Most solar panels are designed to last 25 years. The useful lifetime of solar panels is determined by the amount of performance degradation that the solar PV cells suffer.

Performance degradation refers to the decrease in the efficiency of solar panels as they age. Reputable manufacturers typically guarantee that their panels lose less than 0.7% efficiency per year for 25 years. If the efficiency losses are more than 0.7%, the manufacturer will usually replace your panels under their warranty program.

What are some of the reasons for performance degradation? They include:

• Weight on the panels: Putting weight on the panels damages the structure of the solar cells. The loading can come from factors such as wind, snow, or even someone leaning on the panels.

• Moisture: Since solar panels are installed outside, moisture can slowly gets into the panels.

• Radiation Exposure: Even though solar cells are designed to convert sunlight to electricity, the ultraviolet radiation still damages the cells over long periods of time.

In recent years, the useful lifetime of solar panels has increased. A 2016 report from SolarCity has indicated that most of the systems they are installing should have a useful lifetime of 35 years based on the current rates of performance degradation in their installations.

We should keep in mind that SolarCity typically uses high quality panels which may account for the longer lifetime of their systems. The overall industry average for solar panel lifetime is still hovering around the 25 year mark. However, it is encouraging to see that companies can make solar panels that last dramatically longer.