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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 annual 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.
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 annual 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.
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