Key Takeaways
- Healthy modern panels lose roughly 0.5 to 0.8 percent of output per year, so anything faster than that is worth investigating.
- You cannot detect solar panel degradation without a clean baseline: record your specific yield (kWh per kWp) and performance ratio in the first full year.
- Weather-normalise before you panic. Compare the same month across years, not a cloudy week against a sunny one.
- String-level or panel-level mismatch in your monitoring app points to a specific module or connector, not whole-array ageing.
- A drop that pushes you below your warranty curve (often 80 to 87 percent at year 25) is grounds for a claim, so keep dated screenshots.
Every panel you buy is slowly getting weaker, and that is normal. The trick is telling ordinary ageing apart from a real fault before it quietly eats your savings. To detect solar panel degradation you do not need a lab or an expensive meter. You need your own monitoring data, a baseline from year one, and a bit of patience reading trends instead of single days. I have watched my own 5 kW rooftop system for years, and the difference between "this is just July haze" and "something is actually wrong" almost always shows up in the numbers first.
What panel degradation actually is
Degradation is the slow, permanent loss of output as the cells, encapsulant, and connections age. It is not the same as soiling or a cloudy month, which are temporary. Three mechanisms cause most of it.
- Light-induced degradation (LID): a quick 1 to 2 percent drop in the first few weeks of sun, then it stabilises. This is expected and already priced into good warranties.
- Long-term wear: UV yellowing of the encapsulant, tiny cell cracks, and corrosion of solder joints. This is the slow 0.5 to 0.8 percent per year figure.
- Potential-induced degradation (PID): voltage stress that leaks current to the frame. It can be aggressive, hitting several percent a year, and it is the kind you want to catch early.
The US National Renewable Energy Laboratory reviewed thousands of installations and found a median degradation rate near 0.5 percent per year for modern crystalline panels. That is your yardstick. If your data suggests you are losing two or three times that, the cause is usually fixable rather than terminal.
Why a baseline is everything
You cannot spot a decline without knowing where you started. In your first full year, note two numbers from your monitoring platform: specific yield (annual kWh divided by your kWp rating) and performance ratio (PR), the share of theoretically available energy your system actually delivers.
For most Indian rooftops, a healthy specific yield lands somewhere between 1,400 and 1,600 kWh per kWp per year, and a good PR sits around 0.75 to 0.82. My own system settled at about 1,480 kWh per kWp and a PR of 0.79 in its first clean year. Those two figures became the ruler I measure everything against. Without them, a 4 percent dip looks alarming when it might just be a dustier season.
Reading your monitoring data for the warning signs
Open your app and stop looking at today. Degradation hides in the long view, so switch to yearly or multi-year charts. Here is what I actually check.
The performance ratio trend
PR strips out sunlight and temperature, so a falling PR across the same months in consecutive years is the cleanest degradation signal there is. A slow, gentle slope of a fraction of a percent per year is fine. A step change, PR dropping from 0.79 to 0.71 in a single year, is a red flag for a fault, not ageing.
Specific yield, year over year
Compare the same calendar month each year. My April 2024 output against April 2025 tells me far more than this week against last week, because the sun angle and day length are almost identical. If a matched month has quietly lost 3 or 4 percent while your neighbour's similar system held steady, dig deeper.
String and panel mismatch
If you have string-level or module-level monitoring, look for one string or one panel lagging the rest under the same sun. Uniform ageing pulls the whole array down together. When a single string reads 8 percent below its twin, you are looking at a bad module, a shaded panel, or a loose MC4 connector, all of which are repairs rather than replacements.
| Annual output loss | What it usually means | Your move |
|---|---|---|
| 0.5 to 0.8 percent | Normal ageing | Nothing. Log it and move on. |
| 1 to 2 percent (year one only) | Light-induced degradation | Expected. Reset your baseline after it settles. |
| 2 to 4 percent per year | Soiling, shading, or early PID | Clean, check shade, inspect strings. |
| Sudden step drop | Fault, loose connector, or dead module | Inspect the same week; consider a pro. |
| Below warranty curve | Defective or failing panels | Start a warranty claim with dated data. |
My method for separating real degradation from noise
The single biggest mistake I see is people blaming their panels when the real culprit is dust, a new tree, or a hot week. Here is the sequence I follow before I ever conclude the modules themselves are ageing badly.
