No announcement yet.

New to Forum & Solar - I have a choice to make...

  • Filter
  • Time
  • Show
Clear All
new posts

  • #76
    Originally posted by ButchDeal View Post

    I have seen much closer than that though I wouldn't bet on a TMY monthly matching any one month of real life data. there are too many weather extremes.
    I generally do the comparisons when we get complaints about site not performing as expected, and for the most part they are pretty darn close. We had one or two where a new designer of ours made a mistake and missed shadows a little farther away. A little training there.

    Usually the complaints are just what you say though, or homeowner divided the annual by 12 and expected every month to be the same, we give them the full prediction and they see it increasing on different months with some explanation of temperature etc. Oh yeah or the other big complaint is that they see the bill with labels for consumption and generation which do not match the generation they get from our monitoring, why is their bill not showing all the power? ( I love that one)

    Annually we are usually within 10%, though I think you have been looking at these over longer periods.

    Though I will note you mentioned +/- x% have you really seen the "+" variance be that large? We rarely see the "+" side nearly as much as the "-" side. other than a little in spring and fall, there is usually not much on the "+" side.
    Butch: Thank you for the reply and info. I believe we're mostly on the same page.

    The variations I seem to be seeing appear to be, for the most part, about or approx. evenly distributed about mean values for both the 31 day running periods and the 365 day running periods, but we may be talking about different variables or the same type of variables measured in different ways.

    FWIW, I learned the hard way about the need to be specific when discussing variables of this sort when discussing solar performance.

    On variation between what the various models, PVWatts, SAM, my model (based on a combination of TMY data and the Gueymard 2 band model, and the HDKR model), and what the Sunpower monitoring of my system is showing (after correction for overestimating actual system output compared to the SDG & E meter by 0.5 % or so), my running 365 day average actual production is 9,179 kWh/yr. including a year round late afternoon shading loss of what I estimate at ~ 3-5 % depending on season, with minimum annual average running total production of 8,883 kWh/yr., max. of 9,530 kWh/yr., population std. dev. of 177 kWh/yr., based on ~ 1,055 data.

    The models I use seem to show an annual output of ~ 9,500 kWh/yr. and are for most metrics within ~ 1% of one another, all using a 3 % fouling factor but without any account of the aforementioned shading loss. Based on daily running total output of 365 day average, that would give me an [U]annual[/U] variation of +3.7%/-3.2 %, but being mindful that's only based on ~ < 3 years, or ~1,055 data composed of 365 day running totals.

    Still keeping in mind that's only 3 yrs. of data, that +/- 3% or 4% variation may or may not be representative of long term variation in system performance, but it does appear to perhaps be somewhat symmetric in terms of its distribution, as is my 31 day running total output variation on the adjusted PVWatts same period running totals which variation is, to repeat, ~.+/- 35% or so.