Author Topic: Discrepancy between MTU readings and meter readings  (Read 3808 times)

falcon

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Discrepancy between MTU readings and meter readings
« on: May 21, 2015, 01:42:46 PM »
I just got my TED Pro Home installed on my main panel, and I've noticed something strange. I have another energy meter that's measuring the IR pulse on the top of my Itron meter that blinks every time 1 Ws passes through the meter. Adding that up over an hour gets you the total watts used in that hour. Comparing that to the export data from the TED unit is showing me a difference of something like 25% under that number. This tells me one of two things:

1) The calibration on my TED unit is off by WAY more than the +/- 2% in the specifications.
2) My meter is over-counting.

Now, unfortunately, the meter is what I get billed by, so trying to convince my POCO that their meters (two of them, since I just got moved to a net meter) are off is fighting a very large uphill battle. Also difficult is that I don't have another meter that can measure true power. According to the TED unit my house PF fluctuates between 85-95%.

I did shut off practically everything in the house (except a stray 20W, part of which is the ECC itself), and then plugged in a 375W load as measured by a Kill-A-Watt. The TED's reporting only jumped by 355W, which tells me that it's off by at least 6% right there (assuming I can trust the Kill-A-Watt), but that's still a far cry from the whole house's 25%. Could it get worse off the more current it's trying to measure?

I have checked to make sure both current clamps are secure (they barely fit around 4/0 aluminum wire, by the way), and pointing in the right direction. Is there anything else I should check, or should I just concede they're off and adjust the MTU's kW reading by 1.25?

pfletch101

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Re: Discrepancy between MTU readings and meter readings
« Reply #1 on: May 21, 2015, 05:57:58 PM »
If you are seeing a discrepancy as big as you describe, something is wrong, and calibrating your TED to 'correct' the error is probably not the best response. It is also unlikely (though not totally impossible) that your utility meter is defective. Probably the best way of checking the TED unit is to temporarily move its MTUs to a branch circuit on which you can put a series of known, non-reactive, loads (banks of lights or electric heater) and compare its readings to reality and/or to the simultaneous readings of your Kill-a-Watt.
Peter R. Fletcher
TED Pro Home - main MTUs monitoring utility and PV Solar feeds; 2 Spyders monitoring selected individual circuits

RussellH

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Re: Discrepancy between MTU readings and meter readings
« Reply #2 on: May 21, 2015, 09:39:33 PM »
How is the unit installed in your main panel?  What is the voltage in the main panel?  120V/240V or 120V/208V?

falcon

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Re: Discrepancy between MTU readings and meter readings
« Reply #3 on: May 21, 2015, 11:56:44 PM »
The unit is installed in my main service disconnect with the clamps on the two hot feed-through lugs going to my main breaker panel.  It's single phase 120/240V. The MTU is powered with the black to phase A and the white to neutral, exactly as the manual directs. The only breaker between the MTU and the 200A main disconnect is for my solar installation, and since I was doing this testing at night with the AC disconnect for them thrown, they couldn't contribute to anything.

I'll see about moving the clamps to a branch circuit. I'm really hesitant about having to pull them back off those 4/0 wires, given that it took me half an hour to get it clamped around them last time. I suppose what I could do is just shut the entire house off again and only plug a space heater and the ECC in. It's similar to what I did with the 375W load last night, but that load was more reactive (the UPS to my server rack).

jfpetesn

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Re: Discrepancy between MTU readings and meter readings
« Reply #4 on: May 22, 2015, 06:55:42 AM »
One way to get a rough idea is to check it against your solar inverter readings.  I believe that most inverters are calibrated to +/- 5%.  If your TED is coming up with the same kWh for the day as your inverter is measuring then it is close to accurate.  You could switch your solar and load MTU's and see what you get for a day.  That would rule out the MTU.  Then you could switch the current coils and see if the readings vary.  I think the odds of both MTU's and/or current coils being bad are pretty low. 

