家電錶計算 Home electric meter calculations

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計算總瓦特 Calculating total watts

察您家電錶橫碟轉率計算家裡正在用的電功率(瓦特)

If your house's electric meter doesn't have a large horizontal disk, read no further.

How can you use your home's electric meter to measure the total power of all the electric devices operating at any given time? This is the question of the [link gone] Watt's Your Power page. (However, that page uses rotations per minute, whereas I find seconds per rotation more convenient. So I wrote this page.)

To find how much electricity your house is using at the moment, use the Kh factor as painted inside the meter, observing how many seconds it takes the large black mark on the horizontal disk to rotate once.

Power(in watts) = 3600 * Kh / seconds_per_rotation

Example

On my electric meter it says the Kh factor is 7.2. With a stopwatch I observe the black splotch takes 300 seconds to rotate once when just the refrigerator is busy. The more appliances working, the fewer the seconds it takes for the disk to rotate once. Sacrificing clarity for brevity, and cleaning up tabs, we program:

$ perl -pwe 's@\d+@int 7.2*3600/$&@e' <<End_of_input
700     just UPS, no computer, refrigerator unplugged
300     refrigerator compressor running
115     refrigerator compressor running; computer and monitor via UPS
207     computer and monitor (not blanked) not via UPS; refrigerator unplugged
20      refrigerator compressor running; water pumps

Seconds_per_rotation become watts in the output. 86 watts when just the refrigerator is on, etc.:

37      just UPS, no computer, refrigerator unplugged
86      refrigerator compressor running
225     refrigerator compressor running; computer and monitor via UPS
125     computer and monitor (not blanked) not via UPS; refrigerator unplugged
1296    refrigerator compressor running; water pumps

So it seems that the particular uninterruptible power supply (UPS) I bought for my computer and monitor only uses 225-86=139 watts, only 139-125=14 more than computer and monitor with no UPS. [Does this sound right?] No wonder it only gives less than 5 minutes of power during outages.

I didn't stick around all 700 seconds, so that is a guess. Also there are some small DC transformers on the line skewing that figure.

Disk rotations for one kilowatthour

In [gone web article] The Theories and Modeling of the Kilowatt-Hour Meter, we see KH is known as the "disk constant" and is the number of watthours measured of each revolution of the disk. Ok, how many rotations till we reach an entire kilowatthour?

Kh=wh/rev; rev=wh/Kh = 1000/7.2 = 138.888..

In fact the meter has an "Rr 13 8/9" painted on it.

From: Dave Dahle, 2003
Newsgroups: misc.industry.utilities.electric, etc.
Dan says: Inside my electric meter (model "Type I-70-S" made in 1985 by Zhongxing (Chunghsing) 中興 Co.) is painted Kh 7.2, Rr 13 8/9. Kh=100/Rr, But why is Rr the number of rotations for a hectowatthour, not a kilowatthour?

You're not taking the gear ratio between the disk and register pickup gear into account!

In this case, we can safely assume it's 100 (commonly used on GE, ABB/Westinghouse, and older Sangamo and Duncan meters).

Rr is the number of revolutions of the pickup gear for a full revolution of the rightmost dial.

[So 1000 / 7.2 / 100 = ( 13 + 8 / 9 ) / 10 --Dan]

What does "FM2S" mean?

FM2S means Form 2S - this is just one of a number of standardized styles of electric meters for use on various electrical services (Form 2 is for singlephase 3-wire application). The S means the meter is a detachable version that plugs into a socket assembly - the other option is A for "A-base" which has a built-in terminal block on the base (A-base meters are largely obsolete in North America but standard in the rest of the world).

What does "1 [O slash] 3W" mean?

That just means 'singlephase 3-wire' - the type of service the meter is built for. that symbol is commonly used in the electric utility industry for the word 'phase'.

30(200)A must be 30 amps and what?

30 amps for testing (100% load point), 200A is the maximum load it can safely carry (667% overload).

Say, what is the longevity of the 'average' meter?

Typically, 30-35 years but I have seen some 1940s meters still in use.

The number of notches on the disk seems 100. Is it like that for all meters? Does the big black spotch always take up 5 notches?

Yes, that 100-mark system is fairly universal, although there are some modern exceptions. The idea is that where the disk is being timed against a standard, you would be able to read to within 1% of a revolution once you stop the test. The standard and meter revolutions are then plugged into a formula to get the meter's accuracy. The timing mark does usually take the last 5% of a revolution... the idea being that the transition from black to silver would tell the counter exactly when each revolution starts.

I can use 40 kilowatt hours per every two months as the basic usage at no extra charge. How can I better understand that?

Let's see, that would be equivalent to a

$ perl -le 'printf "%.1f\n", 40 * 1000 / ( 60 * 24 )'
27.8

watt light bulb burning 24 hours a day.


積丹尼 Dan Jacobson

Last modified: 2015-09-12 19:20:43 +0800