- Pull at least two full years of monthly data so seasons cancel out.
- Compare the same month across years, never adjacent weeks.
- Clean the panels and re-check. A single wash often recovers 3 to 5 percent that looked like decline.
- Walk the roof at noon and look for new shade: an antenna, a water tank, a grown-out branch.
- Check each string or panel in the app for one lagging behind its peers.
- Only after all of that, if the whole array is still below baseline, call it real degradation.
Safety first: never open the DC side of your system, unplug MC4 connectors, or probe the inverter terminals with the array live. DC arcs do not trip like AC and can injure you badly. Inspecting connectors and modules on a roof means a qualified installer, proper isolation, and a clear, dry day. Read the numbers yourself, but leave live electrical work to a professional.
Tools that make degradation easier to see
Most inverter apps from SolarEdge, Sungrow, Growatt, and others already log daily and monthly energy. That is enough to track specific yield. If your setup only reports at the inverter level, adding a data logger or panel-level optimisers gives you the string and module view that makes a lagging panel obvious. I compared a few of these in my writeup on monitoring hardware and data loggers, and the jump from inverter-only to string-level visibility is what turns guesswork into a diagnosis.
Export your monthly kWh to a simple spreadsheet once a year. Plot specific yield against time and the trend line tells the whole story at a glance. For a deeper treatment of the metrics, my piece on advanced solar performance analytics walks through PR, temperature coefficients, and how to read them without a physics degree.
When degradation becomes a warranty claim
Nearly every panel today carries a linear performance warranty, typically guaranteeing 80 to 87 percent of rated output at year 25 or 30. Manufacturers publish a year-by-year curve. If your weather-normalised output falls below that line, and you have ruled out dirt and shade, you have a legitimate claim.
This is where your habit of taking dated screenshots pays off. When I documented a genuine slide in one part of my array, I detailed the whole process in tracking panel degradation over three years, and the dated data was exactly what the supplier asked for. If your readings look off for a different reason, my walkthrough on diagnosing solar underperformance covers the faults that mimic degradation but are not.
Keep three things for any claim: your commissioning report with the original kWp, at least a year of monitoring data showing the trend, and screenshots timestamped by your app. Suppliers under India's rooftop programme, including systems installed via PM Surya Ghar, expect this paper trail before they act. You can also cross-check reliability expectations against the field research published by the US National Renewable Energy Laboratory.
Frequently Asked Questions
How much degradation is normal per year?
For modern crystalline panels, about 0.5 to 0.8 percent per year is normal, with a one-time 1 to 2 percent drop in the first few weeks of sun. Anything consistently above 1 percent a year deserves a closer look at soiling, shade, or a module fault.
Can I detect degradation without special equipment?
Yes. Your inverter or monitoring app already logs daily and monthly energy. Track specific yield in kWh per kWp and performance ratio year over year, comparing the same months. A falling trend after you rule out dirt and shade is the clearest sign, no extra meters required.
Why does one panel read lower than the others?
A single lagging panel or string under the same sunlight usually points to shading, a cracked cell, a hotspot, or a loose MC4 connector, not general ageing. Uniform degradation pulls the whole array down together, so a lone outlier is almost always a specific, fixable fault.
How do I know if it is degradation or just dust?
Clean the panels and re-check. Soiling losses recover instantly after a wash, often 3 to 5 percent, while true degradation does not. If output stays low even after cleaning and you have ruled out new shade, then the modules themselves are likely the cause.
When should I file a warranty claim?
File when your weather-normalised output drops below the manufacturer's published performance curve, usually 80 to 87 percent of rated power at year 25 or 30, and you have ruled out soiling and shade. Bring your commissioning report, a year of trend data, and dated app screenshots.
Watch the trend, not the day
Real degradation is slow, steady, and boring, which is exactly why your monitoring data is the right tool to catch it. Record a baseline early, compare like months across years, clean before you conclude, and treat any sudden step down as a fault to chase rather than ageing to accept. Do that and you will know the true health of your array long before it costs you real money. For the full picture on setting up and reading your system, start with our complete guide to solar monitoring and build your baseline from there.
