Can I feed VRMS from emonTX to IotaWatt?

Hello guys,

The problem is that I have emonTX allready installed with AC sensor etc. So the IotaWatt I will use for my second floor, but the electric box is so tight I really have no place to connect the AC adapter.

I will use the IotaWatt to measure my second floor in the house so emonpi is allready getting the VRMS from the emonTX and it should be enough to calculate the right power consumption. Or so I think…

Any ideas?

Bob Lemaire can now be found at, he has set up a dedicated forum for IotaWatt, if you post your question there I’m sure he will get back to you quickly.

You can measure the voltage at any point in your house, however you must remember that high current loads (heating, cooking, EV charging, etc) may cause a significant voltage drop in the cables supplying that load, so it is best to measure at a place that will not suffer from that.

@Robert.Wall Thanks for the reply. I will ask them about this.

So my question for this forum is I’m about to buy another emonTX for my basement with 3 sensors. Do I need to buy the AC sensor as well or can I use the VRMS from my main emonTX for the basement one?

I know that there are drop of voltages if the system is not balanced well. Bu I have a very good electric wiring plus all my appliances are A++ with very good invertors so no heavy consumers of current to destabalize the voltages. So pretty much I measure the VRMS at two spots in my house allready with 2 emonTX both with the AC sensor. There is no point to measure it at another two spots one with the Iota and one with my third emonTX…

I have no idea what the voltage input arrangements of the Iotawatt are, hence I suggested that you ask elsewhere.

As far as the emonTx’s are concerned, provided that both don’t use the a.c. adapter for power (though one can, but it’s preferable not to), then you can use one a.c. adapter for the voltage reference for as many as you have. The only warnings are they must not share any other common connections, or if they do, you must make absolutely certain that all polarities are strictly observed. (I undersand that you might not want to do that, due to them being located in different places.)

I think what you’re intimating is, can you use the Vrms as a number from the emonTx? If you multiply Irms from somewhere outside the emonTx, you can only have apparent power. If you want real power, then for timing reasons, the calculations have to be done sample by sample inside the same processor. So I think the answer to your question is probably no,