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  • Is My New System Undersized?

    Hi all,

    I just had a local installer finish installing a ground mount solar setup at my rural residence in Northern California and I am not sure if I am being inverter limited, or if my system's design is correct.

    My installer (who is a great guy and very easy to work with) insists it is not being limited in any way but is not able to explain it very well and I am still left with some questions.

    My setup:

    75x 325w Canadian Solar Panels
    3x Sunny Boy 7.7 KW Inverters (model # SB7.7-1SP-US-40)

    Panels are pointed 215 degrees, with 22 degree tilt, and zero obstructions or shading.

    Having 75 * 325w panels, puts the system at 24.375 KW maximum production.

    The three inverters which are rated at 7.7 KW each, come out to 3 * 7.7 = 23.1 KW total.

    My installer says that even though the panels are rated at 24.375 KW, and the inverters are rated at 23.1 KW, the system is not undersized at all, and in fact could have at least 5 more 325 watt panels added without any issue. I am left confused as to how this is possible and he is not able to offer an in-depth explanation as to why but does state that his experience has taught him that these inverters can handle much more than their rating and that the 7.7 KW rating on the inverters is not the "real rating".

    He has said that the inverters can do up to 10 KW but the company's technical data sheet shows 7680 watts as being the "AC nominal output power" and 32 Amps as the "max output current". How can the inverters go past their 7.7 KW rating if the max output current can't go beyond 32 amps? 7700w / 240v = 32 amps. Going higher than 7700w would push us past the 32 amps max right away.

    On a recently bright and sunny day, I saw all three inverters showing that they were producing close to 7700w (inverter 1: 7702, inverter 2: 7708, inverter 3: 7704).

    So is my 24.375 KW system limited to 23.1 KW because of the inverters or is my understanding flawed and my system is setup ok?

    THANK YOU FOR READING!

    Attached the spec-sheet for the inverters below.

    SunnyBoy Specs.png

  • #2
    That instantaneous output doesn't sound that low, You will probably not get 24.8 kW instantaneous output more than a few times/yr., for more than a few min./day. on clear days, and this is probably one of them. Bet that output happened around 2 -3 P.M. maybe ? I don't think the inverters are limiting your output. As a matter of opinion, and FWIW, I agree with your installer. Sounds like everything is working about as designed.

    Read up on how solar PV works. You will likely come to agree that things are running nominally. Sounds to me like you have unrealistic expectations of system performance based on ignorance of how PV works and solar in general. Get educated and reality will come into view.

    Read "Solar Power Your Home for Dummies", a fee net download, then get familiar w/ PVWatts, a PV model available for free on the net. Read the help/info screens first - they're also informative and have info that will help you understand as well as get the most out of the model. When running the model, use ~ 10 % system losses. after a few practice runs, choose the hourly output option and look for a clear day's output (the days with high hourly output that's mostly symmetric around solar noon) around the day you had the high readings. If your input to the model matches reality to some degree, the model will give you a rough approx. of what to expect from your system. It'll probably be close to what your system produces, +/- 5-10% or so , maybe less. Between PVWatts and the book, you'll begin to understand why it sounds like you probably have a reasonably well running system.

    Comment


    • #3
      Panels generally will, over the course of a normal day, produce 75-80% of the nameplate STC wattage. So "over paneling" inverters a few % is normal.
      Powerfab top of pole PV mount (2) | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||
      || Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||
      || VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A

      solar: http://tinyurl.com/LMR-Solar
      gen: http://tinyurl.com/LMR-Lister

      Comment


      • #4
        Gentlemen, thank you so much for the feedback! Will definitely read the "Solar Power Your Home For Dummies" guide.

        Regarding the output that I observed, it was definitely a bright and sunny day but it was around 11 am, which is why I began to wonder if I am already limited if we're producing the max at 11 am in early April. Since that day I have seen the inverters hit their 7700 watts max, multiple times a day, so it is definitely something that is happening frequently rather than only a few times a year for a few minutes. To give a better idea of the setup, my panels are high up on a hill with no obstruction and no shading, out in rural California, so if you can see the sun, the sun is hitting the panels. Of course the degree and intensity will vary but the fact that I am hitting that high of an instantaneous output so many times and not going over it makes me think that I am being limited.