I have ended up calibrating my MTU's to my utility meter as that is what I am billed /credited for with net metering.  It has taken a month or so to get my MTU's calibrated correctly.  I have my house (load) MTU at +1.020 and my solar MTU at +1.022 and I am now tracking my utility meter to +/- 2kWh per month.  That is about as close as I can get it.  This required many hourly and daily meter reads to get it this close.

I started by reading my meter each hour and comparing it to my TED hourly exports.  Made fine adjustment then when I was close I extended my meter reads to 6 hours or so and made more fine adjustments until I was close.  Then I went out 2 weeks at a time and so on.  I built a spreadsheet and transferred my data exports to it so it did the math for me.  It was very frustrating at times but I now have it spot on.  Keeps the utility honest too.

I would start with the Inverter readings to get in the ball park.  If nothing else, put both MTU's and coils on the solar and calibrate them to the inverter readings to start.  Then take one off and put it back on the house mains.

falcon

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Re: Discrepancy between MTU readings and meter readings
« Reply #5 on: May 22, 2015, 07:27:26 AM »
I haven't bought the solar MTU yet, otherwise that's probably what I'd do. It'd be much easier to remove the clamps from those #6 wires. :)

RussellH

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Re: Discrepancy between MTU readings and meter readings
« Reply #6 on: May 22, 2015, 09:07:38 PM »
I'm really hesitant about having to pull them back off those 4/0 wires, given that it took me half an hour to get it clamped around them last time.
If those clamps didn't close completely, that would explain a lot.

falcon

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Re: Discrepancy between MTU readings and meter readings
« Reply #7 on: May 23, 2015, 12:27:44 AM »
The clips are around the tab, so I certainly hope they're closed completely. Honestly, if I could, I'd get a 400A MTU/CT add-on set and use the one I have for my solar side, but TED doesn't sell one like that, so I end up wasting money buying a 200A set and 2 400A clamps. I'd say the 200A clamps are more suited to 3/0 cable max.

falcon

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Re: Discrepancy between MTU readings and meter readings
« Reply #8 on: May 23, 2015, 02:40:32 AM »
So, I moved it to my solar panel side and only had to apply 1.008 adjustment on it to be pretty much dead-on with what my inverter readout says. So, I'm down to 2 options:

1) My meter is poorly calibrated
2) The 200A clamps aren't working right with my 4/0 ALU wire because it's such a tight fit.

I can factor out #2 if I get a set of 400A clamps. Maybe I can call TED and see if they'll sell me an MTU with 400A clamps for slightly more than the 200A set. If it's STILL off, I'll be calling my POCO to have them come out and explain why their meter is reading 25% higher than it should be.

jfpetesn

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Re: Discrepancy between MTU readings and meter readings
« Reply #9 on: May 24, 2015, 09:14:58 AM »
I think you are on track.  There is definitely an issue.  I would call tech support and see if they can shed some light on the issue.  It is hard to believe either is the cause but it has to be one or the other.

RussellH

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Re: Discrepancy between MTU readings and meter readings
« Reply #10 on: May 25, 2015, 12:18:43 AM »
ANY gap in the closing of the clamps will affect the reading.  So if clamps aren't loose on the wire after they are in place, then it's likely the clamp is resting on the wire and not closing all the way.

falcon

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Re: Discrepancy between MTU readings and meter readings
« Reply #11 on: May 25, 2015, 01:36:30 AM »
Oh, it's definitely resting on the wire, since it actually marked the insulation. Guess they're not made for all 4/0 wire. I'll call on Tuesday and see what they can do. These work great for my solar side, at least. :)

falcon

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Re: Discrepancy between MTU readings and meter readings
« Reply #12 on: June 03, 2015, 04:39:54 AM »
Yeah, it definitely looks like it was the clamps. The new 400A clamps are working like a charm. Now I have generation and usage rates. Now to figure out what's using the power (although I don't know why I bother since I'm over-generating by ~25kW per day).