Quick Philosophical Energy Monitoring Question


#1

Just wanted to poke the member here with what seems like a simple energy monitoring question, that I believe I’m over thinking, but thought I would pass it out there for your thoughts.

Question: Should solar PV connected CT(s) read as a Negative value (when energized), or should it be Positive when energized?

Reasoning: The reason I ask is that today I feed this value as a positive and subtract the solar output from the grid consumption, etc. I guess I’m wondering if instead it would be better to add the negative value as it is energy creation not usage (positive). One example I have run into is that my inverter during non PV creation periods uses 5-7w to power itself and I wanted to capture this delta which does not work as well with my subtracting the negative number in EmonCMS; which I would think it would.

Again, I think I’m over thinking it but had to ask what others are doing an of they even care. :slight_smile:


#2

I thought I’d wait to see if anyone weighed into this before I put in my two cents. The question is all relative to where you are observing from. If you are hanging out in the main panel box, then what is the difference between the mains and the PV feed? They are both sources of power coming into the box (except that off-duty trickle). From that perspective, the incoming generation should be the same as the mains.

Now the branch circuits are also positive on outflow, so like double entry bookkeeping, your P&L says that net-income = expenses. Net income is the sum of the mains and solar and is therefore reduced by exports - essentially extra power that passes through your little box domain back to the grid.

It’s also perfectly reasonable to have the branch circuits negative. Then the box becomes a zero-sum game.

But on a practical note, there are existing conventions. The OEM folks, whether consciously or not, adopted a convention where the PV is positive. That now works with all of the Emoncms apps like MySolar. So all things being equal, that’s the way I do it.


#3

@overeasy, Thanks for the feedback as it is always welcomed and very informative. You make a good point that such really depends on the perspective or intended purpose in one’s energy monitoring. Insightful in the you thought it out even for sub panels. So, it would be OK for a negative value for the inverter during non-PV energy generation (4-7 watts all night) which does add to the grid usage.

I’m also happy to hear that you have seen others like the OEM group have adopted a standard that is easy to follow and understand. Thanks again for the great product and all your assistance in keeping things running and support of the device!


#4

In fact, I allow my PV to go negative for exactly the reason you bring up–a couple watts of usage at night.


#5

Cra,

Thanks for the reply and good to hear others are just desirous to get good reading from their energy monitoring to make sure every watt counts. :slight_smile: Again, thanks for your input!