        I did go through the PVWatts calculator before we started the project, and had entered in 24.375 KW capacity with 14% system losses (default), a 215 degree azimuth angle and a 22 degree tilt angle. That provided an estimate of 36,500 kWhr per year.

        I do completely admit that I am a newbie to solar setups and am looking to learn more but it can't just be coincidence that I never see the inverters really go past the 7700w max, can it? If they were to only hit that max for a few minutes a few times a year I could understand that I am more or less appropriately sized but the fact that I saw them hitting that 2 days after we installed the system, in April, and then subsequently observed the same multiple times afterwards is what has me worried.

        I just don't know who to trust when it comes to an inverter's maximum AC output specification. Installer says the manufacturer isn't showing the real max (his explanation being that the manufacturer would not be able to sell these units for residential applications if they did not show a lower maximum output). To be honest I am pretty skeptical of that explanation and my installer knows how to install a system but his technical depth and understanding of how all this works at a fundamental level is much weaker than I was hoping for. Our discussions always tend to end up with "just trust me, I have installed a lot of systems before."

        That is a bit unsettling to me and is what has led me to start searching for an alternative opinion. Will definitely read the guide you suggested but would love any more feedback that you gentlemen can provide.

        Sorry for the novel, and thank you so much!

        Comment


        • #5
          Originally posted by PV4Me View Post
          Gentlemen, thank you so much for the feedback! Will definitely read the "Solar Power Your Home For Dummies" guide.

          Regarding the output that I observed, it was definitely a bright and sunny day but it was around 11 am, which is why I began to wonder if I am already limited if we're producing the max at 11 am in early April. Since that day I have seen the inverters hit their 7700 watts max, multiple times a day, so it is definitely something that is happening frequently rather than only a few times a year for a few minutes. To give a better idea of the setup, my panels are high up on a hill with no obstruction and no shading, out in rural California, so if you can see the sun, the sun is hitting the panels. Of course the degree and intensity will vary but the fact that I am hitting that high of an instantaneous output so many times and not going over it makes me think that I am being limited.

          I did go through the PVWatts calculator before we started the project, and had entered in 24.375 KW capacity with 14% system losses (default), a 215 degree azimuth angle and a 22 degree tilt angle. That provided an estimate of 36,500 kWhr per year.

          I do completely admit that I am a newbie to solar setups and am looking to learn more but it can't just be coincidence that I never see the inverters really go past the 7700w max, can it? If they were to only hit that max for a few minutes a few times a year I could understand that I am more or less appropriately sized but the fact that I saw them hitting that 2 days after we installed the system, in April, and then subsequently observed the same multiple times afterwards is what has me worried.

          I just don't know who to trust when it comes to an inverter's maximum AC output specification. Installer says the manufacturer isn't showing the real max (his explanation being that the manufacturer would not be able to sell these units for residential applications if they did not show a lower maximum output). To be honest I am pretty skeptical of that explanation and my installer knows how to install a system but his technical depth and understanding of how all this works at a fundamental level is much weaker than I was hoping for. Our discussions always tend to end up with "just trust me, I have installed a lot of systems before."

          That is a bit unsettling to me and is what has led me to start searching for an alternative opinion. Will definitely read the guide you suggested but would love any more feedback that you gentlemen can provide.

          Sorry for the novel, and thank you so much!
          I was about to edit my prior post to this thread to include a thought about why all 3 inverters seem to have very similar output. I was going to add a comment that one likely reason is that they may be "clipping", or reaching their max. output while the array is producing more than the inverters can handle. However, and depending on several unknown (at this time) variables, mostly the irradiance, wind vector and ambient temp. at your site and at the time(s) of measurement, how much, if any clipping is unknown at this time. One indication but not necessarily a smoking gun is to observe that 7,7 kW output as holding pretty much rock steady for relatively long periods, say an hr. or so or more on very consistently clear and sunny days. The longer the output stays constant, likely the greater the disparity between an array's max. output and an inverter's capacity.

