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  • #31
    Originally posted by JRqwertyui View Post
    Here's my dumb question for the year. When doing the price comparison computing the $ /per Watt and posting on this board, are people using the DC or AC rating of the system ? or the annual output ? One gets a very different number depending on which is used.... often vendors will give you only the DC rating...
    The best way to compare a system $/watt is using the installed price divided by the DC wattage. You can then determine the final cost per watt after you subtract any rebates or Fed tax incentives.

    Knowing what your system is supposed to provide in a yearly AC kWh is also a good marker but that number can vary based on the weather and dirt conditions so don't let a vendor convince you that their calculated yearly kWh value is 100% accurate.

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    • #32
      Originally posted by JRqwertyui View Post
      Here's my dumb question for the year. When doing the price comparison computing the $ /per Watt and posting on this board, are people using the DC or AC rating of the system ? or the annual output ? One gets a very different number depending on which is used.... often vendors will give you only the DC rating...
      Both DC (total panel wattage for the entire system) and annual system output (or output per installed STC kW of panels for example) have uses, and both as well as other descriptors can provide useful information, but they are not the same.

      The most common way I've seen used is to discuss entire system price before any rebates, tax credits., in terms of $$ (or currency) per the entire STC (D.C.) W of panels. Other methods then sometimes/often are added for further explanation(s) and descriptions of system performance or characteristics.

      So, other ways, such as $$/kWh production per time period (per year for example), per installed STC kW of panels also have use. Sometimes the units get convoluted and a bit confusing.

      Most descriptions use or at least start with total $/installed price before rebates/tax credits per total STC W of panels and go from there.

      So, paying $15,000 bottom line to a vendor for a 5 STC kW turnkey system (after any/all of their come-on/B.S. rebates etc.< would have a price usually describes as $15,000/5,000 STC W = $3.00/ STC. W. Often, the STC letters are left out but implied or assumed known.

      If that system then produces 9,000 kWh/yr., the price of the annual generation will be $15,000/(9,000 kWh/yr.) = $1.67yr/kWh.

      The absolute value of the numbers are different because they are describing different things using different units.

      The definition police won't whack your peepee for using more than one definition, provided it's clear from the context what you are talking about. Others can always ask for more information.

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      • #33
        The industrial systems proposed recently here, used the peak AC power the inverters could
        deliver. Same as I do. Some also included a DC:AC ratio, showing how much more the panel
        DC rating total was over the AC. The E-W trackers were much more that 1:1, to compensate
        for panels mounted on a shaft parallel to the ground instead of parallel to the earths axis, here
        at 42 deg Lat. Bruce Roe

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        • #34
          Originally posted by bcroe View Post
          The industrial systems proposed recently here, used the peak AC power the inverters could
          deliver. Same as I do. Some also included a DC:AC ratio, showing how much more the panel
          DC rating total was over the AC. The E-W trackers were much more that 1:1, to compensate
          for panels mounted on a shaft parallel to the ground instead of parallel to the earths axis, here
          at 42 deg Lat. Bruce Roe
          I took the sense of JRqwetyui's question to be what do most folks use as a measuring stick for incremental system cost when discussing system prices. Since the usual, and I'd suggest more common discussion is about and relating to smaller residential systems, either on or off grid, I took that meaning and sense of the question to be either system $$/STC W size, or kWh/yr per STC W (or STC kW), with the former being the more common, but both having usefulness.

          If I have a quote for, say, 5,940 STC W of panels hooked to a, say, 5 kW inverter, and the quoted price from that vendor is for, say, $19,305, sitting on my roof and ready to go, how do I best describe my price per W from the vendor when making bid comparisons ? $19305/5,940 W, or $19,305/5,000 ?

          Suppose another vendor come in with a 5,940 W system hooked to a 6 kW inverter for, say, $19,700 ? What do I do to compare prices and value between quotes ?

          In such cases, I might get a better price comparisons by using a model such as PVWatts that will allow me to SWAG compare the initial cost of bids to the annual output using DC-AC ratios, etc,

          I'd probably do that anyway, but using inverter size by itself, while nice to have is, in itself, insufficient IMO as the sole size descriptor.

          Respectfully,

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          • #35
            Originally posted by J.P.M.
            I'd probably do that anyway, but using inverter size by itself, while nice to have is, in itself,
            insufficient IMO as the sole size descriptor. Respectfully,
            Yes that is true. But that is what is being used for some pretty big stuff here. I see 2MW
            proposals for fixed panels set for our (considerable) 42 deg Lat, but unable to shake off
            winter snow. Another company proposed a 2MW E-W tracker sys able to throw off snow
            but with a very high DC-AC ratio due to a level tracking pivot shaft. These have considerably
            different costs, energy collection potential, (precious) farmland occupied, maintenance level,
            operational noise, and probably more I forgot. But the same inverter capacity is used for
            state/Fed rebate calculation, as well as credit against state mandated renewable energy
            construction. Something else never mentioned is the chosen site relative output degradation
            by clouds, a pretty big factor over this state.

            At some of these zoning meets, I do wonder if I should be doing more to demonstrate what
            is really possible/desirable? Here one rating system could make my sys look great, another
            terrible, none are really apples to apples. I will just say Wed managed 120 KWH from the 15KW
            inverters, some would call that 8 sun hours. Thurs was cloudy, generally acknowledge to give
            10% to 30% level performance, but it made 74 KWH anyway. Meets my goals. Bruce Roe


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            • #36
              Great information, solar panels are complicated as I thought. I am just a newbie and very interested in the solar panel because it's environmentally friendly. I think I need to learn more before converting to solar panel from the usual generator.

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