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Wednesday, June 30, 2021

Heat Impact on Solar Production

In my last post, I quipped that our solar energy production dipped because of the recent high temperatures. Thinking more about this, I decided that I had only looked at a couple of days. This is one of the most common human fallacies. I had an idea, I looked for confirming data, found some, and assumed that means I was correct.

You overcome this, not by trying to prove yourself right, but by looking for credible data to prove youself wrong. Well, it didn't take long. 

Day   Fri   Sat   Sun   Mon   Tues 
Temp (F)   93°  108°  111°  115°  91°
Solar (kWh)   75.1   50.5   73.5   49.4   75.2 

Looking at the table, if you compare Friday and Saturday, it sure seems like the increased temp resulted in lower solar production. Saturday was 15°F hotter and it had ~25kWh lower output. 

Comparing Monday and Tuesday is a similar story. Monday was 24°F hotter and had about 25kWh lower output. 

Problem solved right? Nope. Sunday shatters this correlation. Sunday was hotter than Saturday but had solar production closer to the cooler Friday and Tuesday. Perhaps I was too quick to blame the production fluctuation on heat. 

It is well known that heat can impact solar efficiency, but the impact is not as significant as I thought. Each brand/model of solar panels is a little different, but they all publish the heat impact by listing the efficiency impact for each degree Celsius above 25°C. One reasonable example is negative 0.258% per degree C. So going from 93F to 108F is about a 7°C change. This would be about a 2% efficiency change. This alone does not account for the ~30% decrease from Friday to Saturday. Clearly, something else is happening too. 

Looking at weatherunderground and other sites, these days went from "passing clouds", to "clear", to "sunny." Meaning that, despite all of them being hot, hot days, they had varying levels of cloud coverage. Sunday had the least cloud coverage and solar production remained high later into the evening (8PM) than other days.

So there you have it, the variance can primarily be blamed on clouds, not the heat.

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