Chapter 1 - Introduction
I love my Sense ! In general it is a very accurate tool for looking at my energy consumption, especially when coupled with smart plugs, DCM, etc. But even though I trust it, the old saw goes, “Trust, But Verify”. I decided to look back over my almost 2 years (16,090 hourly observations) with my current Sense to see if I can find discernible patterns of when it led me astray, especially with the most basic of measurements.
The plot below highlights the comparison of Sense Net usage (Total Usage + Solar Production) vs. my utility’s net meter, based on hourly exported data. Perfect correlation between the two would result in all the points being along the perfect 45 degree “utility line”. Instead, there are bunch that a way off, and then another set that are long the “unity line”, but above or below it enough to represent a significant error. I have colorized the points based on the following conditions, to highlight some of the most common indicators that Sense isn’t likely giving you the correct result:
- Total Usage missing (NA) - there aren’t any points with this condition since I can’t really even chart. These probably occurred during power outages - 9 Hours
- Total Usage Negative - This has happened to me, mostly when my Sense Monitor has bugged out. A power cycle has typically fixed. - 18 Hours
- Solar Production missing (NA) - I’m not sure under which situations Sense fails to export a solar value, but I can investigate later - 247 Hours
- Solar Production Positive (and above 5W) - This would mean my inverter is ingesting power rather than producing it. This has happened to me, mostly when my Sense Monitor has bugged out. - 26 Hours
- Always On missing (NA) - Always On is a good indicator that something is wrong with Sense data collection - it is extremely responsive to short burst of bad data. - 850 Hours, though a few chunks of these (24 hours at a stretch), might be from Sense refilling it’s Always On measurement pipeline.
- Delta in Always On between consecutive hours greater than 100W. A sudden change in Always On, especially downwards, is a good hint that Sense might have lost some data readings during that hour. - 67 hours
- Nominal - When everything seemed to be working normally
It’s very easy to see that most of the very large errors occur during the top 5 conditions and that almost all “Nominal” points are pretty much on the unity line, thought there are a few that need further investigation.
If I look at just the points that have passed my high-level “lie-detector”, just the hours / points marked nominal, I still see a few points with substantial errors, though the “lie-detector” has screened many of the most egregiously bad readings out.
And if I look in greater detail at the hours with the largest absolute errors (absolute value of difference between Sense net and PG&E net), I see the largest errors are clustered in time in July, as the brownish yellow points also indicate.
It looks like there are a couple more indicators I can add to my “lie detector”.
- If the hourly Always On is negative, I really can’t trust the Sense result. That seems to be a reasonable assumption, and would remove most of the July problem points.
- I’m also speculating that if the Always On delta is NA (that means the previous Always On was missing - NA), that is a likely indication of erroneous data as well.
Now some of you out there might be saying, "how can you be so sure that these are reliable indicators ? Maybe many of the points with these attributes are actually the “truth”. Don’t worry - I’ll look at the reliability of each these detectors a little later. Right now, I’m going to see if removing those points improves the accuracy against the “unity line”.