Equipment question

Hi guys.

I’m from New Zealand, and I was going to purchase the clamp meter to measure my total energy usage.

I have a 80A feed (230V) from the street and all our vehicles, cooking and heating are electric.

Just wondering how accurate is the clamp meter? Ive done some general reading on Clamp meter technology and it appears that they can at times be quite accurate but at other times not so much. I’m looking for opinions from people who have this set up – how accurate do you find it?

Additionally, I see there is an ac-ac power brick to help increase the accuracy of the energy measurement. This brick is only available in US, UK and EUR style plugs - not AU/NZ plugs. I’d rather not use a power Point adapter - is there one I can get locally sourced with identical spec to the one in the open energy monitor store?


What do you mean by “the clamp meter”? Do you mean the emonPi, the emonTx with an emonBase, the emonTx with the ESP8266 adapter? Something else?

I’ve calibrated an emonTx to be within better than ½% in the long term of my supplier’s electricity meter, that’s on real power using a “shop” 100 A current transformer and a.c. adapter.

There is (at least, it looks identical to the Australian version and we know that is suitable) 9VAC 1A Unregulated Power Supply 7DC Plugs | Jaycar Electronics New Zealand
If you come to use it, the voltage calibration coefficient is 300.0 (No-load = 10.4 V)

Hi Robert, thanks for the info. I am referring to this “non invasive” power meter: 100A max clip-on current sensor CT - Shop | OpenEnergyMonitor

I’m not sure exactly what equipment I should get - my goal is to accurately monitor power usage in the most cost effective way possible. It looks like I will at the very least need an emonPi pre-built which includes the 100A current sensor and the AC voltage sensor that you linked.

Are there any cheaper options or is that it?

Hello @WeAreMetaphysical the emonTx and emonBase combination is a little cheaper than the emonPi whilst still providing full local data logging and visualisation capability, see install guide for an overview here Install emonTx & emonBase - Guide | OpenEnergyMonitor and more detailed information on the emonTx here if needed emonTx - Guide | OpenEnergyMonitor

Even cheaper would be an EmonTX and a serial connection to a Raspberry Pi (or a clone) :grin:.

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That’s what threw me. A current transformer does nothing on its own, it’s not a meter in the sense that most people use the word.

A bit like a DS18B20 is nothing until connected to something to read the data.

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My mistake - new to all this so got my terminology mixed up.

Did some more reading - am I able to use an EmonTX v3 (emonTx - Energy Monitor Transmitter - Shop | OpenEnergyMonitor) + Wi-Fi Adapter (emonTx WiFi Adapter - Shop | OpenEnergyMonitor) to accomplish my goal?

I could run EmonCMS in a container/virtual machine (I already have a fairly complex infrasturcutre with a bunch of VMs etc running on my home server) using this guide github com/emoncms/emoncms/blob/master/docs/

Does all that make sense?

Don’t worry too much about that, but we can only answer the question you ask - not the one you intended to ask. :zipper_mouth_face:

Yes, that must be the cheapest. From what you say, I take it you’ll be happy setting up emonCMS on your own server - it’s just that it is tailored to the RPi and some people have struggled - and I am reasonably certain that the guide will be out of date, because the last update to the software was fairly massive, and the documentation is still being worked on.

I’ve got a Raspberry Pi 4 Model B kicking around here, so if the VM route fails then I’ll just install it on there. Can someone check and validate my final parts list?

I know I’ve picked a UK Power Brick, I likely will get the one from Jaycar but I’d rather have the UK one from the shop for testing purposes.

If you have a RPi spare, and you can get have it close to the emonTx, then you don’t even need the ESP8266, you can use a wired serial connection to the Pi.

But if you do have the ESP8266, then you need a 5 V USB power supply. Either 5V DC USB Power Adapter (Australian Plug) - Shop | OpenEnergyMonitor or you might want to source that locally, you want enough current to supply the ESP8266 (our UK one is 2 A) and it must have a relatively clean output - meaning one intended as a phone charger might have too much noise and ripple on the output. It needs a mini USB plug (or Mini-USB cable 22AWG - Shop | OpenEnergyMonitor

If you’re likely to want to reprogram or otherwise tweak the sketch inside your emonTx or inside the ESP8266, (and you sound well capable) then I suggest you add a programmer Programmer - USB to serial UART - Shop | OpenEnergyMonitor.

There’s no real advantage in having the UK a.c. adapter as well - especially if you get the programmer. It will add quite a lot to carriage costs as that is the heavy item.

The emonTx can measure up to 4 circuits simultaneously, so at some point in the future, you are likely to want to add more c.t’s.

So sorry if I’ve added confusion.

@WeAreMetaphysical if you go to the EmonScripts repo, there are instructions for installing emonCMS on Ubuntu that have been tested and work.

@Robert.Wall - I managed to get emoncms running on a Ubuntu 18.04 VM using the digitalocean guide (EmonScripts/ at master · openenergymonitor/EmonScripts · GitHub)

I think I’ll get the ESP8266 anyways - just gives me more flexibility going forward. I will be sure to locally source a quality 5V 2A Power Supply to run the lot. Will pickup the programmer as well.

So to clarify - I need the 9V ac current sensor PLUS the 5V DC power plugged in with the ESP8266?

Hi Liam,

Not trying to hijack the thread, but rather to give you a quick answer, as I see you’ve posted just 15 minutes ago.

Yes, you need both.

Without the AC-AC adapter, an emonTx will give an approximation of apparent power, not real power, which is what you get billed for.

And I was well asleep then. :sleepy:

And without the 5 V d.c. P.S.U, there isn’t enough power to supply the rather high current demanded by the ESP8266 (a deliberate design decision - it’s current limited as otherwise, it would put too big a dent into the a.c. wave shape that you’re trying to measure).