Well I have to admit that I'm stumped with an energy audit I've been working on for the past two months. Perhaps someone can point me in the right direction...

My client contacted me concerned that his electric bills were atrocious. They were, to the tune of 34,680 kWh per year (average monthly at 2,890 kWh, and a baseline of 2,347 per month!). And after my first visit to his home, it was very obvious to me that, although he has a big house (3700 sq-ft), he and his family of four were energy frugal, being careful to shut off lights when not in use, using power strips for entertainment centers, etc.

He was particularly concerned about the phantom consumption when he and his family would leave for several weeks at a time. When they did he'd shut off everything but the alarm system. And even then he'd see a bill of 1900 kWh for that month, and for what?

So I dove in deep, taking this on as a personal project, since he'd had an auditor from the utility company come out previously and they couldn't find anything. One of the first things I did was to install a PowerSave EnviR monitor on the two mains into the main circuit panel and let that thing collect data for exactly two weeks. I compared this kWh total to what the utility company's meter read and found a discrepancy of about 500 kWh in favor of the utility company. This closely paralleled the additional 10k kWh per year that I could not account for in my projected analysis. Hmmm.... found it! (or so I thought)

So I got on the phone and was soon in touch with the supervisor of the electric meter department. I told him my findings so he and I got together at the client's home one day so that he could test their equipment, which consisted of a transformer/transducer system which reduces the incoming current from the main transformer at the street. After a good hour of testing he informed me that their equipment tested okay. Hmmmm.... bummer.

After many hours of monitoring individual circuits, extrapolating the long-term consumption, I still can't account for this additional 10k kWh per year anomaly. I even looked for hidden conduits coming off the wiring gutter under the transformer, assuming a neighbor might be "borrowing" some power for their growing operation. Nothing. Absolutely nothing.

And now that I've been so thorough with my investigation and extrapolation, I'm left scratching my head, wondering if, even though the utility company tested their equipment as good, perhaps it is not over a long term. I honestly don't know where to go from here... Suggestions?

Tags: accuracy, meter, utility

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It seems like you need to do some measuring at the same spots that the meter is doing it. A pair of current clamps close to their transducers is what I would use, with a pair of multimeters, or if you have a power monitor you like, connect the transducers next to theirs. Turn off all the breakers in the house except one, and connect a known load to that, like a 1000-watt electric heater. Confirm the load and accumulated usage with a separate meter like a Kill-A-Watt. 

Once you have a known load in place, sit there and watch for a while. The meter should tally up kilowatts at the same exact rate as your equipment, and of course as Bud says, you want to confirm that there's absolutely nothing happening if the panels in the house are completely off.

If you're using a 120-volt device as your known load, I would repeat the testing with the load on the other leg if the panel.

Unfortunately, you need to doubt the accuracy of your monitoring rig, despite whatever the manufacturer says. Unless you have very high quality equipment that is well calibrated and beyond any doubt, you need to double-check its readings with other equipment. 

Obviously it's easiest to do this when the owner is not there to experience the inconvenience. You HAVE to  physically shut off all the breakers and make sure that nothing else is connected to the one circuit you are using.

Excellent advice, David. I will do exactly as you described.

I'm on hold for now though while I wait for them to get back from vacation and while I take my three weeks of vacation starting July 17th.

Perhaps by September I'll figure this thing out... :p

Those transformers/transducers are typically swapped out as part of the meter assembly. When a meter is pulled all that's left in the box is 4 lugs for the meter, no electronics.

Not in this case, Bob. The transducer donuts are around the mains. From those transducers are wires running into a transformer. From the transformer, wires run into the back of the meter base. The meter is not your typical 4 lug type. Hence, my mistrust of this sort of system.

The house's incoming wiring is the result of over-sizing based on projected NEC loads. The reason for the transducer/transformer situation is that the disconnect (and associated feed into the house) is sized for (I think, if memory serves) 300 amp service. Typical residential meters are sized for up to (I think) 200 amps. In this type of situation it's necessary for the lower voltage transducer/transformer which essentially reports a fraction of the actual current being consumed and a multiplier (in this case x40) is applied to the reading to get the "actual". It's the "actual" that I'm questioning for accuracy...

