How much variation in panel output is normal?

[Steeler.Fan]

Background: My PV system consists of 2 sets of PV panels (13 Silevo 305W and 11 Panasonic 325W panels). Although my system was installed almost 2 yrs ago, I have only been monitoring output for 60 days. On the very 1st full day of monitoring, I saw substantial clipping at 5 Kw even though my inverter was an SE6000A. After about a week, SE determined that my inverter was set at a maximum of 5 rather than 6 Kw and they fixed the setting. However, during that week and since then I have only observed very brief periods of clipping (at 6 kw) by looking at pvoutput.com graphs for voltage jumps.
When I compare the cumulative output of each set of panels, it comes out close to the 325/305 ratio (1.1 vs actual 1.2). However I see a moderate amount of variation in total output within each set of panels, see attached file. In the Panasonic set, the highest yielding panel was 115.1 KWh vs 94.7 KWh for the lowest. Is this amount of variation in output normal? I sorted panels by highest output at the top.
AKM Solar Production by panel 60d.gif

[J.P.M.]

Background: My PV system consists of 2 sets of PV panels (13 Silevo 305W and 11 Panasonic 325W panels). Although my system was installed almost 2 yrs ago, I have only been monitoring output for 60 days. On the very 1st full day of monitoring, I saw substantial clipping at 5 Kw even though my inverter was an SE6000A. After about a week, SE determined that my inverter was set at a maximum of 5 rather than 6 Kw and they fixed the setting. However, during that week and since then I have only observed very brief periods of clipping (at 6 kw) by looking at pvoutput.com graphs for voltage jumps.
When I compare the cumulative output of each set of panels, it comes out close to the 325/305 ratio (1.1 vs actual 1.2). However I see a moderate amount of variation in total output within each set of panels, see attached file. In the Panasonic set, the highest yielding panel was 115.1 KWh vs 94.7 KWh for the lowest. Is this amount of variation in output normal?

The daily variation in output of an array or a string for consecutive or calendar close days that are only a few days apart will vary pretty much as the sunshine levels fluctuate, one day to the next, and to a much smaller degree inversely as the ambient temp. and wind affects the panels temps. Equally clear, consecutive days with close or the same temp. and wind profiles will look mostly identical in terms of output. Arrays of different orientation will not be the same, but may be close in magnitude but with different shapes on a graph.

I'm a bit confused as to how you're stating your output. A panel with 115.1 kWh output or another with 94.7 kWh output. For what period ? panels or strings. ?

Your graph didn't come through for me. What are your orientations ? (13*305)/(11*325) = 3,965/3,575 = 1.11 If arrays are the same and mostly south or southerly w/out shading, I'd expect the output on clear days to be close to that 1.0 ratio. If two array orientations are symmetric about solar noon, on clear days output will be similar to that 1.1 ratio but off some because of the temp. diff. morning to afternoon.

I'd do a coup[le of PVWatts runs, one for each array, do the hourly output option, and look for clear days in the output near the dates your interested in. Then combine the hourly outputs for both arrays and look for combined output above inverter max.

[Steeler.Fan]

My PV arrays face NW at azimuth of approx 322. (required roof location by my HOA). All of my panels are facing in the same direction, with same wind and temp and with no shading. The figures in the attached pdf table are total outputs (over 60 days) for each panel as summarized by the SE online monitoring program. Of the 11 Panasonic 325W panels, the panel with the highest total output over 60 days totaled 115.1 kwh and the panel with the lowest output totaled 94.7 kwh. The highest output panel produced 122% of the lowest output panel (115.1/94.7). Is that much variation normal or unusual?

[ButchDeal]

My PV arrays face NW at azimuth of approx 322. ...with no shading.

You do realize that your system is self shading, the entire array is shaded by itself, some of it more than others

[J.P.M.]

My PV arrays face NW at azimuth of approx 322. (required roof location by my HOA). All of my panels are facing in the same direction, with same wind and temp and with no shading. The figures in the attached pdf table are total outputs (over 60 days) for each panel as summarized by the SE online monitoring program. Of the 11 Panasonic 325W panels, the panel with the highest total output over 60 days totaled 115.1 kwh and the panel with the lowest output totaled 94.7 kwh. The highest output panel produced 122% of the lowest output panel (115.1/94.7). Is that much variation normal or unusual?

Are all the panels in the same plane ? I still can't get a look at your attachments, but if Butch thinks you're self shading, I'd take him at his word, although I'd not consider an array orientation where the solar incidence angle may ever be > 90 degrees to be, in and of itself, a self shaded array, but I would suggest that a 322 deg. azimuth is not a very good orientation.

Unless you're in the southern hemisphere and got a real deal on the system, I'd bet it'll be a long time until financial breakeven.

Edit: Just checked your location from prior posts. In HI, that array orientation will not have direct sun most of the time, and when it does, it'll be at a relatively poor incidence angle.