          That PVWatts output you post in your 2d para. above seems reasonable, if maybe a bit low. What's your ZIP ? I'm wondering about the time of day - 11 A.M. - for the 7.7 kW readings. At a 215 deg. az. and 22 deg. tilt (and, as an aside, a pretty good orientation for T.O.U. billing rates, at least as T.O.U. peak pricing times are currently), I'd expect max. output to be past solar noon by an hr. or more. If the 7.7 kW output(s) started at ~ 11 A.M. and lasted until say, 3 P.M. or a bit sooner, that would suggest clipping. That does not necessarily however, indicate how much the disparity is between max. system output and inverter capacity. If, and to the degree such a disparity exists at all, and depending on that day's conditions, it may be slight. Hard to tell.

          So, question: how long did those 7.7 kW inverter outputs last that day ? What was the amb. temp. like over the day? Kinda low and heating up a lot as the day progressed ? How was the wind ? This is about the time of the year when many mid latitude arrays in N. America max. their instantaneous output. As a general statement, things tend to stay high for about 6 weeks or so in March/April and roll off as temps. increase into the summer.

          Lastly, a slight clipping condition is not necessarily a bad thing. Example: If slight clipping is present, and that amount of (estimated) clipping results in a small decrease of, say, 1/2 of 1 % in terms of annual output that costs, say $30/yr. in terms of initial lost T.O.U. revenue, and to increase the inverter sizes would have cost, say a few hundred $$ to eliminate with larger inverters, and that 1/2 of 1 % of lost output will disappear anyway because of normal and expected panel deterioration in a couple of years or less, you'll be money ahead. That scenario however, is, to repeat, an example only.

          Comment


          • #6
            Originally posted by PV4Me View Post

            That is a bit unsettling to me and is what has led me to start searching for an alternative opinion. Will definitely read the guide you suggested but would love any more feedback that you gentlemen can provide.

            Sorry for the novel, and thank you so much!
            Your panel to inverter ratio looks appropriately to me. Yes, you might be not be fully realizing the panel's capability in the middle of the day in springtime, but the amount you are losing just isn't worth the cost of increasing the system size to capture it. As you get into summer, you get more production because the days are longer, but the peak power will fall because it is warmer, probably below the level at which there is any clipping at all.

            Long story short... be patient. As you get more familiar with your system's output throughout the entire year, it will put into better perspective your observations so far, and you will uunderstand better why your system was designed the way that it is.
            CS6P-260P/SE3000 - http://tiny.cc/ed5ozx

            Comment


            • #7
              PV4ME,

              I own 2 of the 6.0 version and my 6000w inverter will put out over 6100 prior to clipping. I have 12.96kw on 12kw of inverters. I have seen mine clip during cold sunny days.

              there is current limiting based upon heat sink temperatures. Due to my inverters being installed in the garage they are nice and cool, outputting maximum power. As a side note there is a new firmware recently released to up the output power maximum of your inverter. Mine has seen slight increases with every update. Originally it was 6040 watts then 6050 now over 6100...I think 6150 to be exact.

              your install is similar to a quote I did. The 325w max power panel has a very high voltage and I am curious how your strings are wired. At 80 panels I was forced to use 4 inverters due to voltages being near or over 600 vdc.

              Comment


              • #8
                Dont be afraid to call SMA and ask them about the design. They are also in Nor Cal, Rocklin Ca. I spoke with them recently and they were very helpful. SMA also has a very nice online design tool that displays outputs, efficiency, and makes inverter recommendations. Play around with that, PVwatts, Sunearthtools, RBI Solar, NOAA Solar Calculator... and any other tools. Personally, I found these interactive tools a good resource and a fun way to learn and gain a better understanding of solar systems and their components.

                Comment


                • #9
                  Hi guys,

                  Sorry for the delay in responding. Have some updates. Well it has been about a week since I had the system installed by my installer and I keep having issues.

