Solar output equations


Greetings all
I would like to calculate the output for my panels, inputting the angle of the panels to the horizontal, their orientation, latitude and longitude and efficiency. And anything else required. I would like to get an equation, or equations, to generate the output on a minute by minute basis using an Excel spreadsheet. I would be grateful if anyone could either provide me with these equations, or point me to where I might find them.

I have looked at the “Your topic is similar to …” list from the system but none of the topics seem to address this issue.

best regards


That’s a bit outside the purpose of this website. Try PVWatts. I think you may be a little ambitious with minute by minute results though. Even the gold standard of PV Simulation software, PVSyst, typically calculates hour by hour.


thanks for the comment pjschaffer
What I am looking for is an equation or equations e.g. W/sqm = f(azimuth, slope, hour of the day, day of the year, etc, …) which gives the Watts per square metre at any time of the day, not allowing for anything in the way such as clouds, shadows etc. The reason I seek this is that I have an aberration in my plot of power over the day and a possibility is that it results from an interaction between the sets of panels I have. If I had this equation I could plug it into Excel and model the output I should be getting to see if it identifies the source of this aberration.


Hi Amasa18, i too am in west aus, can you post a screen shot of your problem, i have looked at lots of your days and am not seeing any thing unusual about your outputs, although as you are not showing any extended data, like grid voltage ,inverter string voltages and amps and inverter temperature its hard to be able to tell what your problem is, jims maxim jinkos


PVO can model all this for you, if you have input the azimuth, angles, etc into the system setup - have you looked at the ‘insolation’ button, just below the 5 grey squares below the main chart?


jimdcollie - how do I attach an e.g. jpg file?

willisave - where do I find PVO?

my thanks to both of you for your comments


Hi again
It’s been a long time since I looked at this site and I had forgotten I had connected up my output data. But now I can see it and I can point to the anomaly that intrigues me. If you go to, for example, the graph for 19 May 2020 you will see what I think of as “notch” in the power output graph at about 2:30pm. This day was a cloudless day so I get a nice curve for power output. You can see the same notch in other cloudless days, such as 15th and 14th May, although there is a little cloud noise on the 15th. I, and others have looked at all possible causes, such as shading etc, and can’t see why I am getting this notch. It seems to be seasonal - it fades through spring and virtually disappears in summer, so I think it may be an artifact of the orientation of the panels. So what I would like to do is model the system to see if that shows the same effect, hence my seeking the equations.


PVO is PVOutput - this site!
see and click here…


Thanks again willisave. I now remember putting in the panel data when I joined PVOutput 2+ years ago, although I didn’t read the “fine print” at the time and didn’t adjust for two sets of panels. Anyway, the overall curve matches the plot reasonably well, and gives me what I would have generated if I had the equations and had entered them into Excel, but the curves don’t account for the notch. So it is still a mystery to me.


Hi Amasa18, i have looked at the 3 dates you have supplied and it looks like typical part shade from a tree,i get the same thing on my system in June each year,jim


Hi jim - thanks for getting back to me. My first thoughts were that it must be shade from a tree but a close inspection of all possibilities ruled that out. So still looking. :thinking:


so a pole of some type or a wire for power or TV Ariel brace, it only needs to be thin to cause a shadow, also it might be 50 mts away from your house like the tree next door is on my system , also what is the arrangement and alignment of your panels . also your second array is not showing on your settings page , and the shadow only needs to shade part of one panel for that string to lose output Jim


I had thought it was a shadow of some sort, but there is nothing high enough to the north to cast a shadow. Even at the winter solstice when the sun is low, low, low. Here is the plot

with the notch roughly ringed. As you can see I have now adjusted the settings to show our two sets of panels. 10 panels face 15 deg W of N, 13 face 75 deg E of N.

And just to show a notchless time, here is a plot from 9 Jan last


Hi, please try using this tool to look at your roof from, you may see a vent pipe or something you cannot see from the ground, zoom right in to your roof for a good look, it only need to be small, i had to cut 100 mm of a vent pipe on roof to stop a shadow first thing in the morning, jim


Hi Jim, I do have a kitchen hood vent near the panels but I discounted that as the cause because I thought it was too small. However, given that there doesn’t seem to be any other source I’ll look at it more closely, or rather look at its effect more closely.


Hi again Jim: I have checked the shadows for the last couple of days, and while there is possibly a sliver of shadow from the kitchen hood vent at around 2:30pm there is not anywhere near enough to account for the notch. As we are moving out of notch time I guess I will have to continue to consider this a mystery until early next winter, when I can check again. Thanks again for taking the time to comment.


well if he shadow is on half of one cell ie 250 sq mm then the whole of that string say 16 sq meters will drop to all most NO output, jim


That’s not how modern panels work. Panels are in sections with bypass diodes designed to help maintain production with partial shade conditions. Output will be reduced but the whole array won’t be brought to its knees.


Wattmatters,this is older style of panel and bypass diodes are not shown on the spec sheet,


Thanks Jim
I see no reference to the panel type or age in this thread though.