How long do solar panels last on a farm roof?

Solar panels are sold on a 25-year warranty, which people read as a 25-year life. The two are different things, and on a farm roof the difference matters because what wears out first is usually not the module. This guide uses the one large independent dataset on how fast modules lose output, does the arithmetic on it, and sends you to the pages that cover the parts that fail sooner.

What the best dataset says about degradation

A review by Dirk Jordan and Sarah Kurtz of the US National Renewable Energy Laboratory assembled nearly 2,000 degradation rates from the literature, measured on individual modules and on whole systems. NREL’s publication listing gives a median of 0.5% a year. A later NREL presentation reported a mean of about 0.8% a year across 2,128 rates, pulled up by a minority of high values, and noted that sampling bias may be present. These are averages from many sites and technologies, mostly outside the UK, and say nothing about your roof.

The arithmetic on a 25-year warranty

The arithmetic below is ours and is illustrative. If output falls by 0.5% a year, compounding, a module is at about 88% of its starting output after 25 years (0.995 to the power 25 is 0.882). At 0.8% a year it is at about 82% (0.992 to the power 25 is 0.818). Warranties normally promise a minimum output at set ages rather than a lifespan; the terms the main manufacturers publish are set out, with sources, on solar panel warranty for UK farm roofs.

Annual loss	Output after 10 years	Output after 25 years
0.5% (NREL median)	95.1%	88.2%
0.8% (NREL mean)	92.3%	81.8%

What fails before the modules do

• The inverter. String inverters are normally replaced during a panel’s life; the published figures and warranty extensions are on how long solar inverters last on a farm and best solar inverters for UK farms.
• Livestock-building air. Ammonia and humidity act on frames, cables and seals; the test standards and what to ask are in ammonia-resistant solar panels for livestock buildings.
• Birds and debris. Nesting under an array is a fire and insurance question: bird proofing solar panels on farm buildings.
• The roof under the array. If the sheets will not last as long as the panels, the roof is the lifespan limit: solar panels on asbestos cement farm roofs and combined re-roof plus solar.
• Fire and insurance. Faults and installation quality drive claims more than age: solar panel fire risk on farm buildings and solar panel insurance for farm buildings.

Questions to put to an installer

1. What is the module’s performance warranty, year by year, and who stands behind it if the manufacturer goes?
2. What inverter is quoted, and what does replacing it cost when its warranty ends?
3. How is output monitored, and who tells you when a string underperforms?
4. Is the roof expected to outlast the array, and has that been checked in writing?

Sources

Point	Source	Date
Nearly 2,000 degradation rates; median 0.5% a year	NREL, Photovoltaic Degradation Rates: An Analytical Review (Jordan and Kurtz)	read as quoted 29 Oct 2026
Mean about 0.8% a year across 2,128 rates; sampling bias may be present	OSTI listing for the NREL review	as quoted in NREL material, 29 Oct 2026
Compounding arithmetic (0.995^25, 0.992^25)	Our calculation from the two rates above	29 Oct 2026

The NREL pages could not be opened directly from our fetch tool on 29 October 2026; the rates above are as quoted in NREL’s publication listing and presentation material returned by search. Check the paper before quoting it.