                  Out of the 3 inverters, whenever the sun comes out bright and sunny, at least one of the inverters stops working with errors of "grid overvoltage" and then subsequent "grid connection fault". So on a sunny day one inverter creates 0w of power between noon and 5 pm and then comes on again near the end of the day. This behavior isn't limited to a single inverter, all 3 of them have experienced this issue on different days. It doesn't happen every single day but has happened 4 days already.

                  My installer had previously tested the wire that runs from the inverters to my main panel (was pre-existing from a previous solar install) and had said it was plenty for the amps that we would be pushing but now is saying that the wire length (~ 200 ft) is creating too much resistance and that this is the reason that I am having these issues. Currently I have #3 copper wire running which my installer believed would be good for 130 amps. The installer now wants to upgrade my wire or says the alternative is to "turn the inverters down" so that they don't make too much power and don't cause the issue with the wiring.

                  The zip code is 95672. I did call SMA in Rocklin however they were not very helpful over the phone and kept telling me to ask my installer any questions that I had.

                  Some more details, I am grandfathered in from a previous solar setup so I am not on T.O.U. billing with PG&E.

                  Also someone commented that panels will do 75-80% of their STC nameplate rating. Am I doing something wrong in my calculations because wouldn't 80% of 24.375 KW be 19.5 KW? I am seeing clipping at 23.1 KW (the max of the 3 inverters) so aren't my panels doing way better than the 75-80% of their STC nameplate rating?

                  Each inverter has 25 panels wired to it. 2 strings of 10 each and 1 string of 5 each.

                  Comment


                  • #10
                    So you obviously have issues. Existing wire or not the installer is on the hook for a system and they/you are in violation of the NEC. This should not be acceptable to any electrician with a license...or anyone that can do math... If they give you any crap about using #3 @ 96 amps continuous load they need their license pulled.

                    #1 - Your installer is wrong. #3 Copper @ 75 Dec is only good for 100 Amps...you have 96 continuous amps, code requires 125% as a minimum, then you have a 200+ foot run.... All of this current rating is prior to any de-ratings due to temperatures...
                    #2 - Due to #1 you have no less than 3.15% voltage drop or 7.57 volts of drop assuming exactly a 200 foot run
                    #3 - Your installation is not code compliant due to wiring being too small ignoring the 125% rule and also due to voltage drop not being considered
                    #4 - If all of this is in a 1" conduit you/your installer is screwed due to conductor fill and will need a larger conduit. At least 1.5", I would prefer 2" due to length and if any future expansions are possible

                    You need at a minimum...
                    40 A breakers on each inverter with #8 wire from inverter to 40A breaker (40A breaker is not a minimum, it is required)
                    A panel capable of at least 125A buss rating
                    Wiring with a 75 Deg C amp rating of 125A or higher to support a length over 200'
                    My choice would be at a length of 200' 1/0 copper, 2/0 if upgrades are considered...

                    I will repeat what the code says and many forget... IT IS A MINIMUM STANDARD
                    Last edited by Eleceng1979; 04-10-2017, 07:26 PM.

                    Comment


                    • #11
                      Originally posted by PV4Me View Post
                      Sorry for the delay in responding. Have some updates. Well it has been about a week since I had the system installed by my installer and I keep having issues.

                      Out of the 3 inverters, whenever the sun comes out bright and sunny, at least one of the inverters stops working with errors of "grid overvoltage" and then subsequent "grid connection fault". So on a sunny day one inverter creates 0w of power between noon and 5 pm and then comes on again near the end of the day. This behavior isn't limited to a single inverter, all 3 of them have experienced this issue on different days. It doesn't happen every single day but has happened 4 days already.
                      That's not unusual. High power output leads to voltage rise in the wiring to the main panel, pushes voltage above legal limits and the inverter trips. Note that even if wire gauges and run lengths are carefully designed, you can be pushing the voltage at the service entrance above the trip threshold - especially true with high power systems. We often assume the service entrance is close to zero impedance but that can be a bad assumption, and the time you start to see that is when you are sending (or drawing) a lot of power.

