RESOLVED - IoTaWatt kWh reading much lower than power company reading

Hi Guys,

I have a three phase derived setup here with my IoTaWatt, and when I compare the cumulative kWh readings in IoTa, they are much lower than what I see on my utility meter. I have checked all three phases are correct, when I change phase B or C to a different phase the reading drops down quite a bit, and even goes negative, so I am sure those are set correctly. I am using the PowerTech MP-3027 A/C adaptor, which is configured correctly in the IoTa setup, and the CTs are the 100A 16mm units. I plan to install the IoTa with a full three phase reference system using three reference transformers, but that will be down down the track.

To troubleshoot further, I will borrow my eletrician mates RMS meter with it’s own CT clamp, and compare the current readings between IoTa and the Fluke meter, then verify the voltage readings in IoTa match the Fluke as well.

I haven’t had the IoTa for a full billing cycle yet, which is three months, so I will confirm it once I have enough data to compare, but in the meantime, is there anything obvious that I should check? Rough calculations show the IoTa is reading somewhere around half the kWh of the utility meters (780kWh vs ~1500kWh)


Hi Matt,

Could you post a picture of your board showing the mains CTs, and also your inputs configuration, a picture of the front of the board, and a screenshot of the status display?

Hi Bob,

Thanks for the quick reply!

CTs are temporarily installed via some brickwork that is inprogress - these will be moved to the rear of the panel at a later date:-

Metering panel shot:-

Input config:-

Status screen:-



Here’s a screenshot of my DSO when I was working out the phases. I connected the CTs directly with no ballast resistor (as I was only interested in the relative phases) and the 9VAC output as well. The yellow trace is the A phase, the blue trace at the bottom is the B phase and the magenta trace second from the bottom is the C phase. Obviously the sine wave is the 9VAC output from the Powertech MP3027.

I’ve been going over the documentation and racking my brain trying to work out what I have missed…

…and I have just learned (after some more reading up) that running CTs without the ballast resistor is very bad. I guess I may have damaged the CTs when I had them plugged into the DSO. On the positive side, if they are damaged, I guess they are realtively cheap to replace.

That would be why they are outputting a square wave, as the TVS diode is clamping the output…


The square wave shows that the TVS diodes are working properly. There should be no damage.

The scope approach is interesting, but I’m not real comfortable with what I see in the status display. The power factors on A and B are pretty low. Quite possible given the small loads, but nevertheless suspicious.

If you want to try swapping A and B, inputs 1 and 2 at the IoTaWatt, we may get lucky. That’s so easy to do and you can post the status display.

I’d hold off on that. The numbers you quote are way off. It is not caused by phase-phase voltage variation.

I see that there is a sub-board. All I’ve got to go by are the pictures, so I have to ask, are you sure those cables are your incoming mains? I saw that you have a sub-board. What happens if you turn off the feed to the sub-board? Does it affect your mains reading? Does it stay the same, get smaller or go to zero?

Me too. Being a New England Yankee, I have no direct experience with Australian “boards”. What I’ve picked up over the past couple of years, from doing consults like this, is that there is a wide variety of equipment and connection variation. To that end, looking at the fromt of your board, I see three different boxes at the bottom. All of these are sealed and labelled with stickers “integral energy” which research indicates is the prior name of Endeavour Energy, the power utility.

Do you have solar? The central meter is an EM1000, which research indicates is a bi-directional meter used to measure import/export in a solar installation. What is your rate structure? I’ve never seen true net-metering in Australia. Typically there are separate import and export tariffs. In other words, you pay one rate for power that you import, and are reimbursed (or not) at another rate for power that you export. The IoTaWatt, as installed, is reporting the net power used. It may well be that if you have solar, your bill is indicating the total power imported, which can be substantially more than net.

I’ve had some in Australia say that they get paid nothing for exported power. As a result they have adopted various strategies involving diverters and batteries to store excess energy rather than export.

So do you have solar, and if so, what can you tell me about the export tariff?

Hi Bob,

Status before swapping inputs 1 and 2:-


After swapping 1 and 2:-


I am 100% certain those are the incoming mains cables. They were installed 6 months ago and I was there helping the electrician pull them through when they were installed. The subpanel is run off a much smaller conductor (10mm^2) and it is all contained in the one, orange coloured PVC jacket. I can’t add CTs to that as it requires an electrician to access the cable at the rear of the panel and to remove the outer jacket that contains the five inner conductors (3 phases, neutral and earth)

If I turn the feed off to the subpanel, the mains reading drops by half of what my pool pump is consuming, as that’s the only thing currently running on the subpanel - which is in line with the IoTaWatt reading everything approximately half the actual value. The only other things on the subpanel are two A/C units that are switched off.

It definitely varies quite a bit over here, as the history of our power infrastructure is long and complex - probably a similar story to the US.

Definitely no solar here yet (I wish), and that meter in the middle is for the off-peak hot water system that is controlled by the ripple receiver to it’s left. I suspect the EM1000 was used as that’s all they had on hand and figured it would work fine in a single direction application.

I actually have 2 more IoTaWatt units new in the box, and I will swap one in today and test it out. One thing I want to try is a full reset on the IoTaWatt that is currently installed.

