Solar panel degradation rate: what is normal, and when it is a claim

Solar panels lose a little output every year. That is normal, expected, and written into the warranty. A performance warranty does not promise your panels will keep making their rated power. It promises a floor: a minimum percentage of rated output at each year of the term. You have a claim only when a properly measured output falls below that published line, not when it falls below what you hoped for.

The number that matters is on your panel's datasheet, and it is different for every model.

What a performance warranty actually promises

Almost every panel performance warranty is written in the same shape, whatever the brand.

1. A first year figure. Panels lose slightly more in year one than in later years, so the guarantee names a percentage of rated power the panel will still make at the end of year one.
2. An annual rate after that. A maximum percentage of loss per year for the rest of the term.
3. An end of term floor. The percentage of rated power the panel is guaranteed to still make at the final year.

Put those three together and you get a straight line sloping gently down, from just under 100 percent in year one to a floor at year 25 or 30. That line is the promise. Everything above it is not a claim, however disappointing it feels.

We are not going to publish a single industry figure for the annual rate here, and it is worth saying why. The numbers differ by brand, by product line and by model year, and we only publish figures we have read on a document. Yours are printed on the datasheet for your exact panel model, usually as a small graph with the percentages beside it. Ask your installer for that datasheet, or search the model number on the manufacturer's site.

The degradation calculator will draw your panel's curve and tell you where a measured reading sits against it.

Product warranty and performance warranty are two different things

This trips people up constantly, because the marketing usually shows two numbers with no explanation of why there are two.

The product warranty covers the panel being defective. Delamination, a failed junction box, a frame that corrodes, a cell that cracks without external cause. It is a manufacturing fault claim.

The performance warranty covers the panel underproducing against the published curve, even though nothing is visibly wrong with it.

They usually have different lengths, and the performance one is usually longer. Two examples we have checked:

• Canadian Solar current TOPHiKu6 residential modules carry 25 years product and 30 years performance.
• Silfab carries a product warranty of 12 years from delivery, extended to 25 years when the installation, site and product details are registered through Silfab's portal within 180 days of purchase. The performance warranty runs 30 years either way.

That Silfab detail is worth pausing on. Missing a 180 day registration window costs 13 years of product cover and none of the performance cover. Two warranties, one deadline, different consequences. See the Silfab warranty page.

Your panels are older than your system

Here is the second thing people get wrong, and it changes where you sit on the curve.

The performance term does not always start when the panels were installed. From the documents we have read:

So if your panels shipped a year before they went on your roof, you are a year further along the degradation curve than your install date suggests, and your term ends a year earlier than you think. Nobody sends a letter about this. The warranty deadline calculator will place your real date.

How to tell whether you actually have a claim

This is the hard part, and it is hard because a low reading has many more likely causes than degraded panels.

Before anyone will look at a performance claim, you have to rule out:

Dirt. Dust, pollen, salt, bird mess. A cheap and common cause of a real production drop, and completely reversible.

Shade. Trees grow. Neighbours build. A shading pattern that did not exist at install can develop over a decade, and it will show as a loss at the same time every day.

A failed inverter or a dead string. A production drop that is actually an electrical fault is not a panel claim. On a microinverter system, the per panel view shows this straight away.

Weather and season. Comparing this July to last July is meaningful. Comparing July to April is not.

Temperature. Panels produce less when they are hot. A reading taken on a 40 degree afternoon is not the panel's rated output and was never going to be.

Measurement method. Rated output is measured under standard test conditions in a laboratory. Your roof is not a laboratory. A proper performance test needs the irradiance and cell temperature measured at the same time as the output, so the reading can be corrected back to standard conditions. A number from your app is a starting point for a conversation, not evidence.

Which is why performance claims are the hardest kind to win. The manufacturer will not accept "my bills went up". They want a corrected measurement, taken properly, on identified panels, compared against the curve for that model.

What a performance claim needs

If you have ruled out the above and the numbers still look wrong, the file looks like this:

• The panel model and serial numbers of the specific panels tested
• The datasheet showing the guaranteed curve for that model
• Proof of the start date, which for several brands means the invoice or the delivery paperwork
• A test carried out by someone competent, with irradiance and temperature recorded
• Production history over time, not a single day

Jinko's claim requirements are a good illustration of how document heavy this gets. A Jinko claim wants the warranty certificate plus the purchase and installation invoices. Three pieces of paper from years ago, before anyone measures anything.

The claim checklist tool builds the list for your brand, and what to keep so a claim gets paid covers the capture side.

The labour question, again

Say you win. The manufacturer agrees the panels are below the line and supplies replacements.

Somebody now has to come to your house, get on the roof, take down the failing panels, put up the new ones, and recommission the system. Across the panel brands in our catalogue, almost none carry any labour cover, which means that bill is yours.

There is one exception worth naming. REC's Alpha Pure-R line carries 20 years of labour cover in our catalogue, which is far beyond anything else in it. If labour matters to you when choosing panels, and it should, that is the number to compare rather than the headline years. See the REC warranty page, and note that REC's ProTrust cover exists only where a certified installer registered the installation.

What to do with all this

If you are worried about your existing system, get the datasheet for your panel model, find the guaranteed curve, and put your measured output against it using the degradation calculator. If you are above the line, your panels are behaving as promised and the problem is somewhere else.

If you are buying, compare three things and not one: the product warranty length, the performance floor at year 25 or 30, and the labour cover. Two brands both saying "25 years" can mean very different things once you read which 25 years they mean and who pays the electrician.

Every panel brand we have checked is on the solar panel warranty page, with the date the document was read printed on it.