                      My installer had previously tested the wire that runs from the inverters to my main panel (was pre-existing from a previous solar install) and had said it was plenty for the amps that we would be pushing but now is saying that the wire length (~ 200 ft) is creating too much resistance and that this is the reason that I am having these issues. Currently I have #3 copper wire running which my installer believed would be good for 130 amps. The installer now wants to upgrade my wire or says the alternative is to "turn the inverters down" so that they don't make too much power and don't cause the issue with the wiring.
                      Right. Decreasing wire gauge (i.e. making them thicker) will slightly decrease voltage rise; this may help if you are right on the edge and just need a few volts. To make sure you can measure voltages at both sides of the cabling run. Reducing the output will also help but is obviously not ideal. Some inverters allow you to increase the high voltage disconnect level; depends on the inverter.

                      Also someone commented that panels will do 75-80% of their STC nameplate rating. Am I doing something wrong in my calculations because wouldn't 80% of 24.375 KW be 19.5 KW? I am seeing clipping at 23.1 KW (the max of the 3 inverters) so aren't my panels doing way better than the 75-80% of their STC nameplate rating?
                      Well, he said "over the course of a normal day, produce 75-80% of the nameplate STC wattage." On occasion you will see rated output. Here I see it after rain; the panels are clean and cool, the sun comes out and I see full power at the inverter. But over the course of a day, with normal temperature rise and dirt, I see closer to 75% of their rated output.

                      Comment


                      • #12
                        Originally posted by PV4Me View Post


                        My installer had previously tested the wire that runs from the inverters to my main panel (was pre-existing from a previous solar install) and had said it was plenty for the amps that we would be pushing but now is saying that the wire length (~ 200 ft) is creating too much resistance and that this is the reason that I am having these issues. Currently I have #3 copper wire running which my installer believed would be good for 130 amps. The installer now wants to upgrade my wire or says the alternative is to "turn the inverters down" so that they don't make too much power and don't cause the issue with the wiring.
                        Yes, this makes sense. If the wire is undersized, the voltage will rise at the inverter. If 3 AWG is used to carry 96 A ft 200 ft one way, that would be about 3.1% voltage rise, taking 240 V up to 248 V. That alone shouldn't be enough to trip the inverter, which can tolerate up to 264 V, but more extensive troubleshooting is needed to determine what else might be contributing.


                        Originally posted by PV4Me View Post
                        Also someone commented that panels will do 75-80% of their STC nameplate rating. Am I doing something wrong in my calculations because wouldn't 80% of 24.375 KW be 19.5 KW? I am seeing clipping at 23.1 KW (the max of the 3 inverters) so aren't my panels doing way better than the 75-80% of their STC nameplate rating?
                        You can use PVOutput.org to get an idea of what systems in your area can produce with respect to nameplate. For example, here is a smaller system in 95765 with WSW panels at 30 deg tilt that is recently peaking out at about 85% of nameplate (and suffers from some evening shade).

                        Over 90% for new, optimally oriented systems is possible, especially if you are relying on the inverter's reported production (often a few percent higher than what a revenue grade meter might show).
                        CS6P-260P/SE3000 - http://tiny.cc/ed5ozx

                        Comment


                        • #13
                          For reference most 240v transformers on homes are setup for 240v with max sag/load. This prevents undervolt conditions when 240v open circuit.

                          as a reference my 240v service is 251 unloaded. Once my 12kw of inverters are rocking it raises the whole house voltage to 253v. This is all with less than 25 feet of wire from inverter to meter with almost no voltage drop. So any wire undersized will create issue pretty quick in this example. It is so bad that my 6kw apc ups had problems and I had to open its acceptable voltage range once the solar went online.

                          Comment


                          • #14
                            Originally posted by PV4Me View Post
                            Hi guys,

                            Sorry for the delay in responding. Have some updates. Well it has been about a week since I had the system installed by my installer and I keep having issues.

                            Out of the 3 inverters, whenever the sun comes out bright and sunny, at least one of the inverters stops working with errors of "grid overvoltage" and then subsequent "grid connection fault". So on a sunny day one inverter creates 0w of power between noon and 5 pm and then comes on again near the end of the day. This behavior isn't limited to a single inverter, all 3 of them have experienced this issue on different days. It doesn't happen every single day but has happened 4 days already.

