Subpar Solar Irradiance Undermines East Coast Project Performance

Vaisala 2014 solar study reveals role of extreme weather patterns in growing East-West productivity divide

The East Coast of the U.S. experienced up to 5% lower than average levels of solar irradiance in 2014, negatively impacting overall performance of solar sites in the region. Concurrently, the West Coast enjoyed irradiance levels up to 10% higher than average, widening the productivity gap between projects in the country's two areas of highest solar development.

This disparity is clearly seen in the 2014 Solar Performance Maps of the United States, released today by Vaisala, a global leader in environmental and industrial measurement.

2014 US Solar Performance Map by Vaisala

While it is well-established that the solar resource of the East does not match that of the West, the Atlantic Coast has a large volume of operational capacity and it is therefore of critical importance for project operators in the region to understand the reasons behind below average energy output and whether it is due to malfunctioning equipment or weather variability.

Vaisala's 2014 study illustrates the impact of short-term, month-to-month weather variations on performance at U.S. solar projects and places them into a long-term context. This reveals frequent and significant deviations from long-term average irradiance conditions and highlights the clear requirement to analyze the effect of solar resource variability on both over and underperformance.

From an annual perspective, this year's weather patterns had an adverse effect on a large number of installations along the Atlantic Coast from Florida to Massachusetts, as well as Texas, while the bulk of projects in California and the Southwest saw an uptick in solar irradiance that may have increased overall 2014 production at many sites.

However, these annual variations only offer a high-level view of project performance. Vaisala also conducted a monthly analysis that gives a much more robust understanding of how specific weather conditions affected solar production throughout the year.

"The fact that California, for instance, had higher than average irradiance over the course of the year doesn't tell the whole story," said Gwendalyn Bender, Energy Assessment Product Manager at Vaisala. "We actually have many clients with projects in California coming to us with concerns about last year's production numbers. As seen in our study, monthly low solar irradiance anomalies had a significant impact on generation, even in the West."

The summer of 2014, for instance, was the coolest summer since 2009 and the ninth wettest summer on record within the continental U.S., with cloudy conditions leading to below average solar irradiance. This meant underwhelming solar performance in many regions where there are large numbers of operating projects, and negative media attention for solar projects recently put online - such as Ivanpah, Agua Caliente, Solana, and Topaz.

While the summer was disappointing, it was not the only or the most extreme anomaly of 2014. January, October, and December also saw atypical conditions where solar irradiance was more than 10% above or below average for much of the country*.

In terms of solar operations, routinely evaluating resource variability is critical for understanding in the short-term whether a project is performing as it should be, based on the available resource. This allows both distributed and utility-scale generators to effectively deploy maintenance and repair crews when problems, such as inverter issues or heavy snow on panels, lower production when resource data clearly show the sun was shining.

Vaisala's anomaly analysis is also crucial for viewing recent performance from a long-term perspective. It allows operators to understand whether recent conditions (low or high irradiance, heavy snowfall, etc.) foretell what can be expected throughout a project's lifetime or if it is out of the ordinary. For example, in Texas, where there is growing interest in solar and already a few operational projects, it is useful for developers and operators to understand that this past December's solar irradiance was extraordinarily low compared to previous years due to unusually foggy conditions.

Vaisala is an expert in solar measurement, project assessment, and energy forecasting. It acquired 3TIER in 2013, a company that developed the first high-resolution, global solar dataset and has helped secure over US$5.5 billion in project financing worldwide. For more information on the range of services offered by Vaisala to the renewable energy sector, please visit

Featured Product

Introducing RockIt Smart Slide

Introducing RockIt Smart Slide

Introducing RockIt Smart Slide - the ultimate solution for fast and easy installation of the rail-less RockIt racking system on composition shingle roofs. Smart Slide conforms to UL 441 and TAS 100 (A)-95 for wind and wind-driven rain, providing a super-strong watertight seal that is achieved through compression, in most cases without the need for additional sealant. The waterproofing sealant is embedded deep into the granules of the shingle, thanks to the integrated flexible foam layer that provides cushioning. This ensures a secure fit that conforms to any architectural-style shingle. With UltraGrip Technology™, you can rely on a secure installation, as it absorbs the movement created by thermal expansion and contraction. The pre-installed sealing pads are compatible with all composition shingle roofs, making it the ideal choice, even in ambient temperatures as low as 5 degrees.