Solar panel warranties and degradation: what to expect

Two different warranties come with every solar panel, and they are routinely confused in sales conversations. One covers the panel as a physical object. The other covers how much electricity it still makes. They run for different lengths of time, they are triggered by different kinds of failure, and only one of them has anything to say about ageing.

Product warranty versus performance warranty

The product warranty (sometimes "material and workmanship") covers manufacturing defects: delamination, a failed junction box, frame corrosion, a cracked backsheet. If the panel is physically broken in a way the maker is responsible for, this is the clause you use.

The performance warranty (sometimes "power warranty") is a guarantee about output. It says the panel will still produce at least a stated percentage of its nameplate power in a given year. A panel can be in perfect cosmetic condition and still fail this clause — and it can fail the product clause on day one while producing exactly its rated power.

Two real datasheets, read in October 2026, show the pattern:

Neither of these is "better". They are two documents that happen to be structured similarly, and the figures differ by model within the same manufacturer. Terms vary by model, revision and region, which is why you should insist on the datasheet and warranty PDF for the exact model number going on your roof — not the brand, the model. Ask for the manufacturer's own copy from jinkosolar.com or longi.com; both documents above were read from third-party hosts.

What the measured data says

A warranty floor is a legal commitment, not a prediction. The measured picture is different.

NREL's PV Fleet Performance Data Initiative — a $5.25m programme launched in 2019 — analysed about 2,200 non-residential sites and nearly 19,000 inverters. The median performance loss was about 0.75% a year. The same work found that loss depends on climate: panels degrade more slowly in cooler climates, so hotter ones should expect higher losses. Malta is a hot climate. That is the single most relevant sentence in the whole body of evidence for a roof in Qormi or Xagħra.

So the gap between a 0.40% datasheet line and a 0.75% fleet median is not a trap — it is the difference between a guaranteed floor and a central expectation. Over 20 years, compounding the two rates on a south-facing 32° array at the PVGIS yield of 1,668 kWh/kWp gives roughly 1,540 kWh/kWp in year 20 at 0.4% a year, against roughly 1,435 kWh/kWp at 0.75%. That arithmetic is ours, not the sources'; the point is that the difference is real but modest — on the order of a hundred kilowatt-hours per installed kilowatt per year, two decades out. At the grant-free feed-in tariff of €0.15/kWh for 20 years on systems under 40 kWp, that is around €16 per kWp a year by year 20 if every unit were exported. Self-consumed units displace Enemalta's banded tariff instead — 0.1047, 0.1298, 0.1607, 0.3420 and 0.6076 €/kWh at the 2,000 / 6,000 / 10,000 / 20,000 kWh thresholds, including 5% VAT — so the value of a lost kilowatt-hour depends entirely on where your household sits in those bands. Our savings calculator works from these published rates.

Why published degradation figures disagree

The IEA PVPS Task 13 benchmark is the honest answer to "why does everyone quote a different number?". In a 19-system comparison, 9 systems lost 0.4-1% a year, 3 lost less, and 6 lost 1-4% a year. More importantly, the reported loss rate depends heavily on data quality and on how the data is filtered — heavy filtering can push the result up or down. There is still no single standard calculation method; Task 13 judged the IEC 61724-4 proposal premature.

Treat any single degradation percentage, including the ones in this article, as a figure produced by a method rather than a fact about panels.

What the 2026 REWS grant required

The 2026 Renewable Energy Sources Scheme closed to applications on 12 September 2026, with battery options closing on 13 July 2026. A further round has been announced but not detailed, so nothing below should be read as a live requirement.

Under the 2026 guidelines:

• Applicants had to ask the retailer for a minimum 10-year warranty on the system (s.13). An extended warranty beyond the standard 10 years was not an eligible cost.
• The Annex 3 sample guarantee covered all PV and inverter components and the installation, with full diagnosis, repair and replacement cost borne by the warrantor.
• Output had to not degrade more than 10% from rated output, measured on the PTC rating.
• The guarantee passed to subsequent purchasers or owners at the same location.
• Exclusions: poor operation or maintenance, unauthorised repairs, abuse, accident, fire, flood, earthquake.
• Durability period: 5 years after the grant is paid, during which the system must stay installed and working. Replaced modules, inverters or batteries had to be reported to REWS with the new serial numbers.

That last point matters in practice. If a panel is swapped under warranty inside the durability window, someone has to file the new serial number. Agree in writing who does it.

Transfers, shell form and Gozo logistics

Malta resells property often, and much of it changes hands in shell form before anyone thinks about the roof. Two transfer rules therefore apply, and they are not the same rule.

The REWS guarantee passed automatically to later owners at the same location. The LONGi module warranty is conditional: rights can pass to a later owner if the supplier is told in writing and the transferee accepts the terms. So keep the installation file — serial numbers, datasheets, warranty PDFs, the DC15 compliance paperwork — with the deeds, and do the written notification at sale rather than discovering the condition fifteen years later.

For apartments, condominium rules bind works on common parts, including any later access for a module swap; settle roof access in writing before installation, not during a claim. For Gozo, every replacement panel is a ferry crossing — ask what that does to a warranty call-out, because the manufacturer documents we read do not say who pays for delivery or labour.

Rooftop PV remains permitted development under DNO Class 1(iv), S.L. 552.08, provided the installation is DC15-compliant, so the planning side does not complicate a later replacement.

What to ask your installer

1. The exact model number, and the manufacturer's own product and performance warranty PDFs for it.
2. Which years are covered by which warranty, and what the performance floor is in year 10, 15 and 25.
3. Who carries the installation warranty — the installer, or nobody — and for how long.
4. Who pays for labour, access and delivery on a replacement, which the manufacturer documents do not settle.
5. What the inverter warranty is in writing. We have no verified source on typical inverter terms and will not quote one.
6. What happens to the warranty if you sell, and what written notice the supplier requires.
7. Whether the installer is listed and verified — our verified installer directory is CheckYourSolar.

If you are still at the sizing stage, the free roof planner is SpecYourSolar, and battery-specific questions are covered in StoreYourSolar. No Malta-specific field degradation study exists that we could find, so treat local performance claims — from anyone, including us — as estimates built on foreign data.