                            My installer had previously tested the wire that runs from the inverters to my main panel (was pre-existing from a previous solar install) and had said it was plenty for the amps that we would be pushing but now is saying that the wire length (~ 200 ft) is creating too much resistance and that this is the reason that I am having these issues. Currently I have #3 copper wire running which my installer believed would be good for 130 amps. The installer now wants to upgrade my wire or says the alternative is to "turn the inverters down" so that they don't make too much power and don't cause the issue with the wiring.

                            The zip code is 95672. I did call SMA in Rocklin however they were not very helpful over the phone and kept telling me to ask my installer any questions that I had.

                            Some more details, I am grandfathered in from a previous solar setup so I am not on T.O.U. billing with PG&E.

                            Also someone commented that panels will do 75-80% of their STC nameplate rating. Am I doing something wrong in my calculations because wouldn't 80% of 24.375 KW be 19.5 KW? I am seeing clipping at 23.1 KW (the max of the 3 inverters) so aren't my panels doing way better than the 75-80% of their STC nameplate rating?

                            Each inverter has 25 panels wired to it. 2 strings of 10 each and 1 string of 5 each.
                            Some of this stuff was unmentioned in your initial post. Installation/startup problems are unfortunate but do and can occur with some installations. Depending on what the inverters can accept for stringing, that stringing arrangement seems a bit unusual, but I'm not there, nor familiar with the inverter particulars, and it isn't my design. Still, pre existing wire or not, it does seem to me that "too much (electrical ?) resistance" is something that can be and ought to be known to a pretty good estimate in the preliminary design stage. How big was the prior array, and how old was it ? Any other electrical components/parts reused besides the wire?

                            As for that 85 % number, yes, your array( when it's operating correctly) is doin better than that. The 85% is a rough number, and without putting too fine a point on it, might be thought of as more of an estimate of some sort of average of max. daily instantaneous output/S.T.C. rating. Read the "Dummies" book for enlightenment.

                            At this point, your installer has responsibility to meet the terms of the contract you and the contractor signed, including code compliance as it relates to NEC, building codes and other requirements. Part of your responsibility to yourself is to understand how things work. I'm not there and not taking sides, but if what you write is a fair representation of what you're being told, some things don't sound quite right. I'm sure there's more to the story. Perhaps your grid voltage is too high/low causing the inverters to see a problem and shut down - that may be what the error or trip messages are trying to say. As I wrote, I ain't there.

                            PVWatts for Sacramento /10 % system losses and your stated orientation gives a max. hourly est. of ~ 20.1 kW or so for the period 04/01 - 04/15. Given that, 23.1 kW might be a stretch, but given that the array is new and depending on temps. for the day you saw (if they were low), and if your elevation is significantly above Sacramento, some ground albedo and (maybe) some cloud reflections, that number 23.1 kW) might be achievable as an instantaneous output for a few minutes or more. The idea of albedo and/or cloud reflections seems maybe a bit more likely to me given that you report it as having been achieved at 11 A.M. on an array with a 215 deg. azimuth.
                            Last edited by J.P.M.; 04-10-2017, 07:31 PM.

                            Comment


                            • #15
                              Originally posted by Eleceng1979 View Post
                              So you obviously have issues. Existing wire or not the installer is on the hook for a system and they/you are in violation of the NEC. This should not acceptable to any electrician with a license...or anyone that can do math... If they give you any crap about using #3 @ 96 amps continuous load they need their license pulled.

                              #1 - Your installer is wrong. #3 Copper @ 75 Dec is only good for 100 Amps...you have 96 continuous amps, code requires 125% as a minimum, then you have a 200+ foot run.... All of this current rating is prior to any de-ratings due to temperatures...
                              This is true if the #3 run is in raceway, cable, or in earth. Admittedly, it is unlikely, but if the run is in free air the ampacity is higher and would be OK for 96 A (notwithstanding problems caused by voltage drop/rise).
                              CS6P-260P/SE3000 - http://tiny.cc/ed5ozx

                              Comment

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