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Community and Q&A

Solar Panel Output Less Than Expected

loeffy | Posted in General Questions on

Just bought a house with an existing 5kw solar system.  I reviewed past electric bills, showing the panel monthly output, and it’s lower than I would’ve thought. Around me, most calculators/maps indicate ~5hrs of useable sunlight per day average.  That works out to about 750kwh/mo of production. The bills show half that amount. Am I misunderstanding something?  Is something wrong with the panels?  Or is it something to do with the orientiation?  Due to house/roof design, 3kw are on the SE roof, 2kw on the SW roof. The SE roof is shaded by a taller neighboring house until mid-morning or so.  Could that be the issue?

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  1. Jon R | | #1

    This calculator should provide a much better estimate than "5 kw".

  2. Walter Ahlgrim | | #2

    Do your panels have 1 micro inverter on each panel or one big central inverter?

    If you have the central inverter all the panels are likely wired in series with each other if any one panel gets shaded that will stop production from all the panels in that chain.


  3. Expert Member
    Zephyr7 | | #3

    Those five “useable” hours won’t all be at full power, they’ll vary throughout the day based on the angle of the incoming sunlight, and the season (for the same reason), and even the temperature of the panels. You only get maximum output from the panels when you have full sun coming straight into the panels (rays of light perpendicular to the panel surface). Any deviations in intensity (due to season, cloud cover, etc.) or angle (season and time of day) will reduce the output.

    Half is close to an average, assuming 0% up to 100% and back to 0% again as the sun tracks across the sky, but it’s not linear. You might want to check things. It might be something simple like a lot of accumulated dirt on the panel surfaces.


    1. Muwanguzi | | #7

      These five full hours (sun hours) take into consideration most of the weather conditions, and this may vary by a certain factor depending on a place. The only factor this does not cover are the restraints like the system efficiency, age of the system, dust etc. For design purposes, we normally give this a combined system efficiency of 75-80%, and this reduces to about 3.9 sun-hours per day. A 5kWp system would produce about 600kWh of energy per month which is lower than the expected 775kWh/month.
      Though still, the recent analysis show that the available systems perform to about 6-20% lower than estimated (3.9 sun hours) and main cause of this is accounted to the inaccurate estimation of the radiation data, Researches suggest avoiding overestimating for a short time remedy, and investing in onsite weather stations for a long time remedy.

      Thank you,
      Derrick Muwanguzi,
      Equator solar

  4. user-7264890 | | #4

    1) As Jon suggests, use PVWatts to calculate your expected annual (and monthly production). 5kw x peak sun isn't the whole story. Shade, tilt, and orientation do matter.

    2) Your electric bill may or may not show you how much you produced. Some utilities require a production meter that measures every kwh produced by your PV system, but others only NET meter. In a normal grid-tied system your system produces as much as it can at all times regardless of what you are using. Your house has first access to those electrons produced, and any energy you don't use right away goes out your meter and is counted as credit by the utility. At night you purchase energy as usual from the utility (spending the credits you made during the day). For utilities that don't require a production meter this back and forth from the point of service is all that is shown on your bill. Energy that is used at the moment it is produced does not show up on a net meter only bill. Billing that does include a separate production meter will show the production as a separate meter reading (sometimes on a separate page) and that is an accurate measure of production.

    3) If it was installed in the last few years there many be an online monitoring portal where you can view performance. Ask the previous owners, or if not available, call the inverter manufacturer.

    4) If you do know how much your system has produced and PVWatts tells you it should be producing more than it is, many solar installation companies will do a system inspection/analysis to figure out what's going on. If something isn't working it's almost always dead electronics somewhere. The panels themselves don't generally fail.

  5. Joe Norm | | #5

    the 5 hours usable in your area does not account for shading, soiling, age of system, optimal tilt, etc.

  6. Jon R | | #6

    I would use an ammeter and voltmeter to find instantaneous output. But you have to know what you are doing for this to be safe.

  7. Expert Member
    Michael Maines | | #8

    How are you determining that it's a 5kw system? Older panels may put out as little as 250w each vs. the 305w or more that today's panels generate. Older panels also weren't as good at collecting sunlight at a low angle of incidence--i.e., not perfectly aligned to the sun.

  8. GBA Editor
    Kiley Jacques | | #9

    I think you will find this recent article of interest: Getting the Most out of Your Solar Panels.

  9. Tom May | | #10

    First, you have to determine how they are wired. You may have several strings that tie together at a combiner box. Each string would have to be tested to determine that they are operating correctly. If one panel is not operating correctly, the whole string will not be operating correctly.
    Inspect the connections between the panels if you find a string is not up to par. If you find a problem, then each panel will have to be tested to determine which one is bad.
    Your installation orientation is going to vary outputs over the course of the day. The current will change but the voltage should stay constant. As others have mentioned, even if one panel in a string is shaded, it will effect the entire strings output, but the voltage should still be constant.
    If you have any original paperwork, check that to see how the system was installed.

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