
Example Kh = 6 Wh/rev, N =1 rev, T = 28.00 s
P = (1 rev/28.00 s)*3600 s/h * 6 Wh/rev = 771.4 W

With Rr

P kW = (N revs/T s)*(3600 s/h) *(1/ Rr revs/kWh)
Rr = 166.7 revs/kWh, N = 1, T = 28 s
P = (1 rev/28 s)*3600 s/h*(1/166.7 revs/kWh) = 0.7712 kW

Note that Kh and Rr are related by
(Kh Wh/rev) * (Rr rev/kWh) = 1000 W/kW
so within rounding, my 6 Wh/rev meter is 166.7 rev/kWh.
#4
11-23-2008, 04:29 AM
 Don-fixit Guest Posts: n/a
Re: Home power meter; How much is 1 rev?

John,
I signed up on this site just to say THANK YOU for helping me with this question.
My meter has a Kh of 7.2 so one revolution would be 7.2 watt-seconds.
I never would have gotten the answer by counting the revolutions with a watch in my hand.
Again , thank you,
Don

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