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Not a rule, just something that happens often. I noticed this with my home system, lots of small off-grid systems, and a couple of big systems as well, sometimes we'll get close (or over) STC power but most of the time it's around 70-90%. I guess it has to do with less than optimum angle, sometimes temperature, system losses, and/or the sun's energy not quite matching the source used in the testing labs. Since it occurs so often we mention it so that folks won't assume there's a problem if they see the same thing.
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Good info and thank you for providing a comparison point from your own system. I am good with 70 to 90% "most of the time", but just have not seen "close (or over) STC power" since my micro inverters were replaced and wondered why. I am fully aware of the impacts of weather, shade, temperature and sun energy (Watts/Sq.Meter) and just trying to stage a conversation using reasonable terms.
Up until this conversation, I too have been hearing the "80% rule" as a reason for not doing a little math to better understand module and system performance. Knowing what I have learned over the last few years (hindsight 20/20), I would have asked that "system commissioning" look a little different than just mapping micro inverters, flipping the lever and telling the client, "Look, the meter is spinning backwards!" If this comment insults anyone, I apologize. I know there is a lot that goes into system design and installation that I truly respect, but I had hoped that the commissioning exercise would have been more in depth with some type of "Functional Performance Test" as the Cx industry call it. So... I am trying to learn how to conduct a post system commissioning exercise to confirm if everything is working as it should.
Thank you again for sharing your personal system performance information for comparison.Comment
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I wouldn't give up if I were you - sounds like you have an issue. As I've provided data to prove, I consistently hit STC. Of the 473 days I have logged, 1/3 of them have seen an interval where I was at least at 90% of STC for my less than prefect angle and orientation roof mounted system . If you were only randomly looking at the data from the sunpowerconsole site, then it's very possibly you wouldn't see production near peak. However, since you are looking at peak system production from your monthly emails and you have never seen anything near you peak, (but you had prior to the micro replacements), then that would indicate to me there is an issue. My system peak from my emails mostly matches my own data I'm logging. It appears to be at the micro inverter level and not at the panel level, but I have a clarifying question before I can make that assumption: Post 28 - 5th bullet - you mentioned the panel was tested and it was 359 watts. Is that accurate for a 327 watt panel? Have you had a chance to hover on the info on the sunpowerconsole site from your first screenshot to see what AC vs DC production looks like on a given panel.
I just looked at mine on the sunpowerconsole site for a given panel - power listed is 203 (that is AC). Hovering over the info text, I see:
AC amps .802, AC volts 253, AC kW is .203.
For DC 3.8 amps, 54.15 v = 205.7 watts (for some reason DC kW blank, so you have to calculate).
If you have a big difference, then there is an issue with the inverters (as long is DC watts are under 320 (as that is the capacity of your inverter at and is 96% efficient), if there isn't a big difference the issue could be with the panel - but you would likely need several data points, especially some with 300+ watts of DC output. However to get 300+ watts, you may need to wait until earlier to spring as you probably wont' have any days where panels are producing near STC. For me, I see production over 300 starting in February and running to October)
I think you can use the AC and DC info to isolate the issue, that may help to figure out next steps.
I guess what I've been trying to say - f you truly still have the SPR-E20-327-C-AC panel (that has the micro (MI-C-320-US208/240-10) integrated with the panel and you were able to see close to STC prior to the micro inverter replacement, then you have an issue. The only thing I can think of is that during the replacement, they actually replaced the panels and the micros and you ended up with either a panel that is considerably less than 327 w and/or a micro inverter less than 320 w. Either that or they just replaced the micros with less than 320 watt micros, but you model number seems to indicate that is not the case. I initially thought you may have issues with a few select panels, but since you can validate all the panels are still working on the sunpowerconsole site, that eliminates this option.
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I appreciate your encouragement - and I am NOT giving up. I also agree that I think there is an issue and you have nailed it - by evaluating the V and A (AC and DC) levels, I think the issue might reveal itself. I continue to ask for module level power data from SP, but have not seen it yet, so figuring out a way to get the V and A data is the right course of action. Also, agree that the spring is the right time to conduct this analysis (low temperatures, better sun angles, etc.).
back to the private convo - if I gave you my cell, could I talk to you? or would you prefer another way to talk?Comment
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I just enabled private messaging on my account, see if you can private message me now. I'm not exactly sure how to do it as I haven't done it myself.Comment
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looks like it is still disabled on the site in general. feel free to share a way to contact you and I'll get in touch maybe you could setup a temporary email account if you were worried about it being posted on this site.Comment
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Expecting a power output near STC power may not be unusual but it's usually the quest of someone who doesn't have a lot of knowledge of solar energy or how a PV system works, or what's important in analysing system performance.
I agree that attaining (or exceeding) STC output does happen. I see it with my system occasionally on partly cloudy days with low ambient temp. and wind and with near normal (that is, near vertical) solar beam incidence angle, and then only at the instanteneous readout on my inverter, not with the longer (like 5 minute) time integrated values as my monitor produced.
