# Estimating KWh electicity billing using clamp-on amp meter

Ed Pawlowski wrote:

In this case, I'm asking how a watt calculation made by taking an average RMS reading for a few minutes from a clamp-on RMS amp meter on a supply phase and multiplying that by 120 to get VA would differ from the "watts in use" as being measured by the utility's billing meter during the same few minutes.
The question did not involve the various uncertainties as to whether the meter is employing "demand-meter" metering, or the uncertainties as to whether or not extrapolating a few minutes of average current draw is representative of an entire month's usage.
The answer, as posted by a few sane people who looked at the original question and gave an actual answer, is that yes, VA can be considered as equivalent to watts when all loads are resistive and power-factor is unity (ie - 1).
To the extent that my facility's appliances, devices and equipment have non-linear (ie inductive) loads, my measure of VA == watts *will* be an over-estimate of what the meter is measuring during the time-frame of the measurement.
That over-estimation will most likely not exceed 10% because the worst offenders for having low power-factors are small fractional-hp electric motors, for which I have two (one in each bathroom of this building as ceiling ventilation fans) and for which I would estimate their use to be perhaps 3 hours per day. In addition to those two fans, the building's furnace has a 220 VAC furnace fan motor which I would estimate to be 3/4 to 1 hp and the daily usage (for the present time) to be 1 hour per day or less.

I've been using it since 1988 (that's about 23 years now).

And I've also seen far too many questions asked and then the thread is hijacked by others who want to take the question on a tangent because they don't know how to answer the original question but instead feel a compulsion to post something to prove they are worthy or relevant to the group.
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AND:
I have seen way too many people propose entirely stupid questions and postulate insane methods to try and answer them...
Feeling like you need to offer *correction* to people who reply to your newsgroup posting when they offend some etiquette guideline is just showing that you are too focused on the format of the message rather than the content... Chill out...
I didn't know that in addition to being an expert on accounts payable that you were also an internet psychologist -- good to know...
~~ Evan
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You should ask for a refund
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Just for the heck of it go after hours and turn off your main breaker with friends watching building.
I had a customewr at a local shopping mall they had electrical troubles and found a connection from their meter powering public spaces in the mall, seemingly left over from the malls construction in the 60s. Tenants on that meter had been paying a big chunk of the malls electric bill..
This was identified one night when the customer had a fire. The fire department pulled the meter blacking out a big piece of the mall.
The mall claimed no knowledge:( I believe there was a lawsuit,,,,,,
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@bob haller:
That doesn't sound like it was a professionally managed "mall"...
Sounds more like it was being run by people like Home Guy...
All the malls I have seen inside of have entirely separate switchgear to power tenant versus house circuits...
~~ Evan
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They can often get a limited refund from the power company, who will then add it back into the mall's bill. If not, a suit is certainly in order.
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@Home Guy:
You are the one being the UseNet douche here guy...
Commercial electricity is metered based on peak demand I don't care where you are in the US, it is the way federal laws on the buying and selling of energy are written...
That first given established, you totally blew off the source of the *most* qualified electrical advice on this newsgroup supplied by RBM because you didn't like the 'format' of his posting method and so called 'etiquette' violations when your postings actual content is way out in orbit of some other planet...
Now let us address the specific issues you seem to be experiencing:
-- Your electrical meter is locked within a cabinet enclosure where *you* the consumer are unable to observe the cumulative readings on it at various points during the billing period to determine any abnormal usage issues...
That is *highly* abnormal for a meter to be locked inside a cabinet like that where the indicator of the amount of electricity you are going to be billed for is concealed from you... I would place a call to the local public utilities official and explain your situation and that your meter is hidden away from you where you can not read it but once a month when the power company unlocks its cabinet, that install does not sound kosher -- at the very least an observation hole can be made in the cabinet so you can see your meter...
-- Is there only *one* tenant in this "building" as it sounds like there is only one meter... That is an abnormal way to pay for electricity in a commercial building if there are multiple tenants irregardless of whether the lease terms are gross or net (NN) (NNN)...
With one meter all you would be able to do legally without some sort of sub-metering involved (the emon demon that RBM mentioned) is divide the total cost by the square footage of the building and apportion it to the tenants based on the tenant's square footage...
-- You are so caught up in the minutiae of how you can home brew a way to calculate your power use by simply calculating all the wattage of all the devices and appliances used in your occupancy that you seem blissfully unaware that many things require a "starting current" like your furnace motor and the ballasts for those exterior light fixtures on the mechanical timer -- with a commercial electrical service you get billed for the highest simultaneous demand for current as well as the simple kWh of usage...
It sounds to me like you have a lot to learn before you even attempt to dispute anything with anyone...
It is also waaaay to late to dispute charges for electricity billed like over a year ago -- there is usually a time limitation which covers when you can challenge a utility bill, I have never heard of one that let you go more than 60-90 days after the billing date to initiate a complaint...
Good luck man -- hope you can return safely to earth since you are clearly in orbit somewhere with all of this...
~~ Evan
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My first guess would be that the logical explanation for 1915.26 showing up 4 times would be that it is some form of an ESTMATED reading used when they didn't take an actual reading. The odds of that number showing up 4 times from a true reading would be very low. And if it has, I'd suggest using it to play the lottery.
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" snipped-for-privacy@optonline.net" wrote:

A meter reader always pays a visit to the building either on the first or second work-day of the month. He reads both the electric and water meter (both are inside the building and can't be read from the outside). I know that because we have to escort the reader to the utility room, and I'm the one that does it about 1/2 the time, and when someone else does it they tell me they did it.
There is never a month that goes by without someone coming to read the meter.
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Then I'd have a conversation with the electric company and ask them to explain how 4 months in a year can have the exact same usage, down to the hundreth of a kwh. And how there are two other month pairs where the usage is exactly the same down to less than 1 kwh. It's possible but the probablities are clearly very low. Even if you had a truly fixed load that never varied, you'd expect more variation than that due to some months having more days. possible and we could calcu
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I agree the billing usage numbers look very suspicious.. I would contact the power company, bypassing the regular customer service, by calling the chairman's office and working from there. You can usually get that sort of information from the investor information section of the stock listing for the company.
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Jibe.
I know I sound pedantic, but I want you to look your best when complaining to the electric company, should it come to that point.

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Home Guy wrote:

Hi, I am wondering why you are trying to do this? First what is the power factor in this circuit? If your goal is to increase efficiency to result in savings, look into that. If you suspect meter is inaccurate you can request for a replacement. Newer digital meters are more dependable and accurate. Even my cabin located in the boondogs have digital power, natural gas meters. You already know typical power consumption pattern. So what is the purpose?
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harry wrote:

So I shouldn't assume that, say, a bank or three of florescent lights won't necessarily draw a constant amount of current?
Or a dozen PC's?

The neutral is not smaller (physically) than the other 3 cables. It's the same size.
Most charts I see only go as large as AWG guage size OOOO (almost 1/2 inch diameter). In my case, the cables running from the meter to the distribution blocks (a run of about 7 or 8 feet) are at least 1 inch diameter (OD). The conductor diameter is at least 7/8".

So if I don't multiply my VA number by the power factor, then I'm OVER-ESTIMATING my KWh calculation by 5 or 10%.
Tangent:
Why does my utility apply (add) a 5% "correction factor" to the KWh measurement that comes from the meter?
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Home Guy wrote:

Hmmm, You know what? You ought to monitor frequency. Usually it is not 60Hz, the lower it shows power factor is getting worse. Ideally load should be pure resistive which does not exist in real world. They are mostly inductive load. There is such a thing called Pf correcting device to improve efficiency. My SIL owns/operates mechanical moulding business based on CAD/CAM. His average monthly bill is pretty constant. The amount of monthly bill is pretty predictable. Other source of energy his plant use is NG, mainly for heating.
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Home Guy wrote:

You're pushing a very big rock up a hill to nowhere. Your meter will give you little useful information. You need to KNOW the phase. Sticking your finger in your favorite orifice and pulling out a power factor number is just that...a useless number. You don't even want to think about the orifices you find here.
Why do you care? A decision tree is often helpful. If you think the equipment is faulty, you should enlist the power company. I've found 'em to be very knowledgeable and helpful. They have the equipment to determine whether your meter is faulty. A friendly conversation with customer service should get you a call from a real engineer. If it turns out to be faulty, make sure you get calibration numbers off the old system so you can negotiate a refund.
If you think they're intentionally screwing you, you need to hire an electrician with the equipment to measure WATTS. That's what you pay for. Measuring VA is an exercise in futility. Your "finger" ain't gonna hold up in court anyway.
You might be able to get some advice from the local electrical inspector.
If you think one of the tenants is charging their electric car when you're not looking, your amp meter can point you in the right direction.
I've used current clamps into a computer to log AMPS. Useful for determining relative consumption from the same load...in my case it was a water heater. RMS amps is better than peak or average amps, but still not a short path to WATTS...well, in the case of a water heater, it is, but that's a special case of resistive load.
Depending on how the power meter's made, you can get cheap wireless monitors that you might get the utility to let you clamp on the meter...but that will have the same systematic errors that the meter has. But it might help you find any clandestine loads at odd hours.
Newer digital readout meters have an infrared light that blinks in concert with the load. Mine is one blink per watt-hour. I programmed a pda to read out and log consumption in real time. Again, for my relative use. It has same systematic errors as the meter. But you still gotta be able to "see" the meter...or put a fiber optic cable to the outside.
You can buy clamp-on current transformers that also hook to the volts and measure REAL power accurately. But it's much easier to pick up the phone and have the power company help you.
Did I mention...rock..hill...futility?
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mike used improper usenet message composition style by unnecessarily full-quoting:

I should not see a huge spike in monthly usage during a month when our hvac usage is practically zero. Investigating the reasons for this spike is not path to nowhere.

