Solar Panel Degradation – How Well Will Your Solar Power System Work in 20 Years?

On the day that your home solar power system in installed, it’s probably going to work great. Your panels will most likely meet the production estimates provided by your installer for many years to come. But as with anything else, your system is going to age, and begin to decline in efficiency.

Different products show their advancing age in different ways: Cars become increasingly unreliable and fragile, laptop and cell phone batteries hold less and less of a charge, a TV’s picture begins to fade, and so on.

Solar panels show their age through performance degradation—a decline in how much electricity they generate.

Have you ever looked at a roof that was 20 or 30 years old? Missing tiles, dry rot, mildew—exposure to the elements is not kind to building materials. Now imagine leaving your phone outside for even a few days, let alone 20 years. It’s safe to say it wouldn’t last long.

But that is the challenge faced by solar panel manufacturers: to create a product that will withstand exposure to sun, wind, and rain for decades. And somehow, they have done just that.

The trade-off is that the power production of a solar power cell declines over time. After many years, the transparent surface yellows or browns, hot spots develop, solar bonds degrade, and a variety of other changes occur.

How quickly do solar panels degrade?

The rate of degradation can vary significantly, depending on factors such as date of manufacture, build quality, level of exposure to the elements, and more. A solar panel built in 2005 would likely degrade faster than one built in 2015. A high quality solar panel will probably degrade more slowly than a cheap panel made by an anonymous Chinese manufacturer.

For some time, the general rule of thumb was that panel production degraded at a rate of about 1% per year, compounded. This meant that a panel was expected to operate at 82% efficiency after 20 years, 74% after 30 years, and 66% after 40 years.

However, recent research suggests that this has been overstated to some degree. A meta-analysis of 11,000 solar power installations found that the median degradation rate was 0.5% to 0.6%, meaning for every panel that declined more slowly, another declined more quickly. The mean average degradation rate was 0.8% to 0.9%.

Why the discrepancy?

Manufacturing quality has a huge impact on the rate of solar panel output degradation.

As we mentioned above, the quality of a panel makes a big difference in its lifespan. In studies, low quality panels weigh down the average, degrading at rates meeting or exceeding that old rule of thumb standard of 1%.

Lower quality systems are also more prone to more significant malfunctions which limit performance. For instance, in a study of a 20-year-old solar power system which experienced degradation of 0.8% per year, it is discussed how most strings of modules in the system degrade at 0.4% to 0.6% per year. But a single string degraded at a faster rate, reducing the system’s performance as a whole. Issues like those observed in the study, such as flawed solder connections producing hot spots, can crop up in any system. However, these flaws are more likely to occur in poor to average quality systems.

On the other hand, high quality panels, such as those manufactured by SunPower, show much lower degradation rates. In a 2013 study, to ensure that their degradation models matched reality, SunPower examined 266 large-scale installations of previous generations of SunPower panels, as well as 179 non-SunPower sites. They found that the non-SunPower systems had degraded at a rate of 1.25% per year, while the SunPower systems had degraded at 0.32% per year.

Based off of the data gathered from this research, SunPower anticipates that their Maxeon modules—used in their X-Series and E-Series products—will degrade at a rate of 0.17% per year, and up to 0.29% per year in hot, dry climates. Their expected average degradation rate is less than 0.25% per year, across a broad range of climates and installation conditions.

But doesn’t this all boil down to differences of a few tenths of a percent? Does that really matter? It does, because yearly degradation compounds over time. Below is a comparison of the output of solar power systems over a 50-year period, with degradation rates of 0.25% (SunPower’s expected average), 0.4% (an excellent rate of degradation), 0.8% (the mean average found in some studies), and 1% (the old standard).

Degradation at a rate of 0.25% to 0.4% per year ensures that your solar power system continues to produce more than 90% of its original power production for 25 years. But with degradation of 0.8% per year, production drops below 90% before 15 years have passed. The old standard of 1% degradation means that you barely make the 10-year mark with +90% production.

It’s one thing to estimate future electricity production. But what guarantees are there that a solar panel will perform in the long-term?

Well, until recently, SunPower offered a power warranty for their systems, guaranteeing 95% efficiency for the first 5 years, and then degradation of no more than 0.4% per year for the next 20 years. This would mean that SunPower panels were guaranteed to produce at least 87.6% of the electricity they generated when new.

But recently, SunPower improved their power warranty. Now, they guarantee 98% production for the first year, and degradation of no more than 0.25% per year for the following 24 years. The warranty guarantees that the power output at the end of the 25^th year will be at least 92% of the Guaranteed Peak Power rating.

Compare the rates of degradation described above with SunPower’s old and new warranties in the chart below.

As you can see, even panels with a consistent 0.4% yearly decrease in energy production drop to 90% by the 25^th year, while SunPower guarantees their panels to beat that by about 2%. Meanwhile, “average” panels with degradation rates of 0.8% will have dropped to nearly 80% by year 25.

By buying high-quality solar panels that are warrantied to preserve their capacity to generate electricity over the long-term, such as those made by SunPower, you ensure that you continue to get what you paid for, year in and year out. Investing in high-quality solar panels maximizes your energy savings, as well as your home’s resale value.

If you’re considering purchasing a new solar power system, and want to learn more about how to ensure that it continues to serve you well in the decades ahead, contact Capital City Solar by calling (916) 782-333, or send us a message using our contact form.