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

Continuing the discussion from Basic concepts:

That, of course depends what constitutes a “small fortune.”

This is pretty nice for ~325 USD. (plus cost of CTs and 15-24 Volt AC or DC PSU)
About 100 USD more than the cost of an emonPi, for the unit itself.

They can often be purchased used for considerably less. I bought one today for 25 USD. (Ebay)

It’s a fairly comprehensive device, capable of 0.2% accuraccy.
One of the items on the feature list:

High resolution (24-bit ADCs) with 16kHz sampling provide True RMS
parameters per phase: Voltage, Current, Power, Energy (W, VA, VAR),
frequency, power factor

Like the emonTX/emonPi, the unit’s calibration can be “tweaked” if necessary.
The main difference is the use of RS-422 (Modbus) to communicate.
(RS-422 max cable distance is 4000 feet)
However, USB RS-422 adapters are inexpensive, and if wireless comm isn’t a necessity, it’s an alternative.

But to maintain that accuracy, you need something rather better than the £10 standard CT.

It’s pretty easy to use a metering IC to get quite high accuracy - if you can have a galvanic connection and do all the isolation by radio or at the interface. If you need isolation at source, which unless you really know what you’re doing is an absolute necessity, then good CTs and VTs with low errors are not cheap.

Quite right. I should have specified a better CT would be needed.

Magnelab makes a revenue grade CT (+/- 0.5% linearity, phase angle +/- 0.25°) that costs 55 USD, so a pair of them will cost considerably more than the El Cheapo YHDC job. But users in the US (which includes me) wont likely be able to use the YHDC CT anyway. So they’ll have to use a CT with a large enough wire window, e.g. the Magnelab SCT-0750, of which the 075R is the revenue grade variant and costs a whopping 8 USD more than their -0750 CT.

The unit accepts up to 600V (line-to-line) directly without the need for PTs.

Is that a typo or an incredibly good discount?

Flea Bay. Used, i.e. not new.

I’ve got two others. I paid 75 USD for one, the other cost me 85 USD.
Not bad considering the cost of a new one.

Bargain of the century there! Good shopping. I guess it’s a bit of niche market and you just need to be in the right place at the right time.


They seem to be listed fairly regularly. I bought the first one alomost three years ago, the second one about a year after that. I check every now and then, and when I saw the unit I bought today, I snagged it straightaway. Going to use it to monitor one of my water heaters.

EDIT - While perusing the Aim Dynamics website (they’re a Magnelab distributor) I found this.
It costs 207 USD for a new unit. That’s over 100 USD less than the Elkor WattsOn mentioned above.
Here’s a video clip.

Interesting that they don’t quote any isolation standard like IEC60950, IEC61010 or similar. I’m not sure 2500VAC isolation is sufficient for something connected to 600V. Does it come with a CE or UL tick? If you’re up for it, it might be interesting to crack one open and inspect their safety barrier design. You could see which energy IC they use at the same time.

Both, as well as IEC 61010.
The user manual says: “The line input circuitry is galvanically isolated from the outputs.”

Here’s a pic of the label:

At 25 bucks, it’s a throw-away item. So even if I wind up “breaking” it, no big deal.
I think I’ll have a go at opening it!

Provided they haven’t destroyed the markings on the chip!


Here’s a couple of views of the inside of my WattsOn power transducer. As I suspected, the chip markings have either been removed, or covered.

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)?