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  • System rating vs design total capacity

    Hello, I have a simple question about my system. It was sold to me as a 10.2 kw system 34 300 watt panels. On the form for the power company interconnect I have 3 different figures.

    Nameplate rating: 10.2kW

    Inverter Total Rating: 10 kW

    AC system Design Total Capacity: 7.85 kW

    My question is how do they come up with the last figure of 7.85? Is it due to loss? Incidentally over the past 20 some days it has been on the Solar Edge peak production never goes much above 8 kwh around mid day.

    Thanks

  • #2
    Look at the specs for your panels. The highest number is perfect conditions. Here is an explanation that may help:
    https://newenglandcleanenergy.com/en...sting-matters/
    9 kW solar, 42kWh LFP storage. EV owner since 2012

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    • #3
      Yes on average the DC energy looses about 20% by the time it gets converted to AC due to many factors like line losses, dirty panels, voltage drops, shading, etc. have the panels ever been cleaned? get out your soapy water and hose and go to town you will see 5 to 20% increase depending on how dirty they are.

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      • #4
        Originally posted by Ampster View Post
        Look at the specs for your panels. The highest number is perfect conditions. Here is an explanation that may help:
        https://newenglandcleanenergy.com/en...sting-matters/
        Very good explanation! Thanks

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        • #5
          The nameplate rating is the panel "STC" (Standard Test Conditions). Kind of like a HP rating for an ICE. It's the product of each panel's STC rating and the number of panels. 10.2 kW = 300 W/panel * 34 panels. That gives you what is usually called the array size (in kilowatts).

          The inverter rating is what the device that's the interface between the array and the grid is designed (claimed) to put out. The array size and the inverter are usually but not always rated for something like the same output. An inverter output much smaller than an array size will not utilize everything the array will put out, particularly on sunny days.

          The AC system total capacity is the somewhat obscure part and not as simple to understand. I'd need to see the source to help me better understand what that term means specifically. Because you have a 10 kW inverter, it may have something to do with array orientation or other array specific variables.

          Back to the ICE analogy, an ICE rated at 300 hp will probably never be required or (because of mechanical inefficiencies in the vehicle drive train) never be able to deliver power at the rate of 300 hp to the driving wheel(s). A 10.2 STC kW array under ideal conditions and optimal orientation (~ solar noon for a south facing array on a very cool and sunny day) will probably have a max. power (kW) output of something like 85-90% of that 10.2 kW output.

          Ampster's reference has several omissions and errors in it. For more accurate information, I'd respectfully suggest you download a free (but perhaps slightly dated) version of "Solar Power Your Home for Dummies". Spend an hour or so with the book and the article errors will be evident. After the read, come back and I or others can answer any clarifying questions you may have.

          As another suggestion, to help avoid more confusion, I'd stick with the STC numbers and know that that method does a better job as a comparison tool. PTC numbers claim to better represent "real world " conditions but really don't because real world conditions are always different form one location to the next and also minute to minute at any location and so can't be standardized. STC conditions do a better job as a comparison tool. Also, because PTC ratings use different irradiance levels, temperatures and wind conditions than STC ratings use, they do little more than further muddy the waters for those trying to compare things on equal terms. So, you wind up with different "standards". That situation only adds unnecessary confusion - confusion that peddlers and conmen jump on to screw the solar ignorant.

          Welcome to the neighborhood.

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          • #6
            Originally posted by khanh dam View Post
            Yes on average the DC energy looses about 20% by the time it gets converted to AC due to many factors like line losses, dirty panels, voltage drops, shading, etc. have the panels ever been cleaned? get out your soapy water and hose and go to town you will see 5 to 20% increase depending on how dirty they are.
            At this time the automatic panel washing system (rain) is operating, fairly regular and
            frequently at night. Here DC losses are about 1 2/3 %, AC losses 3 1/3%, Inverter
            losses 5%, near 10% total. Do not think I would call clouds DC losses, but we have
            plenty of them at 61084. The commercial solar systems proposed here have an
            inverter output AC rating, with a DC/AC ratio (generally greater than 1). Maybe
            residential ought to do the same. Bruce Roe

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            • #7
              Crankster64
              Did you find out how your power company calculated that Total Capacity number? I looked up my NEM agreement and for my power compny (PG&E) it is the total of the PTC rating of the panels times the efficiency of my inverter.
              9 kW solar, 42kWh LFP storage. EV owner since 2012

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              • #8
                Originally posted by Ampster View Post
                Crankster64
                Did you find out how your power company calculated that Total Capacity number? I looked up my NEM agreement and for my power compny (PG&E) it is the total of the PTC rating of the panels times the efficiency of my inverter.
                Back in the day, the CA rebates payment amount was based on array wattage using the PTC rating of the array. That number is no more or less representative of annual array output than the STC rating of the array. They differ by what's mostly a constant of proportionality of ~ 0.85 or so.

                I'm only suggesting that having more than one "standard" adds unnecessary confusion to a system that is inherently confusing. The PTC rating system may well result in stated array size numbers that are numerically closer to what are more likely in the field to be max. instantaneous output on a cool, sunless day - but so what ? Think about it. Most days are not completely clear and cool, and even so, max. array output is for no more than an hour or so per day on the best of days. I'm not sure what derating an array's size by 15 % or so gets you in terms of finding the figure of merit that is kWh/yr. output besides a different multiplier for whatever array size is used. But since everyone these days seems to buy and sell arrays based on STC numbers - not PTC numbers, having two rating systems, STC and PTC does little more than muddy the waters for Joe 6 pack. Point is: Choose one system and stick with it. More than that only muddies the waters and does no more than potentially cost the solar ignorant money for no benefit.

                My array was rebated based on 4,567 W for a 16 X 327 W array. That says nothing about an array's likely annual output (even before any accounting/estimates of possible shading) any more than the STC rating says about annual output. Some informed opinion at the time (2013) suggested to me the PTC ratings were used for rebate purposes to result in lower reimbursements per application and so spread/stretch the funding out.

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