[Steeler.Fan]

AKM total panel outputs 61d.gif All panels are in the same plane on 2 parallel pitches of my roof. At the current rate of $0.34/kwh on Big Island, the break even time is substantially shorter than in other parts of the US. In 2010 when I first bought that home, the electric rate was $0.43/kwh. The peak production months according to PVWatts for my location, azimuth and roof pitch are June and July. The 60 day period for my cumulative outputs included the last week of June onward. I have attached the panel layout with cumulative outputs for 61 days for each panel. (Note that my installer didn't orient them properly on the SE layout page.) The 10 panel array and the 3 panel row in the other array are Silevo 305W panels. The other 11 panels in the 14 panel array are Panasonic 325W.

[J.P.M.]

AKM panel output 60d.pdf AKM total panel outputs 61d.gif All panels are in the same plane on 2 parallel pitches of my roof. At the current rate of $0.34/kwh on Big Island, the break even time is substantially shorter than in other parts of the US. In 2010 when I first bought that home, the electric rate was $0.43/kwh. The peak production months according to PVWatts for my location, azimuth and roof pitch are June and July. The 60 day period for my cumulative outputs included the last week of June onward. I have attached the panel layout with cumulative outputs for 61 days for each panel. (Note that my installer didn't orient them properly on the SE layout page.) The 10 panel array and the 3 panel row in the other array are Silevo 305W panels. The other 11 panels in the 14 panel array are Panasonic 325W.

Thank you. Busy just now. Will look over ASAP, probably next 24-36 hrs +/-.

[Steeler.Fan]

Using PVWatts, I calculate that I generate power worth $3700/yr. It will take me 6-7 yrs to break even, and less if the electricity prices continue to rise. I essentially bought a kit and hired an installer, electrician and electrical engineer. My measured production for July was only 100 kwh less than the PVWatts estimate and I think that it was a little more cloudy than usual.

[ButchDeal]

AKM panel output 60d.pdf AKM total panel outputs 61d.gif All panels are in the same plane on 2 parallel pitches of my roof. At the current rate of $0.34/kwh on Big Island, the break even time is substantially shorter than in other parts of the US. In 2010 when I first bought that home, the electric rate was $0.43/kwh. The peak production months according to PVWatts for my location, azimuth and roof pitch are June and July. The 60 day period for my cumulative outputs included the last week of June onward. I have attached the panel layout with cumulative outputs for 61 days for each panel. (Note that my installer didn't orient them properly on the SE layout page.) The 10 panel array and the 3 panel row in the other array are Silevo 305W panels. The other 11 panels in the 14 panel array are Panasonic 325W.

So these are really facing North West instead of South West??

[J.P.M.]

Using PVWatts, I calculate that I generate power worth $3700/yr. It will take me 6-7 yrs to break even, and less if the electricity prices continue to rise. I essentially bought a kit and hired an installer, electrician and electrical engineer. My measured production for July was only 100 kwh less than the PVWatts estimate and I think that it was a little more cloudy than usual.

Use PVWatts electricity costs at your risk. The electricity output numbers seem reliable, but writing only from what I know, the PVWatts numbers for SDG & E rates are not reliable. What does PVWatts say for you arrays' outputs ?

[Steeler.Fan]

PVWatts was only off by 100 kwh for July and I think it was a little more cloudy on Big Island than usual. Yes, my HOA would only approve me installing panels on my NW facing pitches.

[J.P.M.]

PVWatts was only off by 100 kwh for July and I think it was a little more cloudy on Big Island than usual. Yes, my HOA would only approve me installing panels on my NW facing pitches.

From your array layout, if a north arrow would point "up" as is common, it looks like the arrays face southwest. If there was an arrow that pointed north on that layout, which way would it point ? - up, down, left or right ?

Also, what zip /location are you using for the PVWatts runs ?

When you say "PVWatts was only off by 100 kWh for July", what are you trying to say ?

[Steeler.Fan]

My installer didn't layout the arrays properly on the SE monitoring program. I measured the azimuth with a compass and see attached satellite view from google maps.
zip 96743
PVWatts estimated 1,276 kwh production for July and I measured 1,171 but I think that the volcano contributed to some increased haze and clouds.
AKM satellite view.jpg

[SunEagle]

My installer didn't layout the arrays properly on the SE monitoring program. I measured the azimuth with a compass and see attached satellite view from google maps.
zip 96743
PVWatts estimated 1,276 kwh production for July and I measured 1,171 but I think that the volcano contributed to some increased haze and clouds.
AKM satellite view.jpg

If you live on the Big island I hope you are doing ok with all that rain and flooding?

[ButchDeal]

My installer didn't layout the arrays properly on the SE monitoring program. I measured the azimuth with a compass and see attached satellite view from google maps.
zip 96743
PVWatts estimated 1,276 kwh production for July and I measured 1,171 but I think that the volcano contributed to some increased haze and clouds.
AKM satellite view.jpg

Did you correct for True north from Magnetic north?

In any case you have varying shadowing on the array from the roof itself with that install.

Also your installer didn't get the azimuth right OR the physical layout of the array. very lazy installer. I wouldn't be surprised if they didn't randomly assign the optimizer locations thinking that the homeowner would never know ( I have seen this).