Preface Overview
The Summer Paradox: Abundant Sunlight vs. High Temperatures
During summer there are longer days and stronger sunlight. In the Northern Hemisphere, for instance, there can be more than 14 hours of sunshine in a day . This longer time lead gives greater time for energy generation. However, the relationship between heat and efficiency is complicated.
Photovoltaic panels will tend to operate less efficiently as temperatures rise. The temperature coefficient of solar panels, typically -0.3% to -0.5%/oC can be interpreted to mean for every degree a panel is above 25oC (77oF) the efficiency of the panel drops slightly . During extreme heat waves, summer temperatures can result in efficiencies that are 5-7% lower . In addition to the heat, summer also welcomes haze and dust resulting from air pollution that both scatter and absorb sunlight before it hits the panels .
A wonderfully illustrative real-life example of this is from a 300MW solar farm in Fushun, China. The farm produced an astonishing (relatively) 36.2 million kWh in July 2024, and significantly lower than the output of 47.85 million kWh in March, and 40.09 m illion kWh in October. The operator of the farm counted the angle of the sun ever summer to optimally tilted panels is less direct than in the spring and autumn months which caused the farm to produce less energy during the long days.
Winter Challenges: Shorter Days but Cooler Temperatures
Conditions in winter are the opposite of the summer conditions. The first and most obvious restriction is the reduced number of daylight hours. Reduced hours means there's less time for potential production of electricity, and with the sun path also being lower in the sky, sunlight has to travel through more of Earth's atmosphere, meaning it will be less intense by the time it reaches the panels.
Even with these two challenges, though, cold winter air can be a boost in performance. Solar panels perform most efficiently in cooler temperatures. The same temperature coefficient associated with losses in the summer heat is attributed to performance improvements in cooler weather. Consequently, take benefits on an immediately bright and cold winter day. A solar panel may well perform better than expectations.
Winter's impact depends significantly on geography. Northern areas may experience large decreases in production, for example, in San Pedro Town, Belize, where average winter output data per day is 5.09 kWh per kW capacity, versus summer average output of 6.90 kWh and. A residential system in the UK experienced its lowest output of just 10.3 kWh on a rainy winter day.
Beyond Temperature: The Crucial Role of Technology and Location
The gap in seasonal performance is not simply related to the weather; it is influenced by geography and technology.
Panel Technology: Today's designs of solar panels minimize annual losses even during the winter conditions. Bifacial (two-sided) panels generally derive more benefit from reflective light from the snow on the ground during winter. In a study conducted by the National Electric Power Investment Group (State Power Investment Corporation) found that bifacial TOPCon modules have superior annual production in part due to their bifaciality (in excess of 80%). Bifaciality is the panels ability to capture reflective light, which this study noted could contribute 15-25% of winter production from reflected light coming from the back side of the panel, which assists in balancing bottom-line performance to lower summer performance due to the high heat .
Geography: The seasonal pattern of production performance is quite different depending on geography/climate region.
Temperate regions: Spring and autumn are often the actual "golden seasons" for maximum energy production, as those time periods are the ideal conditions for sun angle, clarity of air and moderate temperatures.
Tropical regions: Tropical regions often do not exhibit a large temperature change between seasons, but instead experience wet/dry cycles. Summer (the dry season) is most often the peak production season.
Hot arid regions: Even with high heat during summer, summer remains by far the most productive time of the year with clear skies and long days.
Key Takeaways for Solar Owners and Advocates
The seasonal variability of solar output reinforces a couple of key points. One, since systems should be sized and financed based on annual energy yield instead of peak summer production, your needs can be satisfied throughout the year. Secondly, technologies such as bifacial panels, and tilt adjustments for the rack that would wait to their seasonal position, are helpful if not the best way to optimize generation over various time periods.
Maybe the most significant insight is the increasing value of energy storage and flexibility from the grid. For example, similar to what was experienced in the European heatwave, solar generation can dominate daytime generation, however, the greatest peak demand tends to occur at night to range the day temperature down. It suggests that the future of solar will now start to be less about generating energy from solar when the sun shines, i.e., generation, and more about how well the power is going to be managed and stored for those moments of peak demand.
In general, summertime is great, however, it tends to provide too much sun. On the other hand, the highest solar output from solar panels tends to be during the more mild spring and fall days. Moreover, even if generating in winter is down on overall, the generation is more efficient in colder weather. This knowledge helps better design our system and understand the role of solar energy in delivering clean energy throughout the year.
