The rule of thumb on the long-term performance of photovoltaic panels is that output will decline by about 1% each year. After 20 years in service, panels should still be able to produce roughly 80% of their rated capacity.

But research at the National Renewable Energy Laboratory finds that rate of degradation is probably overstated, according to a post at Engineering.com.

Researchers found the output of the most common type of panels made with mono-crystalline silicon fell by less than 0.5% for panels manufactured before 2000 and by less than 0.4% for panels made after that date. The difference is significant: after 20 years of use, panels manufactured today should be producing 92% of their original rated capacity rather than 80%.

(To read the results of tests performed by GBA senior editor Martin Holladay on an ancient PV panel, see Testing a Thirty-Year-Old Photovoltaic Module.)

Rates vary by climate

Researchers found some variation in the degradation rate depending on where the panels were installed, Engineering.com said. In extreme climates, such as areas where panels were subjected to heavy snow or wind loads or high levels of ultraviolet radiation, degradation rates were nearly 1% per year.

In moderate climates, rates were as low as 0.2% per year; after 20 years of service, those panels would still be producing 96% of their rated capacity.

The NREL report, originally published in 2012, reviewed degradation rates taken from published reports and field testing over a span of 40 years. “As photovoltaic penetration of the power grid increases, accurate predictions of return on investment require accurate prediction of decreased power output over time,” the report says. “Degradation rates must be known in order to predict power delivery.”