It's also important to note that the meters have been swapped out. However, the transducers/transformers were not, although they did test this equipment. Not sure how they did that though.

Sounds like it's a commercial setup of some type, residential has a typical limit of 200A. If the meter is pulled does it disconnect power from the home? It's possible but unlikely that the "donuts" may have been incorrectly sized for the application. Have you done any research online about the "donut system"? I've not seen anything like it on our utility, but we use smartmeters.

The box that houses the meter (and all the other guts) has a bypass switch that routes around the meter, so you can actually yank the meter and power continues to the breaker panel in the home.

As for the donut system in use... when I was onsite with the utility guy (and I don't mean to villianize the guy, he was actually a great human), he explained that he was the guy in charge (many years back) of determining what make/model of transducer/transformer PNM would be using. He said he put two months of intensive R&D into finding the most reliable source and his testing included attempted sabotage to the units and every other test you can think of. The unit that is in place at my client's residence is one of those units he determined was "the best".

Another thought: Since it appears your customer has a commercial power meter (I've never seen current transformers/donuts/transducers in residential) is it possible that they are on a commercial rate schedule? Commercial rates are much higher then residential for most utilities. Of course this doesn't explain the KWH discrepancy. Since you are thinking service is oversized, have you considered downsizing to a 200A meter and swapping out the main breaker? 300A service is most likely 2 panels. I don't recall ever seeing any single phase panel over 200A, even in commercial.

Yep, we're talking kWh, not $ here, so the rate is irrelevant.

We did discuss a changeover of service/meter/disconnect with the utility guy, but after he explained the process in detail (engineering, approvals, contracting, etc), it appeared it might cost the homeowner up to $8k, which he was certainly opposed to.

Same with me, I don't recall ever seeing a single phase panel over 200A. It's possible this is indeed a 200A service and not 300A. I'll call the utility guy on Monday to verify what it is and why he thinks it was originally necessary.

Thanks for the input!

I'm thinking 2 breaker boxes, especially since a commercial meter is installed. I've seen 2 panels @ 200A each connected to 400A service. 2 panels @ 150A each doesn't make sense since a 200A panel isn't that much more expensive than a 150A panel. 2 panels are common in commercial and larger 100% electric homes. 2 heat pumps with 20KW each of backup heat (not uncommon on 100% electric homes over 2500sqft) will put a house in the double panel category. Since a "4 lug" residential meter is limited to 200A, it makes sense. Bypass switches are normally a commercial thing, but they are typically locked by the utility company.

I would agree with BobB. I have a number of large clients that have heat pumps with electric backup and have a total of three 200A panels. Look for another one in the attic.

Hmm, every house has its 'gremlins,' but maybe your clients have: A Hider in the House . It's a mystery. Seriously, I quizzed my utility co. on your problem, & rcvd this info back:

Energy Meters are installed on homes to allow the energy provider to know how much energy was used and produce a bill for the correct amount. Most utilities send someone to the home monthly to read the meter. Systems that automatically read the meter and send the information are increasing in popularity because, although the metering and communication equipment may be more expensive, it saves on the cost of sending someone on-site to read the meter.

For homeowners, reading an energy meter is a good way to become familiar with how much energy a home uses. For example, when trying to decide between two energy "alternatives", like:

"Should I close the windows and run the air conditioner around-the-clock?" or
"Should I open the house at night and air condition only during the hottest portion of the day?"

...reading the electric meter can provide the answer.

Begin the experiment by reading the meter and trying it one way for a few days, and reading the meter again. Then, try it the other way for the same time period and read the meter again. If most other variables were the same during the time periods (e.g., weather and use of the home), one should have the answer to which is the most economical.

Another way to use meter reading is to see how much energy a certain appliance is using. Suppose you wanted to know how much energy your refrigerator uses and the nameplate data telling its wattage is in a hard-to-read place. You could turn off everything else in the house, then take two meter readings an hour apart to find out how many kilowatts it uses in one hour. Or do it for half an hour and double the reading to scale it up to a full hour, or even 15 minutes and multiply the reading by 4.



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