Would almost certainly need to spend a small fortune to find something with greater accuracy

Cool. Thanks for sharing! My guess is it’s an ADE7754 or ADE7758.

Here’s another:
Available in four variants from 137 to 325 USD, depending on features.

From the datasheet:

• Utility Revenue Grade Accuracy - IEC 62053-22 0.5s Class / ANSI C12.20 0.5 Class
• Multiple CT Input Options compatible with any CT: 5A/1A, 333mV, Flexible Rogowski Coil, 80/100/200mA
• 4 Channel current input including neutral current measurement
• Residual current measurement available
• 10-690Vac direct voltage input, fits all voltage rating system with one model
• RS-485 port built-in with Modbus-RTU, standard integration with most systems
• Standard Din-rail mount for ease-of-installation
• Compatible with both 50Hz and 60Hz systems
• Built-in energy pulse output and alarm output
• Optional relay output for alarm and remote control
• Tamper-proof design approved for revenue applications
• Supports three phase three wire (3P3W), three phase four wire (3P4W), single phase three
wire (1P3W two element), single phase two wire (1P2W one element) systems.
• measures neutral current with 4 th CT and provide residual current measurement.

Sorry for bumping such an old thread, but my question seems most relevant here. Would a device like the Elkor WattsOn be capable of doing something similar as the EmonTx? I mean, would I need a single WattsOn device or would I need at least 2 for a single-phase solar equipped system? I haven’t seen a specific configuration app note for this kind of usage.

Hi kobuki,

The WattsOn is capable of everything the emonTx can do, plus much more.

From the manufacturer’s product page:

WattsOn provides comprehensive per phase (as well as cumulative) information, including Volts, Amps, Real Power, Reactive Power, Apparent Power, Watt-hours, VAR-hours, VA-hours, Power Factor and Frequency.

You’d need only one. The WattsOn is capable of three-phase, as well as split-phase operation.

Here’s the app note that shows how it can be used in the manner you’re asking about:

AN0306-WattsOn_Split_Phase_Inverter.pdf (43.5 KB)

The AN describes a 240 Volt “generator” (e.g. a PV system) across both hot legs of a split-phase system.

If I understand your single phase system correctly, you’d need only two CTs
vice the three shown in the app note diagram.

Thanks, but since it’s not split phase and it’s not 3-phase either so I’m not sure the pdf you linked applies here. Could you please sketch or describe in words how it would need to be hooked up and what settings should be used? We currently have a single phase system with N and L1, and the inverter is also connected to this same phase. One EmonTx CT is now on the main phase wire and the other one is on the wire going to the inverter (one of the standard OEM wirings).

Also, could you recommend CTs that work well with these devices and are of good accuracy, and don’t cost $hundreds? The device itself can indeed be acquired used for very decent prices so I might go this route.


What I was trying to say is the device is 3-p capable,
so using it with a single phase system doesn’t present a problem.

Have you looked at the diagram in the app note I attached to my post?

Of course I’ve looked at the diagrams (not just this one, pretty much all recommended configurations), and I’m already familiar with that wiring, but I’m not sure this applies to our system, since it uses 3 CTs on a split phase + solar setup. Hence I came here to ask the experts :slight_smile: If that wiring is relevant, I might be missing a bit or 2 from understanding it, but I assume the Elkor based meter would also require only 2 CTs.

Connections for a WattsOn would be the same as the connections for your emonTx.

The WattsOn is available in three variants. Two current, (mA and 5A) and one voltage (0.333 Volts)
The voltage version can use any 0.333V CT which is nice because they are widely available.

Correct. You’d need only two.

I see. But does the device “understand” what I’m doing? I mean, I should configure it to not assume a different configuration than the one I intend to set up. Like I set it up for a single phase 2-wire config + inverter but it doesn’t detect current on the non-existing phase, and/or would calculate wrong totals or would simply give an error message. I admit I haven’t read the whole configuration document so I might be asking the wrong questions.

About 0.333V CT: can you recommend a decent but not very expensive one (like ones you might be using)?

NO worries. To quote a co-worker, the only stupid question is the one that doesn’t get asked. :grin:

When used on a single phase system, you simply ignore any of the “measurements” that apply
to a 3-p system. e.g. you’d ignore any L-L reading and use the L-N reading. It really is as simple
as that. :wink:

I use a combination of Wattcore and Magnelab CTs. In the US, the wires we use to feed our circuit breaker panels are significantly larger than those used in the UK, (and I’d guess elsewhere, as well) which is the driving reason behind my choice of CT.

So my first question is what is the diameter of the wires your CTs are attached to?

Second, do you want split core, or solid core CTs?

Hm, ok. So you say the device will just work fine with a CT unconnected and I can safely ignore inter-phase metrics. Naturally I can do any calculations necessary on the other end of the Modbus wire.

It’s an European 230V system. We used to have 3 phases somewhat evenly distributed in the house but it’s not needed any more so we simplified the system to a single phase with the solar inverter also connected to it. Amperage is max. 32A so the wires are relatively thin (IIRC the main wire is 6 mm2 / AWG 9 or so). Split or solid is indifferent for me, but split are obviously easier to apply/remove.

OK, that gives you a wide choice.

What’s your “max budget” for two CTs?

Hmm, I’d like for the system to have at least 0.5% accuracy on all metrics including energy, which doesn’t seem unrealistic. I’ve seen split-core CTs around $20-30 each with 0.2% accuracy on paper. Can’t recall the brand, sorry. I’d expect to pay for them around that amount.

Wattcore has split core CTs that are in your price range, but I don’t know if they have a distributor
in Europe.

The Magnelab SCT series is about 50 USD each, but they do have a European distributor.

(Shipping from the US would increase the cost more than just a little)

That’s about all I can offer, as I’m in the US, so I don’t have any more info on European
avaiablility other than what I mentioned above. Sorry I can’t be of more help.

No, it’s a great help already, thank you. I can order from the US or from Ebay used, I don’t care really. I expect to shell out for shipping, naturally. I’m looking for a system price comparable to the price of an EmonPi with a similar setup (that’s around Ł200, preferably less). I think it’s doable, it would have greater accuracy without all the hassles of manual calibration - I would need to do some additional work but that’s fine. I guess I’ll start hunting for suitable CTs and then think about the next step involving a WattsOn.

Sounds good.

I have three of them, and they’ve worked without a single problem for well over three years.
If you need any help / have any questions, fire away!

In that case, another choice is the Wattcore WC-1 series.
Split core, available in several ratings and as a voltage output type.
They usually go for ~25 USD on Flea-Bay.


That listing is for a mA type, so is just an example.

Thanks, I was eyeing the WC-1 series. The listed accuracy is 0.5%, so it might not meet the system accuracy of 0.5%, but that might be a bit far-fetched after all.

Here’s a revenue grade Magnelab CT: