Solar panels are typically warranted for 25 years, and field data shows they lose only around half a percent of their output each year. A 50 kWp array Spirit Energy installed near Winchester in December 2011 had its best year ever in 2022, when it generated 54,600 kWh at 11 years old. On most systems the inverter, not the panels, is the first part to need replacing, usually somewhere between years 10 and 15.
A solar panel is a sheet of glass and silicon cells in an aluminium frame. It has no moving parts, so there is very little to wear out mechanically. What changes over time is how much electricity the cells produce, and that falls slowly.
This is why manufacturers guarantee a panel’s power output rather than how many years it will last. The Winchester system uses ET Solar panels. Their 25-year performance warranty starts at 97% of rated output after the first year and falls to 81.4% by year 25.
Fifteen years in, the warranty guarantees the panels will still produce at least 88% of their original output. If the system had performed exactly at that minimum every year, it would have generated roughly 715,000 kWh by now, whereas the meter on the array has actually recorded 761,000 kWh.
A warranty marks the level a manufacturer is confident enough to put money behind. It is not the end of the panel’s working life. A panel at 81% of its original output in year 25 is still a working panel producing most of what it did when new.
The most widely cited figure comes from a US National Renewable Energy Laboratory review that assembled nearly 2,000 degradation rates measured on individual panels and whole systems over 40 years of field testing, finding a median loss of 0.5% a year.
The ET Solar warranty assumes about 0.65% a year, so it is deliberately more cautious than the measured median.
To put 0.5% in perspective, a system generating 4,000 kWh in its first year would lose around 20 kWh of annual output each year. That is small enough to be hidden by ordinary changes in the weather.
Weather moves annual generation far more than degradation does. A 0.5% yearly loss is small next to the gap between a dull year and a bright one.
A 16.45 kWp system Spirit Energy installed in Finchley, north London, in January 2012 had its best year in 2020. It produced well over 16,000 kWh, about 7% above the figure quoted when it was new. Spring 2020 brought 626 hours of sunshine across the UK, beating the previous record set in 1948 by over 70 hours.
Bad years happen too. The Winchester array’s worst year was 2017, when it generated around 47,000 kWh, under 90% of its estimate. Every year from 2018 onwards it has generated between 53,000 and 54,000 kWh, above the estimate it was given for its first year. One poor year is not a sign that panels are failing.
It does, and a good generation estimate accounts for it from the start.
The Finchley system sits on a roof pitched at just 10 degrees, facing about 22 degrees off south, with no shading. A pitch that shallow favours high summer sun but gives up more generation in winter, when a steeper roof would catch the lower sun better. Rain also runs off a 10-degree slope more slowly than off a 45-degree one, so the panels do not self-clean as well.
The Winchester array is ground mounted on frames in a field. Even there, the horizon and each row of panels shading the row behind were modelled as a 1.81% annual loss. Its first full year of clean monitoring, 2013, came in at 52,158 kWh against an estimate of 52,070 kWh, a difference of under 0.2%.
When the design factors are modelled properly, long-term performance tracks the estimate closely. When they are ignored, a system can under perform from day one, and that is often blamed on degradation later.
A smaller 3.99 kWp system was still beating its generation estimate by 7% after 11 years. The panels were fine, but the inverter needed replacing and the monitoring went with it.
That is the usual pattern. The inverter converts the panels’ DC electricity into AC for your home, and it is a box of power electronics working hard every daylight hour. The panels just sit on the roof.
If you are working out whether solar pays back over 25 years, include one inverter replacement somewhere between years 10 and 15. It is a predictable cost rather than a sign the system is failing.
Start with the estimated annual generation figure in your original quotation. Compare it with a full calendar year of readings from your generation meter or monitoring app.
Look at three or more years together rather than a single year. A gradual dip of a percent or so over several years is normal. A sudden large drop, or a monitoring screen that has gone blank, more often points to an inverter or monitoring fault than to the panels themselves.
Older systems on the feed-in tariff can often be extended rather than replaced. In a recent video, Spirit Energy upgraded a feed-in tariff system to a larger solar array with a Tesla battery while keeping its tariff earnings. If you’d like to know what your own roof could generate, or whether an older system is worth upgrading, you can request a quote below.