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Need help understanding how module watts translates into watts fed into the grid

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  • Need help understanding how module watts translates into watts fed into the grid

    My apologies for what must seem like a knucklehead question but I'm extremely new to this. I want to get an idea of whether my newly installed system is producing about what would be the expected output of watts in regards to offsetting my electrical consumption. I live in Palm Springs in the southeastern California desert where cloudy days are headlines in the local newspaper. My contractor installed a SolarEdge SE7600A-US inverter, thirty LG Mono-X LG335S2W-G4 modules rated at 335 watts each, and an equal number of SolarEdge P400 optimizers. All of the equipment is brand new (installed just over a week ago). The modules are mounted on a flat roof at what I believe is a 7 degree tilt, all facing south. Shading is never an issue. My assumption is that the rated output of the setup is 10,050 watts out of the modules under optimum conditions.

    What I cannot figure out, at least so far, is what I might expect in terms of AC power production into the Southern California Edison grid. I am aware that maximum output in watts right out of the panels is rarely going to be hitting the 335 watt mark, and I am aware that there are all sorts of losses along the line from the modules to my electrical panel due to the conversion and all sorts of other issues. But on a bright clear late summer's day around 12:30pm, what might I expect in terms of AC power output as measured in watts? The last time I checked under these conditions, the "Pac[W]" (AKA "Power Prod" on another screen) reading on my inverter's display was 7,744, which I assume is what's used to reduce the amount of juice I'm paying Edison dearly for. If that's the case, and if the conditions I've described are pretty decent for production purposes, then is "maximum AC output in watts under best case conditions = 0.75 x total rated module DC output in watts" a standard rule of thumb? I'm just looking for a ballpark figure here. If 7.7 Kw from this system around noon is within the expected range, then I'm happy. Should I be happy?

    Thanks,


    Nick in Palm Springs

  • #2
    Your inverter's output is limited to ~7600 W, so what you are seeing is the most you will ever see. If you had a bigger inverter, you'd get more, but for a variety of reasons, 7600 W might have been the best choice for you. Loading up 10 kW onto the 7.6 kW inverter is kind of a stretch in such a hot location, but hopefully your inverter is mounted someplace where it can stay relatively cool.

    CS6P-260P/SE3000 - http://tiny.cc/ed5ozx

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    • #3
      Generally, you only get about 80% of panel nameplate harvest, and when the sun angle is bad (anytime other than solar noon + - 10 min) production falls off.
      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

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      • #4
        Your assumption about output is mostly not correct. You will rarely hit the STC rating of your array. You need to be educated about what you bought, and how it works, as well as its limitations and strengths.

        Start by downloading a free, but slightly outdated copy of "Solar Power Your Home for Dummmies", or get an updated version at bookstores/Amazon for ~ $20 or so.

        After reading it, you'll have the basic knowledge you'll need to make (better) sense of the answers you get here and thus, use this information resource more efficiently.

        BTW, I'm quite familiar w/the Coachella Valley, having lived there for several years. SWAG: expect max. output for your system very ~~ 75% or so of STC output for this time of year.

        Read the book for more info on why.

        Welcome to the neighborhood and the forum of few(er) illusions.
        Last edited by J.P.M.; 09-08-2017, 10:28 PM.

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        • #5
          With a 7% tilt your winter production will be considerably less than you are seeing now. Your panels will be cooler than summer which helps production but the short days and low angle of the sun don't match up well to a nearly level array.
          2.2kw Suntech mono, Classic 200, NEW Trace SW4024

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          • #6
            Originally posted by NPS View Post
            If 7.7 Kw from this system around noon is within the expected range, then I'm happy. Should I be happy?
            I just took a superficial look at PVWatts (a program which attempts to estimate production based on location and configuration) and in September there is no reason to not be happy, You are 'within the expected range' as best as I can tell.

            As to the question "Should I be happy?". Well, if you are happy with the panels mounted on a flat roof then I think you should be happy. If you had other places in mind, either a roof with a pitch or a ground mount system with a lot of elbow room, then you might be a little miffed. As littleharbor mentioned, you could be getting more winter production with more tilt. Also, the inverter size might limit you even a bit more in peak summer than it is now.

            PVWatts is a very easy program to manipulate and understand, you can find it here: http://pvwatts.nrel.gov/pvwatts.php but if the flat roof is the best place for the panels, then I wouldn't even bother with it.
            Last edited by AzRoute66; 09-09-2017, 03:07 AM.

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            • #7
              Nick,

              I now have a question for the forum that I will ask for you: "If I have panels at 7 degrees tilt, and I am hitting the inverter ceiling at solar noon in mid-September, what can I expect from mid-May through July? Is it going to be max'd out from 10:00 AM to 2:00 PM? What percent of the power is not going to be getting through? Are these questions I should be asking the designer/installer?"

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              • #8
                It seems you have enough panels to keep your inverter at capacity despite the low angle your
                roof mount gives, DC wiring losses, and converter efficiency. You have the additional advantage
                that as the summer sun rises and sets more to the north, the low angle works better. Sounds
                like a decent design, if the above were the intent. The AC wiring losses won't show up at the
                inverter readout.

                With the huge range of momentary situations for a PV solar system, there will always be some
                component that is limiting production at that moment. That doesn't mean a "fix" is needed. Bruce Roe

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                • #9
                  Originally posted by AzRoute66 View Post
                  Nick,

                  I now have a question for the forum that I will ask for you: "If I have panels at 7 degrees tilt, and I am hitting the inverter ceiling at solar noon in mid-September, what can I expect from mid-May through July? Is it going to be max'd out from 10:00 AM to 2:00 PM? What percent of the power is not going to be getting through? Are these questions I should be asking the designer/installer?"
                  Hard to get real specific, but the question is valid.

                  I have some modeled data for PV performance using TMY3 data for Thermal, CA I used for design of my solar magnum opus in Borrego Springs.

                  Using that data, a 7% tilt and 180 az., May monthly output per installed STC kW is ~ 180 - 190 kWh. In Sept., the monthly output per installed STC kW is ~ 140 - 150 kWh.

                  Annual output per installed STC kW is ~ 1,775 kWh/yr. per STC installed kW.

                  The PVWatts data with 10% system loses is a bit less optimistic at 162 kWh/month and 134 kWh/month, May and Sept., respectfully, with an annual output of 1,569 kWh/yr. All estimates include adjustments for temperatures. My model runs fairly close to SAM for the most part, and was modeled as a ground mount, which may explain some of the variation in output, one model to the next.

                  The Thermal, CA temps. are a bit higher than Palm Springs, but the rest of the data, including insolation estimates are quite close.

                  Using TMY3 data for Thermal, CA, May's aver. temp. is 27.2 C. Sept.'s average temp. is 30.8 C.

                  Using a 7 deg. tilt angle, estimated average operating cell temps. for a ground mount are 45.8 C. for May, and 51.9 C. for Sept., but those are a bit deceiving as they weigh early A.M./late P.M. temps. equally with mid day temps., so caution is advised about usin g them with some judgement.

                  Given what I've measured on my roof at zip 92026, I'd expect/SWAG a roof mounted system in Palm Springs to run maybe 5 - 8 C. warmer than my ground mount desert array estimates of cell temp.

                  ADD: The P.O.A clear sky global irradiance estimate for that orientation of 7 deg. tilt, 180 deg. az. is ~ 262 kWh/ m^2 for May, and ~ 214 kWh/m^2 for Sept.
                  Last edited by J.P.M.; 09-09-2017, 02:58 PM.

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