Tracking Down the Biggest Errors (Residuals)
Here’s the table of the 13 hours where the residuals (errors vs. the fitted line) were greater than +/- 100Wh. I’m viewing them in date order since a couple of them seem to have correlation in time. You’ll also notice that I have included the AbsDiff (Absolute Difference) between my hourly utility reading and my Sense reading, and that it closely approximates the corresponding residual, except for the sign (I did the subtraction in the opposite order). If I regard my utility reading as golden, this AbsDiff is really the true error, so it’s good to see that they are close.
I’m first going to look at all three of the hours on May 5th since I actually know what caused those errors - my electrician was working in my main panel and turned off the power several times. Here is the Sense Power Meter for those 3 hours - definitely the cause of error with big dropouts and inversions everywhere ! But the Sense monitor started functioning normally once he was done.
If I look at the waveforms, two more of the biggest residuals in the list also have gaps, that I can’t explain. Those are cases where my network or the Sense monitor might had a problem. You’ll also notice something else about the second waveform. After a few months, Sense reduces the resolution of the saved data available from the web app, hence the less detailed lines, presumably to save storage and AWS costs.
The remaining 8 worst residuals seem to stem from another cause. The common thread between all of those hours is that they are all during the charging of our Model S, thus during very big changes in power usage and high currents (20kW @ 80A at max). Intriguingly, none of these hours are ones where the car is charring for the entire hour, but rather hours where charging is either starting and completing. The residuals are negative (Sense greater than fit line) when the EV charge is ramping up and the residuals are negative (Sense less than fit line) when the EV charging ramps down.
You’ll also notice that Feb 19th is the breakpoint for the reduction in webapp time resolution. Feb 20th has half second resolution while Feb 19th is at 1 minute resolution, so Sense must keep around hi-res waveforms for about 3 months.
