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Solar owners can see weaker performance even on clear, cloudless days.
Photo Credit: iStock
When temperatures rise, solar panels can do something that seems counterintuitive: produce less power right when sunlight appears strongest.
That was the frustrating experience one U.K. solar owner described after temperatures hit 100.5 degrees Fahrenheit (38.1 Celsius) in London, with an infrared camera showing panel surface temperatures reaching 177.8 F (81 C). They took to Reddit to seek answers, and commenters suggested that while some loss is normal, there could be a fixable issue causing a bigger impact than what’s normal from heat alone. 
The owner’s complaint centered on a steep drop in output during the heat wave.
In a post on Reddit’s r/SolarDIY, they said a two-array system tied to Victron and Pylontech equipment was delivering far less than expected: “The main array is running at 40% of what it should be! (3.2/8). I know heat is a problem for panels but a 60% loss is pretty awful.”
While solar panels will easily pay for themselves over a long enough period of time — typically 7-10 years, according to EnergySage, after which it’s all profit — it’s still frustrating when a particular day is hamstrung by circumstances outside of the homeowner’s control.  
The setup uses 20 400-watt panels on one maximum power point tracker and 12 300-watt panels on a second. The larger array usually reaches about 5 kilowatts in bright summer heat, while the other gets to roughly 3.5 kilowatts because those panels are “less than ideally placed.” 
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During the heat spike, though, the best output was only 3.2 kilowatts and 2.1 kilowatts.
An update highlighted just how hot the system had become. After checking the panels with an infrared camera, the poster wrote, “Just checked panels with infrared camera. 81C in some places. Dems working hard.”
Replies generally agreed that heat can reduce solar production, but several people said temperature alone probably would not account for a 60% decline.
One commenter wrote, “Solar cells lose about 0.35% per degree Celsius” — mostly accurate, as the typical ranges cited depend on the type of panel, but it’s generally 0.3% to 0.5% per degree Celsius above the panel’s rated temperature. Another added, “Heat affects output as others have said, but not anywhere near 60%. You have other issues to look for.”
Most solar installations come with strong warranty protections, something that all interested in solar should ensure before moving forward. Those picking a solar lease or power purchase agreement, as offered by companies like Palmetto, can also get included maintenance since the company owns the panels and is contractually obligated to maintain production — often to 90% of the panels’ expected annual generation, depending on the company and its terms.
Nameplate solar ratings are based on standard test conditions, including a cell temperature of 77 F (25 C), not the much hotter conditions panels can reach in direct summer sun. In real operation, panel surfaces can run well above ambient air temperature, and as the cells heat up, voltage falls, and total power drops with it.
Solar owners can see weaker performance even on clear, cloudless days. That is one reason production records often happen on bright but cooler spring days instead of during the hottest stretches of summer.
The poster also indicated the battery bank was not limiting generation. They said their 32-kilowatt-hour batteries were empty in the morning, the batteries themselves were at 89.6 F (32 C) despite being rated to 131 F (55 C), and the garden equipment room had forced ventilation.
Other users described losses that were noticeable but less severe.
One said a standalone 465-watt panel, tested by itself in very hot weather, was producing about 38% of its rating.
Another, writing from southern Spain, said a plug-in solar kit had dropped from more than 10 kilowatt-hours per day in June to around 8.6 kilowatt-hours per day in August at 98.6 F (37 C).
Some summer underperformance is expected, but a drop this large can justify troubleshooting beyond the weather.
Checking for dirty panels, shade, wiring issues, voltage drop, inverter behavior, and high component temperatures can help distinguish ordinary heat derating from a system problem.
Dust buildup was one possible explanation raised in the discussion, especially after a long dry period, and commenters noted that bird droppings or partial shading can have an outsized effect. Depending on the inverter setup, too, panels may drop in output together with shade or other obstructions primarily affecting only one panel, if they are sharing an inverter rather than each functioning on their own inverter. 
The poster replied that the panels had been cleaned and that production improved afterward.
If the production curve shows abrupt dips instead of a smooth bell shape, that may suggest inverter throttling, overheated electronics, or an intermittent connection rather than straightforward heat loss.
When the poster suggested testing a spare panel with water, another commenter warned, “Don’t hose it when the panel is scorching hot. Can cause the glass to crack,” and the poster agreed, “Should have said — yes. Of course. Thermal shock will kill it.”
Extreme heat does not mean solar is failing, but it does show how real-world conditions can differ from brochure numbers.
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© 2025 THE COOL DOWN COMPANY. All Rights Reserved. Do not sell or share my personal information. Reach us at hello@thecooldown.com.

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