Reported wattage is WAY too high? - solved

Hi community,

I have just got my board up and running and I am getting some weird results that is probably some user error.

I’m in Australia so its 240v. I have a 9vAC pack from

The voltage ref has been calibrated with a true RMS meter and is holding the correct voltage.

However I am just getting the wrong wattage results.

Here is my config

Here are my results

killawatt    IoTaWatt
0.0          0          (Sometimes -2)
0.2          0          (LED globe)
1.9          5          (LED globe)
5.8          12         (LED globe) Pf:0.290095
26.1         54         (incandescent bulb) showing Pf:0.852915
1923         4820       (kettle) Pf:0.999775

Is it user error? Does anyone see something obvious?


You definitely have different results. Not sure what is wrong or right. Measuring 5 watts, at 240V, with a CT that has a range of 24,000 watts, may not be realistic. So I don’t get too excited about the first three or four in your list.

Let’s look at the last two. What is the wattage of that incandescent bulb and what is the nameplate wattage of the kettle?

Where did you get your CTs and are they really SCT013-000 or are they one of the voltage type SCT013? Could you post a picture of one of them that shows the model number please?

Both an incandescent bulb and a kettle should have a power factor of 1.00. You are showing some5hing much less, presumably from the killawatt because IoTaWatt doesn’t display it to 6 decimal places.

Lastly, do you have three phase power?

I brought another meter into the equation just for reference

Kettle (1850-2200w)
4762w Pf:0.999684 - IoTaWatt
1730w - EnviR
1873w - Killawatt

25w Bulb
56w Pf:0.806151
25.1w - Killawatt
24w – EnviR

I got my CT from ebay… (yer I know) but I paid extra to hopefully get the genuine one.

Yes I assumed the globe should have a pf of 1.00 which is why I used it and included the results) so something weird there.

I do have 3 phase however the entire house runs on one phase (other phases were for a pump which is long gone). All devices are using same power outlet and are on the same phase.

I got the SCT013 as I see it is what you were recommending. What CT would you recommend for around 20A? Would like a higher resolution of wattage.

Thank you for your time.

V[quote=“dotnetdan, post:3, topic:455”]
Kettle (1850-2200w)
4762w Pf:0.999684 - IoTaWatt
1730w - EnviR
1873w - Killawatt

25w Bulb
56w Pf:0.806151
25.1w - Killawatt
24w – EnviR

I don’t understand where you are getting these PF from. This implies they are from IoTaWatt, yet IoTaWatt doesn’t output PF to 6 decimal places. So there are some details missing. Could you post some pictures that illustrate how you are setting up this experiment?

Where is the CT located and does it go back to zero when you remove the bulb or kettle?

Can you post the config.txt file?

Do you have more than 1 CT and if so, can you clamp several of them to the same cable and show the status display with all of them reading the kettle load?

I am getting the PF from your rest endpoint as it was not showing up on the status page.
(Love this endpoint BTW!)
Would love it if you could add a decimal point to the watts also :blush:

The CT is on the Active just before the bulb

I only have one CT now. (I should have got more) I will purchase some more.

It is starting to sound like It may not be a config issue. The board was made by me however I followed all your info and all resisters are 0.5% (

The fact that I am getting the correct Hz and correct power figure, I thought this was a config issue.

Again thanks for your time

config.txt (1.4 KB)

Ok, now I get it. This is a homemade unit. Now I see you hinted at that (got my board running) in the first post, but I missed that. I’ve changed the category of this thread to home brew Iotawatt.

So now this opens up a host of new possibilities, but let’s see if I can nail it on the first try. What value voltage reference shunt did you use?

If your answer is 1V, that’s your problem. But don’t fret. Just add the json object: “refvolts” : 1.0
To the device json object in the config.txt file.

config.txt (1.4 KB)

1 Like

You sir, hit the nail on the head…

Thank you very much for your help and this fantastic project.

I would like to buy you a beer

That won’t get your numbers to absolutely agree with those plug meters. They don’t use transformers. As I understand it the kill-a-watt uses a shunt to measure current and a voltage divider to get voltage. That’s a good approach for a relatively low wattage device like that, and is probably more accurate in the 5-10 Watt range. AT a minimum, you should get some 800 or 1000 turns CTs if you want to try to match those things.

Another thing I noticed is that you are using the USB output of the power strip to power the IoTaWatt. That’s not a good idea. Those things are meant for battery charging and typically have poor regulation and ripple control. A raspberry-pi power supply or any of the CUI 5V USB supplies will give better results. You had mentioned getting a -2V spurious reading with no load, and that is characteristic of ripple in the power supply.

Otherwise, nice job with the board. Takes a steady hand to populate that SMT.


I knew that it should be a better representation than the plugin meters. The IoTaWatt got close but matched the bulb wattage closer.
I then moved it onto my grid solar unit which is 100% correct. The IoTaWatt was about 1% off which I am really happy about.

I know about the power supply. I thought it would only effect the reliability of the unit and not so much the readings. I will get a Pi supply going forward.

Can you recommend an ebay available CT that has at least 800 turns and is good for ~20A?

Look at the tables.txt file. If you are willing to go with solid core and add your own cord, the HWCT004 od DL-CT08CL02 are fine. Stay away from the YHDC SCT006. I use echun CTs which are excellent. I haven’t seen them on ebay, but you can deal with them directly. I believe the contact for web sales is Vicki. For split core I use the ECS1050 and for solid core I use the ECOL09.

There’s no getting away from the fact that your accuracy is only as good as the CTs and with a 14 channel monitor, you are going to have to pony up.

4 posts were split to a new topic: Question about ECOL09 CTs