Adjustable panel efficiency or a scaling factor for insolation calculation


#1

I’ve noticed that the insolation curve calculation shape is a very good match to my system generation, so I’m pretty sure I’ve got all the angles, azimuths and other geometry described correctly - however the scale seems to be consistently under-estimating the output from the panels. I’m using 335W panels, but get very good agreement with the insolation curve if I set the panel power to around 400W instead.

See https://pvoutput.org/intraday.jsp?id=64310&sid=57175&dt=20181122&gs=0&m=1 for an example.
Panel Max Power 335W:

Panel Max Power 395W:

(Flat top due to maxing out the 6kW inverter - and I do have some minor shading in the morning, dropping that section until around 9am below the perfect line).

I’m wondering if the insolation calculation is using some form of assumed panel instantaneous conversion of energy (15%?, 16%?) which is lower than the conversion efficiency of more modern panels? (in the case of my LG335 panels, 19.6% according to the datasheet)?
If so, could this be a parameter that could be added to the System specification along with the Panel Max Power parameter, to be used in the Insolation calculation?
If not, could some other scaling adjustment factor be added somewhere to correct the insolation curve to match observed panel output on a cloudless day?


#2

Hi i think you can adjust it using this in settings Panel Coefficient
0.03% Percentage of production lost per degree above 25C,
most older panels were at 25c many of the newer one,s are at 45c .
this is panel temperature NOT air temperature, my panels run from -5c in the early morning winter time to 75c in full sun UV 12+ in summer time.
the NOCT for the the Lg 335 is taken at 45c at 0.03 % and PV org has it set at 25c so geting the outputs to match is it think not possible as there is know way to know at what temperature the panels are running at, with the old 25c it often worked out better as it was closer to Air temperature. and there is no setting for adjusting for greater panel efficiency with the newer type of panels.
yes you can recalculate ,go to days tick days to do, go to more and click recalculate max 20 i think,
if you make the number 0.0 then that may improve the efficiency of your panels i think


#3

i have the same results

i have change the Panel Coefficient to 0.03% and updated the days i was looking at but it didn’t change. is this something i can backdate?


#4

In order to predict how much power your system is expected to capture under optimum conditions you should be very specific about what panels your are using in your PV system. The efficiencies are well documented by the manufacturers and are usually very accurate. Systems with more than one orientation tend to be less predictable due to additional sources of errors.


#5

Yes - and have done so - but there is no place within PVO to tell the PVO system the efficiency of the panels.This could be used to scale the insolation calculations. Thats the point of the request.


#6

I am assuming that BB has access to the specs. for all the panels he sees ID’ed in my settings. My Insolation values typically run above 120% on sunny days but I now have a Pyranometer which I use to calibrate my system. Using the formula:

Power Out = Total Panel Area X Solar Irradiance X Panel Efficiency

21 Panels X 1.64 m^2/Panel X 18.3% Panel Efficiency = 6.30252

Using today’s Solar Irradiance and my Power Out I calculate a value of 6.28459 for this constant. If I adjust my Panel Efficiency to 18.25% this brings the constant in line with that value. A difference of 0.05% in the efficiency given by the manufacturer.
This indicates that the Insolation calculated by PVOutput is an underestimation of the actual Solar Irradiance. On mostly clear days my systems operates at 120+% of Insolation given by PVOutput. On mostly cloudy days it runs below 50%. So I would put the Insolation given by PVOutput at 17% underestimated. This result may vary with location so beware.
I have examined several other systems near mine and similar results are seen. However, this result may vary with locations other than coastal Southern California.


#7

What type of pyranometer do you use? I am passionate for weather forecasting and related things, one sensor I do not own is a pyranometer and way to measure clouds coverage.


#8

I have the WS-2000 weather station on my home about 20 ft from my solar panels. It is probably the entry level and didn’t cost very much. I am pleased that it does as well as it does. I have compared it’s output to my solar production and it seems to be adequate for my purpose.
I did look into other types which may be of higher quality but opted for the WS-2000 because of cost, wiring to install more accurate meters as well as getting the data to a website like PVOutput.


#9

Added to the "possible shop items list " in case of overseas trip, thanks.


#10

If you would like to see output of WS-2000 it is on WU id is KCASANCL94