Expected output and efficiency from panels

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foosman
foosman Solar Expert Posts: 38 ✭
in Advanced Solar Electric Technical Forum #1
I just got my system monitoring online and have been looking at the numbers. I'm trying to decide if it looks reasonable before I hand over a check to the installer. Here's what I'm seeing:

- South array, 3.24kW, peaked at 2242W and produced 16.7kWH.
- East array, 7.92kW, peaked at 5983W and produced 43kWh.

Doing a simple calculation I get 69% efficiency and 75.5% for the respective arrays. This initially strikes me as low.

The installer believes I should see a derating factor of 0.85 (although the default for PVWatts is 0.77). If I understand correctly this does not account for temperature and is the value at STC (temp = 25C). Adjusting for temperature (it hit 104degrees today, 40C), I would lose approx. -0.5%*15=7.5%, worst case.

I'd appreciate feedback if the losses/efficiency seem reasonable. Thanks.
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  • Solar Guppy
    Solar Guppy Solar Expert Posts: 1,989 ✭✭✭
    #2
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    Re: Expected output and efficiency from panels

    sounds right to me, heat really knocks down performance ... 104F, that's 10 degrees hotter than where I live in Florida! so your panels are probably running near 75C on the roof. So its not a 7.5% loss, more like 75C - 25C = 50C * -0.5% or 25% hit.

    Its interesting your data backs up what I have been saying, east arrays are better as they harvest there energy in the coolest part of the day, my arrays show similar performance differences. I would say all looks normal
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  • foosman
    foosman Solar Expert Posts: 38 ✭
    #3
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    Re: Expected output and efficiency from panels
    Solar Guppy wrote: »
    sounds right to me, heat really knocks down performance ... 104F, that's 10 degrees hotter than where I live in Florida! so your panels are probably running near 75C on the roof. So its not a 7.5% loss, more like 75C - 25C = 50C * -0.5% or 25% hit.

    Its interesting your data backs up what I have been saying, east arrays are better as they harvest there energy in the coolest part of the day, my arrays show similar performance differences. I would say all looks normal

    Thanks!
    Yes, this is totally different from what I expected, as the heat seems to play a bigger role that expected. The east facing array hits a peak at around 10:30AM while the south hits a peak around 1:30. This is about a 10degree difference on the ground in temperature, i.e- 88degrees at 10:30 vs. 98 at 1:30, and then continues to climb. The south starts to roll off slightly around 3PM; however, it is probably still getting almost direct sunlight...most likely the temperature impact.

    At least on hot days, the east array seems to produce more energy per rated watt than the south. I'm curious how it stacks up over time...so it will be interesting to watch.
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  • Dave Angelini
    Dave Angelini Solar Expert Posts: 6,749 ✭✭✭✭✭✭
    #4
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    Re: Expected output and efficiency from panels

    I use a 35% design loss in the rated solar power in summer up here in the mountains. I would agree with SG. East to solar noon always produces more KWH than Solar noon to west on a tracked array FYI. Equipment is much cooler!
    "we go where power lines don't" Sierra Nevada mountain area
       htps://offgridsolar1.com/
    E-mail offgridsolar@sti.net

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  • foosman
    foosman Solar Expert Posts: 38 ✭
    #5
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    Re: Expected output and efficiency from panels

    Yesterday it cooled off a bit and we had some cloud cover. I got a peak power output that was > 10% higher than the previous day.

    Good to know the system is doing as expected. Again, thanks for the feedback!
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