As if displaced desert tortoises, burned birds and political theater around the loan guarantee program weren't enough problems for the Ivanpah concentratingsolarproject, the owners are now dealing with lower-than-expected electricity production.

From January to August of this year, the 392-megawatt plant generated far less than the electricity expected. For some, it's yet more proof of the technical limitations of solar -- and an example of another bungled government investment in clean energy.

But there's a simpler explanation for the early woes faced by Ivanpah: a spurt of bad weather.

"The developer [BrightSource Energy] understood the full implications of having a bad year when planning. "They were disappointed it wasn’t sunnier, but they weren’t surprised," said Gwendalyn Bender, an assessment services project manager for Vaisala, a company that measures industrial processes, including weather for renewable power plants.

It wasn't just weather. BrightSource said some early equipment issues "impacted plant availability" as it ramped up to four-year targets. But a poor summer made things worse.

Vaisala (which bought the resource analysis company 3TIER last year) produces data sets on resource variability for wind and solar project developers. When analysts at the company looked at this year's solar irradiance, they found wild swings in the number of sunny days in the Western U.S.

The map below shows a strong departure from average irradiance levels in Southern California and Nevada over the last fifteen years. The Ivanpah project is located in Southern California, near the border with Nevada -- right in the blue part of the map.

The wind and sun are free resources. But if the weather isn't cooperating, developers can take a big financial hit. Ivanpah, which is operated by NRG Energy and partly owned by Google and BrightSource, generated millions of thousands of dollars less than expected.

The project owners are now taking advantage of cash grants from the Treasury to help repay their initial loan from the Department of Energy. They're also burning more natural gas to make up for the drop in electricity output.

"Industry-wide figures suggest that a 5% negative anomaly in solar irradiation during the summer months for a CSP or PV site can translate to a 1% to 1.5% decrease in annual revenue for a solar developer or operator, highlighting a clear industry requirement to take a more comprehensive approach to gathering long-term weather data," wrote Vaisala its analysis of 2014 solar irradiance.

Here's how that variability broke down by month this summer:

"A lot of developers that aren't analyzing an entire time series can be totally blindsided when they have a big issue," said Bender in an interview. "You can be really surprised when there's a bad year and not understand what's going on."

NRG, Google and BrightSource are likely very unhappy with initial performance. But with more than a decade worth of data, they knew a dramatic change was a possibility. 

Bender said that most solar project developers assess resource availability based only on one modeled year of irradiance data. In order to account for change, developers commonly assume a 2 percent drop in resources. But as the 2014 California/Nevada data proves, the change can be far more dramatic.

The problem can go the other way as well. If developers assume too much variability risk because of limited modeling, they can "leave money on the table" when securing financing, said Bender.