Working out all the feeds for Generation, Import and Export

Hi there,

So I recently got an emonPI and an emonTX with the idea of monitoring both electric consumption for my house and electric generation from my solar panels.

Quick details on the setup:

I have the emonTX running off batteries attached to the house meter outside in its box transmitting to the emonPI.

The emonPI is connected to the input (DC+?) of the solar panel inverter. I can’t however access the generation output, it feeds via I think a multi-core cable into a cut-off switch and meter and then off somewhere into the house. There is a separate meter for FIT export so I’ve added a pulse sensor here.


First question is how to set this up in emonCMS so I get actual generation, import and export values (especially in things like My Electric).

So far I have:

I’m also unsure because as far as I understood the house also feeds off the solar generation as well as exporting it to the grid.

Second question is really a curiosity, my solar inverter for example says it’s generating ~600W but the value detected on the line is usually <8W, is that normal?

Welcome to the OEM forum, Lloyd.

I can’t answer your questions about setting up emonCMS, but one thing struck me immediately:


Have you attached the emonPi’s current transformer to the d.c. cable between the panels and the inverter?
Because if you have, I’ve got some bad news for you: NO transformer - and that includes your current transformer - can work on direct current. The laws of nature simply don’t allow it. That would explain your reading of 8 W - it is just noise picked up out of the air, or coming in via the emonPi’s power supply.

If you want or need to actually measure the actual generation, you’ll need to find a single core cable leaving the inverter and clip the c.t. on that, before it joins the rest of the house wiring. Does your FIT meter have 4 single-core cables in and out? That’s one place where you might be able to clip a c.t. The pulses from your FIT meter will be the best you can get if you cannot find a suitable cable.

That will give you the current, it will give you an approximation of apparent power (because it assumes a nominal voltage, and your voltage will fluctuate, possibly quite widely, over the course of the day, but unfortunately it cannot tell you real power (what your meter shows) nor the direction of power flow, so you cannot tell whether you are importing or exporting power.

All in all, I think you’re going to have to look carefully at your wiring and find places where you can monitor so that you can gather rather more meaningful data. If you’re not completely confident with electricity, I’d advise consulting a qualified electrician.

That’s correct. You can think of your house wiring as one pool of electricity. Once your panels generate some electricity, it goes in, and it’s got to go somewhere. If you don’t use it, it goes out to the grid. If you want to use some and your panels aren’t generating enough, it flows in from the grid. The grid will always supply or absorb the difference between what you generate and what you use.

Thanks for the response, total noob so looks like it won’t be as simple as I’d first hoped!

Yes it seems I did! :frowning:

Excuse my naivety but would this not be possible using the AC-AC/VRMS attached to the emonPI?

Yeh I was afraid of that, this is a new(ish) build with everything neatly squirreled away so getting at things is a pain.

The electricians left their card though so maybe I’ll poke them see what they can do.

Unfortunately no. What you need is the phase relationship between voltage and current, and that in effect means both must be measured in the same device - either both in the emonTx or both in the emonPi.
See Learn→Electricity Monitoring→AC Power Theory→Introduction→An Introduction to AC Power for a detailed explanation.