Confused by how Sense handles Solar

#1

Okay so forgive my ignorance but I did some searches and can’t quite figure it out so I decided to ask…

I just installed sense yesterday. 200Amp main service with the CTs clamped around the service mains (before the meter) since my meter connects to my panel via buss bars that I couldn’t get the CTs around due to the way the panel is configured. In addition, I also have Solar and have those CTs properly clamped around the wires that feed the solar breaker in the panel. Sense is reporting fine, no issues there. It did report an issue during the initial setup that it corrected itself which I believe is perfectly normal.

My question is simple… My solar is feeding 5,900W into my panel right now but Sense is also showing a draw of 2300W. I know I’m not using 8,200W so clearly I’m not understanding how this works… How can Sense detect load when electricity is being backfed into the grid? Shouldn’t the Sense app show a negative energy use? My meter says I’m feeding into the grid 3,600W so it’s reading the backfeed correctly. I guess I just don’t understand how Sense actually works… How can it detect load when my solar is generating more energy than the home is using?

Thanks,
Jeff

#2

Thats what mine looks like when generating 6500W and using 1800W, the difference is fed into the grid.

#3

Sense, in essence, monitors your net usage and your solar backfeed. It adds your net usage results to the solar backfeed to present Total Usage. Of course, the computations are quite tricky since we’re talking about AC voltages and currents, but some things make the math simpler:

  • The voltage stays sinusoidal (maybe with a few harmonics) and doesn’t vary much in RMS magnitude.
  • Current does vary widely, but current coming from the solar doesn’t change quickly relative to the house usage, so it can be viewed as nearly constant over a couple of cycles.
#4

So what doesn’t make sense about that?

You’re generating 5,900 W, using 2,300 W which results in 3,600 W being returned to the grid. So yes, your 2,300 W you’re using is within the 5,900 W you’re generating, so you have 0 net use (or -3,600 W going to the grid I guess) Now if you would be generating 2,300 W and using 5,900 W you would have a 3,600 W net use (5,900 - 2,300)…

#5

How is it possible Sense can detect “load” when it’s being washed out entirely by “generation”? That’s the part I don’t understand… If my house is using 1kWh but my solar is producing 2kWh then I would expect Sense to see a negative number going through the CTs that come off the main service feed. Yet it sees the 1kWh load as if the electricity isn’t really being backfed.

I’m really having trouble explaining what I’m confused about which makes sense since I’m confused about it… :slight_smile:

Jeff

#6

@jeffro01,
Solar generation is fairly stable in output from a time perspective when compared to the types of events Sense looks at. If you were to look at the 60 Hz waveforms Sense sees you would see something like this (if I haven’t completely forgotten my 2-3 power generation labs in college:

  • Solar current - 60Hz sinusoid with relatively slow smooth magnitude changes (clouds drift in front, then gone), and a phase close to 0.
  • Solar voltage - 60Hz sinusoid, in tight phase with Net Usage voltage, staying close to 120V RMS magnitude
  • Net usage current - crazy waveform with a 60Hz underlying periodicity, and a phase that moves around, reflecting most of the variation in Net Usage. I’m assuming Sense has some way to observe overall phase of current waveform with respect to voltage, but it would be tricky.
  • Net usage voltage - 60Hz sinusoid staying close to 120V RMS magnitude, with some injected harmonics.

Bottom line is that most of the attention needs to be placed on Net usage current, since its magnitude and phase relationship carries most of the information about the devices in the house.

2 Likes