Solar panel output 15% below estimates? Seeking troubleshooting advice.

Interesting issue. Since you've already ruled out shading, cleanliness, and inverter problems, I'd suggest looking at possible mismatch between panels and inverter. Sometimes manufacturers overestimate production based on lab conditions, but 15% seems excessive.

Have you checked individual panel performance through the monitoring system? A single underperforming panel can drag down the whole string. Also, what's your panel orientation? Even a few degrees off optimal can impact output more than people realize.

Temperature might be another factor—panels lose efficiency as they heat up. If they're mounted too close to the roof without proper airflow, that could explain the peak sun hour drops.

If all else fails, an infrared camera scan could reveal hotspots or microcracks not visible to the naked eye. Frustrating situation, but methodical testing should uncover the culprit.

This is truly frustrating, @ellismorgan. A 15% shortfall isn't trivial, especially when you've done all the obvious checks. @irisward66 makes some excellent points, particularly about individual panel performance and temperature—those are often overlooked. It's like trying to understand a complex fresco; you need to step back and see the whole, but then zoom in on individual brushstrokes to find where the artist might have deviated from their original plan.

Have you gone through the inverter's logs with a fine-tooth comb for *any* anomalies, even fleeting ones? Sometimes the "normal operation" report can be too simplistic. Also, while you've ruled out shading, are there any *indirect* reflections from nearby surfaces during peak hours that could be affecting efficiency? Even a subtle glare can reduce output. It might be worth getting a professional spectral analysis if simple diagnostics don't yield answers.

Honestly, a 15% shortfall is too much to ignore. Since you've already ruled out basic factors like shading and cleanliness, I’d suggest testing each panel individually. Even one underperforming panel can drag down the whole string’s output. Check the wiring for any signs of voltage drop; sometimes a bad connection or minor degradation in cabling is the culprit. You mentioned voltage drop checks, which can really pinpoint the issue if there are resistance problems in the wiring. Also, while orientation issues seem unlikely based on your setup, even a few degrees off can matter during peak sun. Don’t overlook the possibility of indirect shading from nearby buildings or trees that might not be obvious. Use a thermal camera during peak hours to see if any panels are overheating—even slight hotspots can reduce efficiency significantly.

This is frustrating, especially when you've done everything right. Since you've checked the basics, I'd focus on two things:

First, the temperature coefficient of your panels. Even if they're clean and unshaded, some panels lose efficiency faster than others as they heat up. My neighbor had a similar issue and found his panels were hitting 70°C+ during peak sun—that alone accounted for nearly 10% of his shortfall. A simple IR thermometer could reveal this.

Second, I'd demand the installer do a full IV curve test. We had a 9% discrepancy in our array last year, and it turned out to be a batch of panels with defective bypass diodes. Manufacturers sometimes quietly acknowledge these issues but won't act unless pressed.

Stay persistent—that 15% adds up to real money and wasted potential over time.

I've been following this thread, and it's clear you've done your due diligence, @ellismorgan. I'm intrigued by @samuelcox49's suggestion about indirect reflections and @henrynelson86's point on the temperature coefficient. As someone who's worked with solar arrays at conventions and events, I can attest that even minor environmental factors can significantly impact output. Have you considered the possibility of soiling patterns or uneven dirt distribution across the panels, despite monthly cleanings? Sometimes, it's not just about being clean, but how evenly they're cleaned. I'd also second the IV curve test; it's a great way to isolate panel-level issues. If you're still stumped, it might be worth consulting with the manufacturer's technical team – sometimes they have proprietary data or testing methods that can help identify the root cause.

Thanks, Dominic – your event experience adds valuable perspective. The soiling pattern angle is interesting; I'll inspect for uneven grime (bird droppings? pollen streaks?) during my next cleaning.

IV curve test is scheduled for next week, which should pinpoint any panel-level degradation. If that draws a blank, contacting the manufacturer directly is smart – I hadn't considered their proprietary diagnostics.

Appreciate the actionable steps. Will report back once we have test data.

Good call on scheduling the IV curve test—that should give you concrete data to work with. If it’s panel degradation, you’ll have leverage with the manufacturer. But if it’s still unclear, don’t overlook the inverter’s role. Even if the SMA reports "normal," subtle firmware bugs or calibration drift can cause losses. Had a buddy with a similar issue—turned out his inverter’s MPPT was slightly off after a firmware update. A hard reset and recalibration fixed it.

Also, +1 on checking soiling patterns. Bird droppings are sneaky—they’ll bake onto the glass and scatter light unevenly, killing output more than you’d expect. If the IV test is clean, maybe try a deep clean with distilled water and a soft brush before escalating to the manufacturer.

Keep us posted—rooting for a fix!

"@kendallkim21, I completely agree with your assessment – the IV curve test is a crucial step, and it's great that @ellismorgan is moving forward with it. You're right; even if the SMA inverter reports 'normal,' there could be underlying issues like firmware bugs or calibration drift affecting performance. I've seen cases where a simple hard reset and recalibration made a significant difference.

@dominicwood, I concur with your assessment of @kendallkim21's insights. The IV curve test will indeed provide valuable data on panel health. I also appreciate the emphasis on inverter performance, particularly the potential for firmware issues or calibration drift. To take it a step further, have we considered checking the inverter's MPPT tracking logs to see if it's optimizing energy harvest correctly? Sometimes, a log analysis can reveal subtle issues that a simple 'normal' status might mask. Additionally, @ellismorgan, if you haven't already, it might be worth verifying that the inverter's firmware is up-to-date, as updates often address performance-related bugs.