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June 14, 2009 / Jim Fenton

Photovoltaic, Part 3: Results

We have had the photovoltaic system in place for about 2 1/2 years now.  What are the results?  The simple answer:  We haven’t paid more than a metering charge since.  But there’s a lot more to it than that.

In the first year, the system generated 7380 kWh of energy, 3977 kWh from the south array and 3403 kWh from the west array.  Our net energy usage for that period was 1182 kWh (we used 1182 kWh more than we generated), but our bill was zero because we generated more than we used during peak hours.  Here in California, Pacific Gas & Electric averages the usage for “net energy metering” customers on an annual basis, which takes into account seasonal variations in usage.  They send monthly statements summarizing this, such as this bill at the end of the first year:


As I mentioned in the last post, the inverters have serial interfaces that allow one to read, in real time, the status of the unit.  Xantrex has free monitoring software (for Windows) available, so when the system was first installed I had it hooked up to my Windows PC in order to generate a year’s worth of baseline data.  The Xantrex software generated a CSV file each day with the day’s measurements, plus another file that summarized the daily output of the system.  I was able to generate graphs fairly laboriously by importing the data into Microsoft Excel.  But it was great to see what the system was doing and to verify that both arrays were producing power as expected.

For the most part I stopped collecting data after the first year, in part because it seemed like a contradiction to keep a PC running mostly to collect data from the photovoltaic system.  But I do have a Linux system that runs all of the time (it is my mail server), so earlier this year I made a programming project out of importing the old data into MySQL and writing an application that would query the inverters periodically and insert the results into the database.  The result now is that it’s fairly easy to generate graphs like the following:

Graph for May 17, 2009

Graph for May 17, 2009

I had been noticing this year that the output from the system seemed a bit lower; I had remembered having several 30 kWh by this time in 2007, but none this year.  I compared the graph above against the baseline two years earlier and found the earlier graph to look almost exactly the same, but higher.  The output of the system had decreased about 8.5% over the two years.  What happened?  It appears that, even though the panels didn’t look excessively dirty, they’re quite sensitive to dirt.  On June 1, I cleaned the panels with a long-handled mop and squeegee, and it appears that things are back to 2007 levels (we have had an unusual cloudy spell so it’s difficult to be more definite).  I expect to be cleaning the panels more often in the future.

You’ll also see that the output from the west array isn’t that much lower than the south array.  The difference is more pronounced in the winter (due to sun angles and shorter days) but I suspect that the west array is actually contributing more financially due to time-of-day metering.  Morning fog can also cause the west array to be more productive than the south array.

A similar graph taken during the winter months tells a very different story:

Solar data for January 8, 2007

In addition to the shorter day and lower sun angle, this graph shows the impact of shading on array output.  This is probably most visible on the west array about 2 PM.  As the sun angle goes down, objects further away can shade the array from the sun.  Even relatively small partial shading of the array has a major impact.

Here’s a summary of the output of the system over the course of a year (2007):

System output for 2007

System output for 2007

As you can see, the output of the system varies widely between winter and summer.  In addition, you can see more consistent results during the summer, due to the relatively dry and sunny summers here in California.

One of our neighbors to whom I showed our data collection and analysis setup described it as having significant “toy value”.  He’s right; I have had a good time collecting, analyzing, and showing off the data.  But it has also taught me a lot about the sensitivities of the system to things like shading, solar panel dirt, and alternative panel placements which I hope is useful to others as well.


Leave a Comment
  1. Ellen / Jun 15 2009 9:35 am

    Good info. If I had a system like that, i’d be charting it, too; heck, I already take my PG&E data and graph it!

    Very interesting about how dirt-sensitive they are. Good info.

  2. Tim / Jun 30 2009 4:54 pm

    Very good info. check data after 5 years? That would be interesting to see long term system performance.
    thanks, Very cool!


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