An unnecessarily dramatic statement.
To say that a clamp-on amp meter can't give useful information is hyperbole.

To the extent that my aggregate power factor is less than .95 or .9, yes, then I need to know the phase.
Are you suggesting that my effective power factor is likely to be less than .9?
What is the power factor of 10 to 20 year-old florescent lamp ballasts? Or a 1 hp, 220 VAC fan motor? Or a 10 year old refridgerator? Or a typical desktop PC power supply?
Those are the largest (and probably only) non-resistive loads in question here.

Because I pay the bills. What a stupid ass question that was.

I've already stated that I've contacted them, and that I expect to encounter difficulty in having them ever admit that their metering equipment could be faulty or even undertake a process to evaluate the meter, but I will pursue every course of action and give them every chance to determine that.

In the pages and pages of materials and contracts that exist for this utility, describing all manner of service obligation and liability, billing, etc, I find nothing in print that defines a process whereby a billing meter is tested or what is done if a meter is found to be defective.
There is absolutely nothing I can find in writing even contemplating the possibility of a meter that does not measure correctly.
I believe that issue is a political "hot potatoe" for all municipal electricity suppliers, something they'd rather not have to deal with and hence they largely remain silent about it.

I believe that they never "intentionally" screw anyone, but that instead they put up a front that their meters are always correct, all the time, and reinforce that by not mentioning the possibility of erroneous meter operation anywhere in any printed material they make available, let alone define in writing a process or methods to test a meter that the client believes is suspect.

The worst I can do by measuring VA is to OVER-ESTIMATE my watts used by 5 or 10% - unless you think it's likely that my aggregate power factor is less than 90%.

Making my own measurements would be a first-step. I never said I'd use those measurement in court (that is your hyperbole again).
If indeed it got that far, then I would investigate my options have having an acredited third-party measurement performed, and that would only happen if my local utility did not perform their own tests that I was satisfied was unbiased and accurate.
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@Home Guy:
Ah, the true picture emerges -- an accounts payable rep who thinks that because they pay the bills they understand how everything works...
You seem to not understand the regulations which protect consumers and control how power is sold in your state -- therefore you are unaware of your potential remedies in this "situation" if one really exists...
Instead of spending your time researching something actually useful which might shed some light on what is actually going on (if anything really is at all) you have chosen to ask stupid questions which are clearly not on the proper wavelength to make any sense to someone who actually understands electrical issues AND you are chasing after something *YOU* can do which would support *YOUR* claim that your electrical meter is not functioning correctly when there may in fact be a procedure to follow which has already been defined by the public utilities commission (or equivalent in your state) which would almost always involve bringing in an uninterested third party with the proper credentials and equipment to assess what if anything is happening in this whole convoluted story...
It seems whenever you get some sound advise that would make sense in the real world, you attack the contributor because the person didn't respond with the specific answer you were looking for in your especially preferred format... So you critique based on newsgroup etiquette and posting format rather than the supplied content -- keep doing that and you will be properly labeled as a troll and written off as such...
~~ Evan
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Hmmm, BOTTOM LINE>>>>every single dime. Sounds like OP is Scrooge, will be only happy when he gets free power. His building may be 100 years old containing industrial revolution era stuffs. Proper course of action would cost \$\$\$ which is not in his book. My take on this thread.
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Home Guy wrote:

Investigating is good. The method you're using is the rock to nowhere. Get a tool that can do the job...and he works for the power company.

Yes, I am. If you have a bunch of CFL lights and nothing else running, your aggregate PF can be 0.6. During the day, when the lights are off and motors are running, it might be 0.6 in the other direction. The power meter only cares about what's happening NOW. There's a lot of legislation in place or on the way to make NEW stuff do internal power factor correction. But it's gonna take a while to make a difference. Power factor is a mythical number that assumes that voltage and current are both perfect sine waves that are out of phase. Take a look at the load from your computer. You might find that it's a bunch of narrow spikes that bear no resemblance to sine waves. A "kill a watt" meter will give you a power factor number, but the crest factor may be WAY bigger than 1.
The only number that makes ANY difference is the one after the \$ on your bill.
Quit messing around and get the power company out to look at it. Show them the evidence you're bitching about here. Only they can do anything about it.

What's on the paper is inconsequential until you get into a court of law. Fret over that when it happens. CALL THE POWER COMPANY...you don't appear to have the skills or equipment to make a challenge.

It's my assertion that GETTING THE POWER COMPANY TO INVESTIGATE is the first step. The second step is to call whatever agency regulates the power company. Attempting to measure it yourself is way down the list. You ain't got the equipment to prove 'em wrong.

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