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Please help me understand my production rate vs my system size.....???? Confused...

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  • Please help me understand my production rate vs my system size.....???? Confused...

    Hi,
    so, the good news is that i recently took the leap and bought a 10.06 kW system (capable of generating 12,000 kwh annually). I have 33 individual 305watt panels. I have a solaredge inverter.
    As i monitor the power generation, i've notice that my MAX production on a sunny warmish day is 8kW. it sometimes dips to 7.4, 7.5, 7.9, but never 8.1 or above....which leads me to believe that it's capped at 8.0.

    So, here's my confusion: 33 x 305W gives me ~10kW. My MAX generation rate is showing at 8kW on the App. I'm trying to understand what there's a difference.....should it say 10kW on the App when it's nice and sunny?

    Sorry for my noob-ness but i thought someone here might be able to explain in a way that i'd understand.

    thanks in advance!

  • #2
    No mystery at all. Panels do not generate their rated capacity. Panel specified capacity is under ideal test conditions in a laboratory called STL. Only in space will a panel produce full power. Real world is roughly 80 to 90% for a few minutes around solar noon. If you watch from sunrise to sunset, at first light you may only produce a few hundred watts. As the sun gets higher in the sky, power increases gradually until you hit solar noon which will be peak power of 80 to 90% of the spec. Then a few minutes afternoon power gradually decreases until sunset. If you plot power vs time you will have a Bell Curve. Something like this 3 Kw system in summer. Note solar noon occurs at roughly 1:30 pm. You must be one big energy hog to use 33 Kwh/day like me. National average is just less than 10 Kwh/day
    Last edited by Sunking; 04-10-2018, 03:17 PM.
    MSEE, PE

    Comment


    • #3
      Originally posted by Sunking View Post
      No mystery at all. Panels do not generate their rated capacity. Panel specified capacity is under ideal test conditions in a laboratory. Real world is roughly 80 to 90% for a few minutes around solar noon. If you watch from sunrise to sunset, at first light you may only produce a few hundred watts. As th esun gets higher in the sky, power increases gradually until you hit solar noon which will be peak power of 80 to 90% of the spec. Then a few minutes afternoon power gradually decreases until sunset. If you plot power vs time you will have a Bell Curve. Something like this 3 Kw system in summer.


      Yes, i've seen the graph, and that makes great sense. thanks.
      But in an 'ideal world', is what i'm stating below accurate?

      305Watt is the maximum amount of power that can be produced by one panel in an hour under ideal conditions. 33 x 305Watts equals ~10,000 Watts (ie. 10kW) of energy that can be produced under ideal conditions in an hour.

      Comment


      • #4
        Real life will never be "ideal" so you should expect a lower than 100% of the nameplate wattage as output for any pv panel.

        The Imp value is the highest amount of amps the panel can produce based on an Irradiance of about 1000w/square meter. There is also a factor due to temperature of the cells.

        Both of those variables are taken into consideration in the "lab" while the panel is being tested. You will never see "lab test" conditions in the field so the panel will never put out 100%.

        Comment


        • #5
          No you are confusing energy and power.

          Power = Watts, the rate at which energy is being consumed.
          Energy = Watt Hours = Watts x Hours.

          Example lets say you have a 100 watt light bulb and you run it for 10 hours. 100 Watts x 10 Hours = 1000 Watt Hours. The light uses 100 watt and if used for 10 hours consumes 1000 watt hours (1 Kwh) of energy.

          So your system is calculated to generate 12,000 Kwh/year. So your system on average generates 12,000 Kwh / 365 day = 32.8 Kwh per day. That means you have roughly 32.8 Kwh / 10 Kw = 3.28 Sun HOURS per day on average. So you are looking at the wrong numbers. You want to know how much energy (watt hours) you generate in a day, not how much power (watts) the system generates at a given moment in time.

          It is like saying you are asking that you have a 250 hp engine in your car. How far can I go and how long will it take me to get there? Complete nonsense right?

          So the answer to your question is your panels will never generate 10 Kw. At best around 8 Kw and if they generate 8 Kw x 1 Hours = 8 Kwh. You need to get your head wrapped around the difference between Power (watts) and Energy (Kwh). They are not the same thing.
          Last edited by Sunking; 04-10-2018, 03:52 PM.
          MSEE, PE

          Comment


          • #6
            Originally posted by sharpd1 View Post

            Yes, i've seen the graph, and that makes great sense. thanks.
            But in an 'ideal world', is what i'm stating below accurate?

            305Watt is the maximum amount of power that can be produced by one panel in an hour under ideal conditions. 33 x 305Watts equals ~10,000 Watts (ie. 10kW) of energy that can be produced under ideal conditions in an hour.
            305wh not watts and NO it can actually produce more than that if it is very cold out, but you should not expect it to produce that particularly on warm days.

            NOTE it is a 305watt PV module capable of spitting out 305watts at STC and in an hour would generate 305wh (see where the h goes)
            it is like this 305w X 1h = 305wh
            OutBack FP1 w/ CS6P-250P http://bit.ly/1Sg5VNH

            Comment


            • #7
              Those of us with some shade expect the edges to be clipped off that nice, south-facing-panels curve, till
              the sun rises high enough to clear the obstacles. Bruce Roe

              Comment


              • #8
                Originally posted by sharpd1 View Post

                Yes, i've seen the graph, and that makes great sense. thanks.
                But in an 'ideal world', is what i'm stating below accurate?

                305Watt is the maximum amount of power that can be produced by one panel in an hour under ideal conditions. 33 x 305Watts equals ~10,000 Watts (ie. 10kW) of energy that can be produced under ideal conditions in an hour.
                The operative word here is "ideal". The short and practical answer is usually something like this: Output of a 305 W panel will not be 305 W/panel under most conditions.

                Reason: You are confusing "ideal" conditions - which do not exist by definition - with something called "Standard Test Conditions" = S.T.C. (which do exist by definition).

                A new and clean panel's output is measured under controlled conditions with an irradiance source similar but not identical to the solar spectrum of 1,000 W/m^2 radiant energy flux in the plane of the array (P.O.A), no wind and with a cell temp. of 25 C.

                Most or all actual operating conditions will be different from that test condition. Usually, but not always, under clear skies and normal (perpendicular) irradiance and little wind, cell temps. will be the biggest factor that in lower than S.T.C. output, with cell temps. probably ~~ 25-30 C. above ambient and each deg. of cell temp. above 25 C. decreasing panel efficiency by something like ~ 0.5 %/deg. C. or so.

                Those cell temps. are influenced mostly by :
                1). Irradiance level (more P.O.A. irradiance increases cell temp. just like a black surface sitting in the sun gets warmer).
                2.) Wind (more wind decreases cell temp. just like when you blow on your food to cool it off).
                3.) Dirt/fouling on the array. More of a variable and perhaps, but not always of a minor nature, but in a somewhat more complicated way (more dirt generally decreases irradiance on the array with that irradiance decrease lowering output, but also decreasing call temps., with those temp. decreases tending to increase cell efficiency and thus increase output. The net result of all that usually being a decrease in output).

                It is also possible, and probably likely, that during some times, like around this time of year, on cold, windy and clear days, conditions may temporarily exist that will allow an array to exceed S.T.C. output. Those conditions won't last long at elevations close to sea level in warmer climates.

                Output in excess of S.T.C. output can also regularly exist at higher elevations where P.O.A irradiance levels can and do regularly exceed 1,000 W/m^2 and with amb. temp. relatively low, array orientations favorable and the wind is kicking ass. add to that ground snow cover albedo adding further P.O.A. irradiance increases. Think of places like Santa Fe, NM or in the Rockies with south facing arrays at 60 deg. tilts.

                Comment


                • #9
                  Thanks guys. I understand that i would only get 80-90% efficiency...that's why i used the word 'ideal'. i never said i expected it....I was trying to understand it at the 'ideal level'.
                  With that said, I'm not sure i've gotten a yes or no to this yet....so let me phrase the question again:

                  under IDEAL (lab/never attainable/whatever we want to call it) conditions, is this correct?

                  305Wh is the maximum amount of energy that can be produced by one panel in an hour under ideal conditions. 33 x 305Wh equals ~10,000 Wh (ie. 10kW) of energy that can be produced under ideal conditions in an hour.

                  Also, i'm still trying to understand why the max that i'm showing as producing is 8kW....Why doesn't it show as generating above 8.0kW on the App? is it capped by the inverter? is that normal? Anything I should/can do to make that more?


                  ps. i generated 55.7 kwhs yesterday super happy and excited!

                  Comment


                  • #10
                    Originally posted by sharpd1 View Post
                    Thanks guys. I understand that i would only get 80-90% efficiency...that's why i used the word 'ideal'. i never said i expected it....I was trying to understand it at the 'ideal level'.
                    It doesn't really work that way . "ideal" doesn't exist. The value you are talking about is STC (Standard Test Conditions) which is a very fast instant flash of light. The PV module does not have time to warm up and heat cuts performance. The light is very bright and most places in the US do not get that level of light any time. Light intensity (insolation) varies considerably with the weather and particulates.
                    The efficiency of the system also cuts down on production with losses in wires, inverter etc.


                    Originally posted by sharpd1 View Post
                    under IDEAL (lab/never attainable/whatever we want to call it) conditions, is this correct?

                    305Wh is the maximum amount of energy that can be produced by one panel in an hour under ideal conditions. 33 x 305Wh equals ~10,000 Wh (ie. 10kW) of energy that can be produced under ideal conditions in an hour.
                    No. 305W can be produced (usually +5w,-0w) on that module when it was new, clean, and under STC. STC means a "flash" of light. if it runs for an hour, then the module will heat up and performance will drop. Also that is in DC, If you measure it in AC then you have losses in the longer wires and inverter.

                    Also "10,000 Wh (ie. 10kW)" these are two different things 10,000 wh = 10kWh NOT 10kW


                    Originally posted by sharpd1 View Post
                    Also, i'm still trying to understand why the max that i'm showing as producing is 8kW....Why doesn't it show as generating above 8.0kW on the App? is it capped by the inverter? is that normal? Anything I should/can do to make that more?

                    ps. i generated 55.7 kwhs yesterday super happy and excited!
                    What inverter do you have?
                    are the modules all facing the same direction?
                    are there shadows?
                    OutBack FP1 w/ CS6P-250P http://bit.ly/1Sg5VNH

                    Comment


                    • #11
                      Originally posted by ButchDeal View Post
                      No. 305W can be produced (usually +5w,-0w) on that module when it was new, clean, and under STC. STC means a "flash" of light. if it runs for an hour, then the module will heat up and performance will drop. Also that is in DC, If you measure it in AC then you have losses in the longer wires and inverter.

                      Also "10,000 Wh (ie. 10kW)" these are two different things 10,000 wh = 10kWh NOT 10kW



                      What inverter do you have?
                      are the modules all facing the same direction?
                      are there shadows?

                      Thank you.

                      sorry, typo on the 10kW vs the correct 10kwh

                      Inverter is SolarEdge....my representative told me that it's the "best one they have and that it's about 2k extra that he's including for free"....for what it's worth......could have been a complete sales pitch.
                      All modules are facing the same direction, yes. facing south west -ish....
                      There are no shadows from 10am to 430ish this time of year. i was producing 8kW from 1130ish to 430 yesterday (almost flat line) and then dropped to 5.3kw by 6pm and 0 by 8pm.

                      just trying to understand why it never goes above "8.0kW output"...

                      Comment


                      • #12
                        Originally posted by sharpd1 View Post
                        Inverter is SolarEdge....my representative told me that it's the "best one they have and that it's about 2k extra that he's including for free"....for what it's worth......could have been a complete sales pitch.
                        Ok so from that information it sounds like your inverter is the newer HDWave version and is the 10kW model so SE10000h Which means that your inverter is capable of spitting out 10kW at any time. BTW the next one down is the SE7600h which is capable of only 7.6kw so you do not have that one.


                        Originally posted by sharpd1 View Post
                        All modules are facing the same direction, yes. facing south west -ish....
                        There are no shadows from 10am to 430ish this time of year. i was producing 8kW from 1130ish to 430 yesterday (almost flat line) and then dropped to 5.3kw by 6pm and 0 by 8pm.

