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Solar Panel Output Decline Over Time

Solar panels slowly produce less as they age. Knowing what is normal helps you decide whether a lower number on your monitoring app is aging or a fault.

What The Research Found

The Department of Energy OSTI record for "Photovoltaic Degradation Rates, An Analytical Review" (NREL, June 2012), by Jordan and Kurtz at the National Renewable Energy Laboratory, reviewed degradation rates of flat plate modules and systems reported from field testing over the 40 years before 2012. The abstract says nearly 2000 degradation rates, measured on individual modules or entire systems, were assembled from the literature, showing a median value of 0.5 percent per year.

That is a median across many technologies, ages, and locations, so a single system can sit above or below it. We did not read the full paper and we are not using it to predict your output.

Normal Decline Versus A Problem

What You See	What It Points Toward
A small drop over several years, compared in the same month	A slow drop is the kind of change the median rate above describes, though weather and shade also move the numbers
A sudden step down on one day	An equipment event, not aging. See Solar System Not Producing.
Flat or zero lines, or missing data	Often a communication or inverter issue. See Solar Monitoring Stopped Working.
Visible cracks or glass damage after a storm	See Solar Panel Snow And Hail Damage

Compare Fairly

1. Use the same month in different years. Sun, clouds, and temperature change by season.
2. Note any new shade, such as tree growth or a neighbor's addition.
3. Check Solar Panel Cleaning for what the manufacturer says about washing before you assume the panels are the issue.
4. Keep exports of your monitoring data. A service company can use them.

When To Ask For A Review

If the drop is bigger than you expect, or sudden, ask the manufacturer what the product and power warranties say, using Solar Warranty Help. We did not find a single percentage that applies to every system and we are not offering one, so use the warranty document for your model.

If you want a local service company to look at your system, describe it in the system check form. We name the one company before anyone contacts you.

Related: Solar System Not Producing, Solar Monitoring Stopped Working, Solar Warranty Help, Solar Monitoring, Repair, And Inspection.