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Six Years of Production Data (6.9 kW SunPower System Phoenix, AZ)

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  • Six Years of Production Data (6.9 kW SunPower System Phoenix, AZ)

    I now have over six years of production data for my system which was turned on May 18, 2012. The data presented is over an annual period from June 1 to May 31. Linear regression suggests a roughly 0.78% per year loss however, the chart appears to indicate a faster decline over the first two years and slower decline over the subsequent four years. Obviously, the fluctuation in weather year to year is another factor that is not considered and may be significant. However, we do have a lot of clear skies here.

    ScreenHunter_02 Jun. 02 12.01.jpg

    ScreenHunter_03 Jun. 02 12.02.jpg







  • #2
    To reduce variance from weather, I'd stick to looking at the figures for months when cloud cover is least likely, which ought to be when the figures are highest. I don't see any statistically significant reduction in April or May output from one year to the next. Even so if you exclude the partial result of May 2012--how did you produce your highest monthly output with half the month spent off-line?

    In June and July, your panels are really cooking in the AZ heat. PV voltage (and thus power output) gets lower with temperature. You might want to look at how hot it was in June of each year. Note that July 2016 production was actually better than previous years, which is not consistent with any of this being from panel degradation.

    Good news for your panels, I'd say.

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    • #3
      Originally posted by BackwoodsEE View Post
      To reduce variance from weather, I'd stick to looking at the figures for months when cloud cover is least likely, which ought to be when the figures are highest. I don't see any statistically significant reduction in April or May output from one year to the next. Even so if you exclude the partial result of May 2012--how did you produce your highest monthly output with half the month spent off-line?

      In June and July, your panels are really cooking in the AZ heat. PV voltage (and thus power output) gets lower with temperature. You might want to look at how hot it was in June of each year. Note that July 2016 production was actually better than previous years, which is not consistent with any of this being from panel degradation.

      Good news for your panels, I'd say.
      Actually, how I presented the results was perhaps not the clearest. Each column in the table starts in June of one year and ends in May of the following year. So I have not included any May 2012 data: the column labeled 2012 actually goes from June 2012 to May 2013.

      You make a good point about looking at individual months. Our driest month is June (.02" rain) followed by May (.11") and April (.28"). July and August are quite variable in rainfall due to thunderstorms from our monsoon. So there's bound to be more fluctuation for those months from year to year and that may be reflected in the July 2016 outlier. Maybe June is the month to look at over the years but it actually seems more variable than the months of April and May. I may pull up data on weather for those months over the years and see if that adds to the understanding.

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      • #4
        a couple of the helpful guys on the site said that the 1st couple of years is where u see the most decline of efficiency , and then it levels off... and ofcourse there is weather...

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        • #5
          I have a 6.63kW system in Gilbert. My last three years (SRP fiscal) show I'm keeping up with you.

          12158 kWh June-May 2016
          11865 kWh June-May 2017
          11971 kWh June-May 2018
          Dave W. Gilbert AZ
          6.63kW grid-tie owner

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          • #6
            Originally posted by Ian S View Post
            I now have over six years of production data for my system which was turned on May 18, 2012. The data presented is over an annual period from June 1 to May 31. Linear regression suggests a roughly 0.78% per year loss however, the chart appears to indicate a faster decline over the first two years and slower decline over the subsequent four years. Obviously, the fluctuation in weather year to year is another factor that is not considered and may be significant. However, we do have a lot of clear skies here.
            I think that's what one would expect. Most panels (at least that I looked into) are warranted with something like a 2-3% reduction in performance the first year, and something like 0.5% every year there after (numbers will vary by manufacture and panel of course - these are just some example numbers.) But expect the first year to be the biggest drop, you may have had a pretty good 2nd year and bad 3rd year do to weather, making it look more like a 2yr drop, but you can see it settled out and is trending down at a slower rate than the initial year.

            Funny thing, looking at Sun Power (X&E series), It was a flat 95% production the first 5yrs, then a 0.4% reduction per year after that for 25yrs (P series is 3% 1st year and 0.6% every year there after.) And then they claim put a chart of a traditional warranty up there, of 90% for first ten years and 80% for the next 15yrs. I can honestly say from my looking through panels, I didn't see any with that poor of warranty, I'm sure one can find examples though, but I don't think that as typical, at least for quality panels.

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