Interestingly, when a pressure washer was being used yesterday, I noticed that it was drawing - you guessed it - a little under half it’s rated power too. It’s an 1800 watt unit, pure on/off operation (no high medium low or anything) and the power draw was under 900 watts IIRC.

Hi again Bob,

I just plugged in my 2nd IoTaWatt unit, reversed inputs 1 and 2 back to the way they originally were and here is the status:-


It seems to be back to square one - reading identically to the first unit before I swapped inputs 1 and 2.


OK Matt, no solar, so that rules out a difference between import and net. That’s progress.

Swapping the two lower pf CTs doesn’t really help, so nothing there.

The new information is that in addition to the Wh being about half, individual appliances are coming in at about half. Now I see two possibilities:

First, the CTs may be defective. Very unlikely.

Second, the phase assignments are wrong. More likely, and a typical symptom of this is readings that are half what is expected. I recently published a newer instruction for assigning the mains phases. Can try going through the procedure to validate your phase assignments? Then, if anything changes, retry those appliances and see if it reads more in line with the nameplate?


I have just been through this (in truth stll going through it !)

I 2nd Bobs recent instructions in the Blog about how to get your system calibrated.

The other thing that is also worth doing so you have faith in the CTs is to get an extension lead and cut off about 6 inches of the outer sheath - plug a hair dryer into it and put the CT onto the brown wire.

This will let you test each of the CTs for the expected outtput.

ALso if you are going with 3 IOTAWATTS over time it is probably worthwhile getting a clamp meter (have a look on whirlpool in the electricity forums there were some links recently to some cheapish ones on ebay) and then finally i would also purchase a cheap Killawatt meter etc from Jaycar

I used one of these with the extension lead and a hair dryer to go through all of my CTs and calibrate them all before i bit the bullet and removed the burden resistors.


I want to point out that @craigcurtin had third party voltage typr CTs that he field modified and subsequently checked with a calibration procedure. 99% of you will have AccuCTs from the stuff shop. They are highest quality 0.5% CTs and require no calibration.

Yep - i was more pointing out to him that he could have ultimate faith they were orented correctly and also not damaged by doing this.


Prospect County Council - got my first job here as a cadet engineer when I left high school.

Looks like a standard 3 phase board thats been upgraded a few times.

I tend to agree here, plus the other thing that is throwing me off is that I don’t really know which one of the four cables is neutral. I think I’ll just wait until I can get a sparky to come and open up the panel, so I can be 100% sure that the CTs are installed on the right cables. Then I’ll know for sure that part is ruled out.

We IoTaWatt works so it has to be down to something specific in my installation that hasn’t been set up correctly.

We bought the house a few years ago now, and it was a dual phase installation with the original porcelain fuses etc. The panel itself probably contains asbestos given it’s vintage. Earlier this year, I added about 20kw worth of reverse cycle aircon, upgraded to three phase (63A per phase max) and had RCBOs for added safety.

Interestingly, it seems that Prospect County Council covered a huge part of Sydney back in the day. I’ve seen many panels made by them over the past 40 years or so.

Well, I think I have worked it out. When I install the CTs with the arrow going in the same direction as the power, they are actually reversed!

The phases were all completely wrong as a result…

Physically reversing the CTs, and then going through the processes above seems to have sorted it out. The power readings are what I expect, and the power factor is now reading far more sensible values of around 0.80.

I’ll keep and eye on it and compare to my utility meter when I have some more data to work with. Also, I reset my history with the following commands, as the existing data was totally suspect.



Sounds like progress. Identifying the neutral is important. If one of your CTs is on it, you should see it change as you switch your load on another line, although probably not an equal change. If you don’t have a CT on the neutral, switching a single phase load should not cause any change on the other two.

To properly erase history, use:



After 24 hours or so, it all looks to be spot on now. I only bought three 100A CTs so there is nothing on the neutral now. I identified the neutral with a volt stick, since it’s effectively bonded to ground in several places, it shouldn’t have any potential difference on it.

Given that I am using a factory built IoTaWatt with the factory sensors, I am confused as to why the arrow on the CT is backwards - surely I am not the only person that has been confused by this one? Maybe something could be added to the documentation outlining that they need to be reversed?

Also, thanks for creating and supporting such a great open source product. Kudos to you Bob!


I’m glad to see this one resolved.

It’s alternating current, so even though you may conceptualize the power flow into your house, and think the arrow on the CT should agree with that, the current is alternating evenly in both directions. It is only when you observe it relative to the voltage reference that it takes on the characteristic of power flow in one direction or the other, depending whether the current is in phase with the voltage or opposite.

Your Aussie plugs are polarized, and that’s great, but the plug on the US VT and the Euro VT are not. If you were using one of those VTs and reversed it in the socket, it would have the effect of reversing all your CTs.

So it’s all relative. To complicate it further, there is no standard for how the polarity of the plug should relate to the barrel and pin of the plug (which is a DC plug). I have a sample of each of the Aussie VTs that I recommend - AC910 and MP3027 - are exactly opposite. In other words, your CTs will point in one direction with one and the opposite with the other. In the US or Europe, you can just reverse the VT in the socket. You could have simply checked reverse for your VT.

There is likewise no standard for the how the CTs should be wired to the 3.5mm jack. The YHDC and Echun work one way, and the AccuCTs work the opposite.

So that’s why the procedure that I referenced says:

Complicated as the above explanation is, it’s as simple as that.

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