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  • #46
    Originally posted by Mike 134 View Post

    I never compared prices, Price for the 7600 with a revenue grade meter option was $1986.
    Thank you.

    Just wondering what the cost differential savings for the inverter might be vs. the value of the possibly clipped electricity by getting the smaller inverter.

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    • #47
      Originally posted by SolTex View Post
      I totally agree that having one industry wide accepted standard for rating solar panels is a good thing. Although, even under the current rating system, there are actually TWO ratings available for most quality panels: STC and NMOT.
      NMOT is very similar to NOCT (nominal operating cell temperature) That is a rating when the cell is as hot as it would be in the sun on a day that was 25C. However, you can calculate NMOT/NOCT from STC if you have the temperature coefficients (which is part of almost every panel's data sheet.)

      There's also PTC, which is a rating at a more reasonable insolation (i.e. less than the ideal of 1000 w/m2, which you rarely see.) Again, it is easy to calculate PTC from STC.
      Why did the industry settle on the STC rating
      Because it's easy to test. The panels can be at room temperature (no ovens involved) and they can be tested with a flashlamp in under a second.
      But it's not the accepted rating system that is the problem. It's the fact that when a customer is sold a solar system of a given "size" based on the STC rating they are, in MANY cases, not advised that they will probably never see the power output level they think they are paying for.
      By the same measure, they are not told they will see 10,800 watts AC out of a 10.8kW STC DC system. And this is mainly because no one cares what the peak AC output is.

      And it's a pretty useless number anyway. I mean, the installer has to know that number to accurately size breakers and conductors and safely install them in a panel, but the person whose house it's on doesn't care. What he DOES care about is how much power it will generate over the year. And every good installer provides that number.

      And no matter how accurate you get on the AC production, again, it won't matter. In fact, if you drop the yearly production number and instead get very accurate about the max AC production, you are doing the customer a disservice. Due to panel orientation, local weather, trees, soiling etc a max power number doesn't tell you much about what max energy production will be. A system where the installer tells you "this is a 10,800 STC system that will give you over 8500 watts max output!" but only does that for an hour a day due to the trees all around is not a well installed system, and they will likely be unhappy with it.

      And even when you know yearly production there is still a lot of influence on total cost/savings based on whether you are on a TOU plan, the details of your net metering agreement etc.

      Over the past decade there has been a trememdous amount of work done on forecasting software that gives you a very accurate prediction of your total power bill reduction based on all the above. Good installers not only have this software, but they are experienced at using it (which is important.) This gives potential customers an accurate way to predict how much they save, and thus make better decisions on price tradeoffs.

      As an engineer my big goal is to understand the VI curve of the panel as well as the maximum power over the conditions it will be used in. The available data (Voc, Vmpp, Isc, Impp, Pmax @ STC, temperature coefficients for Vmpp and Impp, fusing and max voltage ratings) does that for me. I would be very opposed to any change in that, because then you'd see some panels rated one way and some panels rated another way, and you wouldn't be able to trust values on a datasheet any more.

      In most solar installation companies I have talked to there is "that one guy" who designs their systems and takes all that into account. Usually it's only one guy even at bigger installers because it's really pretty easy nowadays, and both the forecasting software and the installation manuals for the inverters they use tell you in great detail what you can (and can't) do. And because most installers has a few standard designs they try to use for everyone.

      So what we have now works pretty well.

      It may be true that a "competent solar installer" will give the customer an estimated annual kWh production number, but I would not agree that's the only number people care about.
      Agreed. They also care about total savings, which is more complex (but again comes from forecasting SW.)
      A new solar system is a very expensive purchase. Most customers will be extremely interested (at least initially) in closely monitoring the system to "see how it is doing". In most cases, the customer is provided a app to install on their computer or their phone. Most app's will tell them what the system is producing on a daily, weekly, monthly and yearly basis. In many cases, this leads to the exact situation described by the OP of this thread: a customer who was sold a 10.8kW system looks at the app and discovers that their output starts clipping at the 9.0kW level. Who can blame that customer for feeling like they got screwed?
      Well, I can, if they never asked. It's like buying a gas car that is rated at 300hp but putting it on a dyno and finding out that it only generates 260hp at his home in Denver. Did that car buyer get screwed? Whose responsibility is it to educate him on the effects of altitude on a normally aspirated gas car?

      Of course, the simple solution to this problem would be education. Every potential solar customer should educate themselves about how solar systems work and about the difference between DC panel output and AC inverter output.
      Well, I think every customer should have the opportunity to educate themselves. Whether they do or not is up to them.

      The 82 year old widow two doors down recently got solar because she also got an EV and it seemed to make sense to her. She doesn't care about maximum production, or STC vs PTC. She just likes that her power bill went down.
      Last edited by jflorey2; 04-01-2023, 01:42 PM.

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