#1 June 6, 2016 08:32:43

juliec
From: Staffordshire
Registered: 2016-06-02
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Instantaneous peak output

I have a 3.62KW system and a geo solar monitor so I can see instantaneous peak output. This seems to peak at 2.4 kW under the best sunny conditions . I accept that I might never see 3.62kW but this seems a long way short. Does anyone else have any observations on instantaneous peaks ?

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#2 June 6, 2016 08:55:43

prodigymad
Registered: 2015-02-06
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Instantaneous peak output

I've got a 2.4kwp system on my garage roof and for the last week or so the highest peak output has been as high as 2.34kw.

The really weird one was on the 20th May were I apparently had a peak output of 2.568kw!!

This is via the inverter logs though. I do have a Geo Minim monitor and a Wattson Solar Plus and they both show very similar, accurate readings.

Have you got access to view the actual inverter to see what's showing there? It does seem a bit low.

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#3 June 6, 2016 09:24:15

juliec
From: Staffordshire
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Instantaneous peak output

Thanks for your reply - the inverter shows the same. My panel are flat rather than at an angle so that why I do not expect to see the theoretical peak but this does seem a long way short.

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#4 June 6, 2016 18:04:49

morrisok
From: Sussex
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Instantaneous peak output

Hi,

Where is your inverter? Mine is in the loft and gets quite hot. I don't normally see more than 3.2kw from a 4kw system. I see much higher in early spring though.

I assumed that was what you get for putting the inverter in the loft. Now you're making me worry there is something else wrong!



Kevin

URN 131, 3.92KWp 12x 327W E20 SunPower panels Sunnyboy 4000 inverter, facing ~south, no shading issues. Location: Sussex, http://pvoutput.org/list.jsp?userid=12216

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#5 June 6, 2016 21:00:27

pnews99@hotmail.co.uk
Registered: 2015-06-03
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Instantaneous peak output

my inverter is in the loft a 3.6 kw also a samil and regularly get around 4 kw , sometimes at night the geo monitor will light up and give a instantaneous peak reading near max i assumed this was a change on the grid voltage.
when i first got the inverter i used the inverters on line monitoring system and this was identical to the geo monitors readings and fit meter readings

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#6 June 7, 2016 11:24:07

bhommels
Registered: 2011-10-03
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Instantaneous peak output

The main reason for rarely seeing the nominal output is the thermal derating of the panels.
Depending on the make and type, panels produce 0.35 to 0.5 % less power for every degree above the nominal 25 C. Needless to say that when in full sun, the solar cells get quite hot, up to 50-60 degrees easily, and therefore not capable of producing the nominal power output.
For example, a 300Wp panel with 0.5%/deg derating can only produce 265W when the cells reach 50 C. Note that it also works the other way, so 315W can be expected at 15 C.
The nominal output figures are established in a “flash test”, where the panel output power is measured when exposed to 1000W/m2 irradiance for a very short while, i.e. before the panel has the chance to heat up.
This is also the reason why most people see the highest instantaneous output in May: while the sun is already high in the sky, the air is still cool, and the sun can emerge rapidly from behind clouds, illuminating the cool panels. The presence of highly reflective white clouds also helps the intensity.
Inverter derating happens when the device reaches a set temperature and starts to throttle its output in order to keep the temperature within limits. This is a much more sudden effect, and happens typically when the surroundings prevent proper cooling of the inverter, such as in a loft on a hot summer day.

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#7 June 8, 2016 07:43:06

juliec
From: Staffordshire
Registered: 2016-06-02
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Instantaneous peak output

My inverter is in the garage and so quite cool. The thermal degradation comments are interesting. I'm still waiting to hear from the installer, but will pursue this if they fob me off

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#8 June 8, 2016 12:10:24

gbrain
Registered: 2011-11-09
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Instantaneous peak output

According to my inverter I recently hit a new instantaneous peak output of 4070w from my 4Kwp system. This isn't the first time I've had over 4Kw but as explained by bhommels, this level of generation quickly diminishes as the panels heat up. My system can hold approximately 3.4Kwh during the optimum hours of the day.

The fact that I have just hit a new peak output after nearly 5 years running bolds well for future panel performance.

URN143 - 4.00Kwp - 20x 200w Caymax panels - Aurora 3.6 Inverter located in loft - facing perfectly south on 35deg pitched roof - minor shading in winter when sun is low early morning/late afternoon – Location DE74

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#9 June 9, 2016 09:37:29

bhommels
Registered: 2011-10-03
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Instantaneous peak output

@juliec: depending on what inverter you have, the derating figures might be available on the web. SMA have a nice document explaining the effects on yield here:
files.sma.de/dl/7418/Temp-Derating-TI-en-13.pdf
and the actual figures for their Sunnyboy domestic inverters can be found here:
files.sma.de/dl/1348/WirkungDerat-TI-en-40.pdf

As you are saying that your panels are flat, I presume they are placed horizontally? This alone decreases the maximum power output to 79% of panels tilted by 38 degrees, which is the optimum for maximum power output (not annual yield in the UK!). Having the panels lie flat also eliminates the self cleaning action from rain, so debris and dust will accumulate on the surface, further reducing the yield. Also, flat panels cool less well than tilted panels, increasing the thermal derating mentioned earlier.

Taking the above into account, your 3.62kWp system expected max power output is 79% x 3.62 kWp = 2.86 kWp due to the 0 degree tilt angle. The observed max power output is 2.4kW, suggesting a cell temperature of 60 degrees when assuming 0.5%/deg derating, which is more or less what is expected.

Not meaning to sound rude, but it seems there is nothing wrong with your system. There are some nice gains to be had, though! Installing a structure that tilts the panels to 30-35 degrees could well pay for itself in a few years I would think.
(the optimum panel angle can be estimated from PVGIS, or perhaps the folks in Sheffield could make a suggestion!)

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#10 June 9, 2016 13:32:12

juliec
From: Staffordshire
Registered: 2016-06-02
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Instantaneous peak output

Thank you for your comments - I have read around this and now understand that there can be thermal degradation issues with both the inverter and panels. I've emailed both companies to seek further information. The thermal characteristics of the panels are on the technical sheet.
http://www.solarworld-uk.co.uk/fileadmin/downloads_new/produkt/sunmodule/datenblaetter/en/mono/mono_black_275-285_en.pdf

but the parameters are not explained further. So I will wait to see.

I'm aware that flat is far from ideal but that was my only option, but it means I cannot see them ! then again I sometimes wonder if there are actually 13 panels up there !!!!

In case anyone is interested I've attached a file with some within day profiles.

Attachments:
attachment Solar profiles.docx (337.6 KB)

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