Through four years of work and a “massive” data set, NREL researchers say they have discovered that extreme weather can have small but noticeable effects on photovoltaic (PV) system performance, but not enough to suggest that PV is inherently unreliable or vulnerable to extreme weather.
With PV systems designed to operate for decades, even small losses in production can lead to a system becoming unprofitable, the NREL noted, but small changes in production can be hard to measure. Its report, Extreme Weather and PV Performance, aimed to address this challenge by compiling data from an “unprecedented” number of PV systems across the U.S. – 25,000 inverters across almost 2,500 commercial- and utility-scale PV sites in 37 states and U.S. territories.
By comparing the performance of systems in the PV Fleet data set against a National Oceanic and Atmospheric Administration (NOAA) map of extreme weather events, the researchers studied how each system’s performance was affected when an extreme weather event occurred within 10 kilometers of its location.
It’s no secret that solar panels will degrade over time, although the causes and average rate of PV performance loss are often debated, NREL said. In 2022, the PV Fleet Performance Data Initiative found a national median loss in performance of 0.75%/year, which the NREL said confirmed similar values reported by previous studies that analyzed smaller data sets. The analysis also found that that systems in hotter temperature zones exhibited about twice as much performance loss as those in cooler climates (0.88%/year and 0.48%/year loss, respectively).
“This median loss in performance is a crucial number,” said Chris Deline, a group manager for PV field performance at NREL and author of the PV Fleet publications. “First, it shows that our fleet of PV systems, on the whole, is not failing catastrophically, but rather degrading at a modest rate within expectations. It’s important that we quantify this rate as accurately as we can because this small but tangible number is used in almost all financing agreements that fund solar projects and provides critical guidance for the industry.”
In the latest report, researchers found that short-term outages caused by extreme weather, such as outages due to PV modules being disturbed by strong winds or inverters being damaged by flooding—have a minimal impact on most systems.
Over the 2008–2022 time range studied, researchers found that the median outage length after an extreme weather event was two to four days, resulting in only a 1% median loss in annual performance. A very small number (12 systems out of 6,400) experienced much longer outages of two weeks or more. Most outages occurred because of flooding and rain, followed by wind events. And most systems in the data set only experienced one weather-related outage.
However, aside from short-term impacts, extreme weather like wind, hail, or snow can cause cracked cells or other forms of degradation.
Researchers say a “clear” trend emerged in the long-term performance of PV systems after exposure to extreme weather. Following extreme weather events above a certain threshold – hail greater than 25 millimeters (1 inch) in diameter, winds in excess of 90 kilometers/hour (56 miles/hour), or snow depths greater than 1 meter – systems showed greater annual performance losses. Weather below these thresholds showed performance losses similar to the PV Fleet’s average.
In a finding that NREL says should suggest more stringent testing, even systems composed of modules qualified through International Electrotechnical Commission (IEC) 61215—the industry standard that includes a test for resistance to impact by 25-millimeter-diameter hail—showed higher performance loss rates when exposed to that same size hail in natural settings.
The researchers also noticed that in structures damaged by winds over 90 kilometers/hour, some systems avoided damage, possibly due to wind shadowing from adjacent structures slowing down wind speeds.
“We don’t feel any of this analysis suggests that PV systems are unreliable or especially vulnerable to extreme weather,” said Dirk Jordan, a distinguished member of the research staff at NREL and author of the PV Fleet publications. “PV has demonstrated that it can provide backup power and save lives when surrounding infrastructure is damaged by extreme weather events. Yet, there are further measures we can take to improve the quality of equipment and especially installation best practices to increase resilience to these weather events.”
The first step for module manufacturers and PV testing organizations, NREL says, is understanding the thresholds at which damage can occur. This will allow for proper design and realistic testing to hopefully mitigate impacts from extreme weather events,
Recent manufacturing trends like larger modules, thinner cells, and thinner front glass may make some systems especially vulnerable if not designed or tested appropriately, NREL said.