A new anomaly study by 3TEIR examined how extreme weather events affect irradiance, which is the amount of sunlight that hits the land.  The study found that solar irradiance was off from its long-term norm.  Weather report maps indicate a strong correlation between weather events and their effects on irradiance this past summer.

3TIER, a renewable energy assessment and forecasting company, released solar performance maps of the Continental U.S. on Oct. 9 for the months of June, July, and August of 2013.

“One surprise of the 2013 solar performance study was that areas of the country typically associated with summer sunshine had below normal irradiance, while the Pacific Northwest, often stereotyped as cloudy and rainy, had one of the clearest summers on record,” said Dr. Mark Stoelinga, Senior Scientist at 3TIER. “This underlines the fact that solar project underperformance is a risk—even for sites in sunny, desert regions during peak production months.”

“The massive installations now online in the southwestern U.S., as well as the tight cluster of projects along the Northeast, definitely saw reduced production this summer,” added Gwendalyn Bender, Energy Assessment Product Manager at 3TIER.

The results suggest that solar power is subject to climatic variability and that such variability should be factored into the financial structure of solar projects since the variability of weather conditions can affect the profitability of a solar project.

“For some of the projects, this was only their first year of operation, which makes it particularly important to understand why output was lower than expected,” Bender stated. “Since the equipment is still under its first year manufacturer’s warranty, determining whether underperformance was caused by weather or by panels and inverters is critical.”

The company bases its solar performance analysis on its global solar dataset. The dataset includes continuous, hourly records of irradiance that span the globe for well over 15 years. The company updates the results on a monthly basis for more regions.

3TIER found that during this particular time period, irradiance was off by about 20 percent from the norm.

While irradiance and other factors are considered when projects are undertaken, the annual variability from the norm might not be considered when making financing decisions regarding solar projects, regardless of whether they are photovoltaics or concentrating solar power projects. The research from 3TIER suggests that annual variability of such systems should be considered when financing such projects. It's important to note, however, that the production of solar projects has not been calculated to determine whether or not the amount of energy produced by the projects varied as much as the irradiance did.