                        just trying to understand why it never goes above "8.0kW output"...
                        SolarEdge does not make any 8kW inverter. check for yourself:
                        https://www.solaredge.com/sites/defa...tasheet-na.pdf


                        your system is facing SW so you should expect the peak to be after noon, the more west the latter the peak should be.
                        If you want to post some photos of the graph or give access we can take a look.
                        production dropping at 6pm would be fairly normal depending on late shadows.
                        OutBack FP1 w/ CS6P-250P http://bit.ly/1Sg5VNH

                        Comment


                        • #13
                          Originally posted by ButchDeal View Post

                          Ok so from that information it sounds like your inverter is the newer HDWave version and is the 10kW model so SE10000h Which means that your inverter is capable of spitting out 10kW at any time. BTW the next one down is the SE7600h which is capable of only 7.6kw so you do not have that one.




                          SolarEdge does not make any 8kW inverter. check for yourself:
                          https://www.solaredge.com/sites/defa...tasheet-na.pdf


                          your system is facing SW so you should expect the peak to be after noon, the more west the latter the peak should be.
                          If you want to post some photos of the graph or give access we can take a look.
                          production dropping at 6pm would be fairly normal depending on late shadows.


                          Thank you very much.
                          Unfortunately, i think i actually do have the 7.6kw version of the inverter. I called Tesla asking this question and that's what i was told. He also told me that it is configured "to produce above the rated kw..." hence the 8kW output.
                          We have a limit on how much solar we can have here in Northern Virginia....a 10kW system is the max you can have before Dominion energy will start imposing ridiculous penalties....
                          Could that have something to do with them putting in the 7.6kw inverter?

                          here is my curve for yesterday. ps. i don't know how/why it shot up to 16kw for a short while....
                          Attached Files

                          Comment


                          • #14
                            Originally posted by sharpd1 View Post



                            Thank you very much.
                            Unfortunately, i think i actually do have the 7.6kw version of the inverter. I called Tesla asking this question and that's what i was told. He also told me that it is configured "to produce above the rated kw..." hence the 8kW output.
                            We have a limit on how much solar we can have here in Northern Virginia....a 10kW system is the max you can have before Dominion energy will start imposing ridiculous penalties....
                            Could that have something to do with them putting in the 7.6kw inverter?
                            No Dominion is looking at the DC size. The only reason to put in a smaller one particularly the SE7600 would be that is is the largest that can fit in most panels without major more expensive electrical work. It is the flag ship model with lots of options. Take a look at the inverter and you should see the model number on the side of it.



                            Originally posted by sharpd1 View Post
                            here is my curve for yesterday. ps. i don't know how/why it shot up to 16kw for a short while....
                            Well that production curve does not come from the inverter but tesla monitoring and there is a hickup in it were it is showing some production early (the big spike and then valley).

                            Also looking at the array it is shaddowed. The higher roof with more larger array is going to shadow the lower two sections till the sun gets latter (more west) in the sky,
                            By that time the sun is lower in the sky and you will have shadows from the trees.


                            OutBack FP1 w/ CS6P-250P http://bit.ly/1Sg5VNH

                            Comment


                            • #15
                              Originally posted by ButchDeal View Post

                              No Dominion is looking at the DC size. The only reason to put in a smaller one particularly the SE7600 would be that is is the largest that can fit in most panels without major more expensive electrical work. It is the flag ship model with lots of options. Take a look at the inverter and you should see the model number on the side of it.


                              Well that production curve does not come from the inverter but tesla monitoring and there is a hickup in it were it is showing some production early (the big spike and then valley).

                              Also looking at the array it is shaddowed. The higher roof with more larger array is going to shadow the lower two sections till the sun gets latter (more west) in the sky,
                              By that time the sun is lower in the sky and you will have shadows from the trees.

                              Here's a better report direct from the solarcity app....looks more accurate.....
                              You're certainly right about everything noted above regarding shadows and trees and etc.

                              I guess as long as i'm producing close to the estimated 12,000 kwh annually, i shouldn't care if the generation is maxed out at ~8kW at any given moment.....right?
                              Attached Files

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