My Sunpower monitor (with the old 5 minute intervals) never showed an input to the inverter greater than 5.129 kW (321 W/panel) and that was a real fluke of conditions under a steady but hazy sun with more short wavelength irradiance coming through which meant that more of what got through the atmos. was utilizeable by the cells which use short wavelength irradiance best, and lowered arrray temps. because less infrared wavelength energy heating the array. All that had the effect of enhancing performance in a very non standard way and lasted about 15 minutes, all around the time of mininmum incidence angle on the array for that day (1305 - 1320 hrs. P.D.T, 03/17/21). The P.O.A irradiance for that 15 minute period averaged 1,180 W/m^2 and the temp. corrected G.H.I. for the period was measured by the pyranometer as 1,032 W/m^2. All those irradiance readings are to be viewed with suspicion however, as the pyranometer's sensor being a silicon device like the panel cells, is more sensitive to the shorter (ultraviolet end) of the solar spectrum. All that info is interesting but those data are of little use in trying to trouble shoot a system as they are highly transient, and quite unusual. After that event, the P.O.A. irradiance dropped back to the more common sunny day values.
If your questioning where the "80 % rule" comes from, well, for one thing, it isn't a rule at all, except maybe a rule of thumb (which is, by definion, only good for measuring thumbs). It might be instructive for this discussion to talk istead talk about how and why solar panels hardly ever output their STC rating, but that's more effort and spoonfeeding than I've got time or inclination for. Suffice to say that similar to a ICE in a vehicle, A solar panel will most likely never put out its rated power or attain its top speed.
Under bright, clear atmosphere, cloudless sun conditions, with a beam incidence angle on the array of close to normal to the array/panel, a well designed equator facing array operating at maybe 20 - 30 C. above ambient air temp. might well be expected to operate somewhere between, say, very roughly, 75% to 85 % or so of it's STC rating, and that will vary a lot with season, environmental conditions, array orientation, albedo and other things.
FWIW, under such optimum conditions and seasons, and when it's clean, my array operates at an output effieiency between ~ 0.824 and 0.884 or so of a spec sheet efficiency 0.2005, depending on a bunch of variables. Those are the types of data that you want to have if you want to determine if an array is operating nominally or not, not some off standard condition wild ass numbers and panel max. outputs without details of opersating conditions and that represent little more than one off flukes. Also, without environmental monitoring of at least P.O.A irradiance, ambient air temp. at the array and wind vector at the array, what you get will be meaningless junk.
I still think the OP may have inverters that are a problem, part of which (I suspect) may be no more than mislabeling (stuff happens), but even though I've seen mislaleled panels a few times, that's pure conjecture on my part. However, I've also not seen any discussions in this thread about array orientation or shading that would influence output, or any talk about operating (environmental) conditions.
As usual, take what you want of the above. Scrap the rest.Comment
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I don't have much pride of authorship, but to avoid confusion, I believe you're referring to my post and data, not Mike's.
Also, that was an average cell temp. over the entire array, not a panel temp. There is a difference. When under irradiance, operating Cell temps. are usuallly a bit higher than array surface temps as f(wind, irradiance, fouling, other stuff) by, very roughly, maybe 1 - 4 C.
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Thank you for clarifying the author - lots of words here to absorb and I obviously made a mistake. I also appreciate your detailed last post - I will review in detail. I might spend the next couple of days just absorbing all of the information presented in this thread. you really are a "solar fanatic" - meant in the kindest way. Your patience with me is also appreciated. I also just bought a small 20-watt panel for a trail camera with battery backup the other day and just playing with its orientation during different parts of the day and measuring the volts and amp output really swings the numbers considerably.Comment
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You're welcome. No big deal. Like I wrote, no particular pride of authorship here, just keeping things straight.
As for patience, I just think your spinning your wheels chasing after max. output without understanding all the variables that affect the output of a solar panel or an array, and I hate watching wasted effort - there's so little of it put forth these days.
A respectful suggestion: If you haven't done so already, download a free - but slightly dated copy of "Solar Power Your Home for Dummies". Or, buy a revised ed. at bookstores/Amazon for ~ $25 U.S.
Good luck, stay curious.
J.P.M.Comment
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I don't think you understand what the OP is doing - no one is not chasing after max output and spinning wheels. He (and I) are trying to ensure the panels and micros are all performing correctly. You and Mike have indicated he should be satisfied with hitting roughly 80% of STC, even though he previously was hitting over 90%. Yes panels degraded slightly each year, but not much. I also track data (requires no effort on my part it is all logged automatically) to ensure each panel and micro is working correctly. I have proved with data over and over that the panels can hit near their capacity on a regular basis (and have indicated my panels are not optimal in terms of orientation and angle and can also happen in the middle of summer). Excluding Nov - Jan, my panels hit anywhere from 90 to 102% of STC at least one interval in a day over 50% of the days in that period. I'm sorry your system is not performing as well as often.Comment
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