The solar myth exposed: Why the heatwave is making your panels less efficient

As the UK experiences what is being described as the hottest May on record, renewable energy specialists at Dwellow are reminding homeowners that while sunshine is excellent for solar generation, extreme heat can actually reduce solar panel efficiency.

With temperatures soaring to 35C across parts of the UK this week, many homeowners may assume their solar systems are performing at their absolute peak. However, according to Dwellow founders Ian McLaren and Adam Raw, solar panels operate most efficiently in bright sunlight combined with cooler temperatures, not necessarily during heatwaves.

“Solar panels generate electricity from sunlight, not heat,” explained Ian McLaren, founder of Dwellow.

“There’s a common misconception that hotter weather automatically means better solar performance, but in reality, excessive heat can reduce efficiency. A bright, sunny 20°C spring day with clear blue skies can often produce better generation than an extremely hot 35°C day.”

Most solar panels are tested under Standard Test Conditions (STC), of around 25°C. STC is the global industry benchmark used by manufacturers to measure and rate the peak power output of solar panels. Once panel temperatures rise significantly above this level, performance begins to decline because the silicon inside solar cells becomes less efficient at converting that light into usable electrical voltage.

Typically, solar panels lose around 0.3% to 0.5% efficiency for every degree Celsius above 25°C. During extreme UK heatwaves, roof-mounted panels can easily reach surface temperatures of 60°C or more, potentially reducing output by 10-20% compared to their optimal operating conditions.

Adam Raw, founder of Dwellow, said in-roof solar systems are particularly vulnerable during periods of extreme heat.

“In-roof systems sit flush within the roof structure, which looks fantastic aesthetically, but they generally have less airflow behind the panels compared to on-roof systems,” he said.

“That reduced ventilation means heat becomes trapped more easily, causing the panels to run hotter and therefore slightly less efficiently during very warm weather.”

Traditional on-roof systems are mounted above the roof covering, allowing air to circulate underneath and naturally cool the panels.

It’s worth noting that reduced solar output during extreme heat is not always due to the panels themselves, as inverters can also limit system performance through thermal “derating” when they overheat, particularly if they aren’t installed in a cool, well-ventilated area.

Despite the reduction in panel efficiency during very high temperatures, Dwellow stresses that they still perform extremely well during sunny weather overall and are still one of the most effective ways for households to cut energy bills and reduce carbon emissions. 

Despite this temporary reduction in efficiency, extended daylight hours and clear blue skies during UK heatwaves still tend to produce some of the strongest solar generation days of the year overall. 

“The good news is that even with some efficiency loss during peak heat, long sunny days still provide excellent solar generation,” added Ian McLaren.

“What matters most is solar irradiance, which is the amount of sunlight reaching the panels, rather than air temperature alone. That’s why the UK’s clear, bright spring days are often ideal for solar production.”

Dwellow says the recent hot spell highlights the growing importance of smart solar system design, ventilation, and battery storage as the UK experiences increasingly extreme weather patterns.

Homes with solar and battery storage systems can expect a drop in electricity bills of between 50% - 80% depending on their system size and set-up, usage and battery size.

Get a no obligation, free solar estimate in two minutes at dwellow.co.uk.