The next thing to do is to compare Sense against the other data source, our hourly feed from SolarCity, showing production from our 4.2kW solar system that uses a 2013 vintage SolarEdge inverter. All the data is going to be negative (energy consumption is positive, production is negative), so this might also give a little more insight into the negative tail of the net usage correlation scatter plots earlier.
My first scatter plot of the hourly solar data, with coloring set to the month was little confusing. The “line” was more of an ellipse. And the coloring really didn’t offer any clues as to why.
But shifting to coloring based on time of day showed a discernible pattern. Sense gave higher readings than SolarCIty in the morning, SolarCity gave higher readings in the afternoon - a systemic error. But why ?
Looking at the aggregated daily data, the ellipse goes away, indicating the the error cancels itself out on a daily basis. Note that I have gone back to coloring by month for the daily plot.
A few things we can see from the daily plot.
- There is good correlation between Sense solar data and SolarCity, though the slope of the correlation line is slightly less than one. I’m not going to calculate it by regression until I remove erroneous outliers for which I can find a measurement issue, but the slope differential bears out a 4% SolarCity/SolarEdge inverter over-optimism saw with earlier measurements.
- The hourly measurement ellipse is gone. That indicates there were offsetting measurement differences between Sense and SolarCity, that cancel out when aggregated on a daily basis. I suspect that there is an interval assignment difference between the two measurements of 15-30min. Perhaps the SolarCity measurement is for hour centered around the timestamp, while Sense measurement is for the hour following the timestamp. That would explain the ellipse.
- We have a number of big hourly differences between SolarCity and Sense, but only a few big relative differences between SolarCity and Sense at a daily level. That means that the big daily mismatches are likely caused by the accumulation of sequential hourly errors.
A quick look at the hourly differences in a histogram show a different view on the ellipse, a typical distribution the width of the ellipse, with a small number of outliers worth investigating.
The daily histogram tells us that we really only need to look closely at a small number of mismatches. The histogram once again highlights the systemic optimism of the SolarCity data - the mode of the distribution is offset bay about 800W from 0.
Now we have looked at direct comparisons between Sense and both “utility” sources. Next, it is time to do some detailed mismatch analysis between Sense and PG&E data.