March 12, 2018 Solar Energy Written by Greentumble Editorial Team
Effect of temperature on solar panel efficiency
Imagine one of those searing hot days,

when all you can do is to sip a margarita somewhere in the shade. How would you perform on a day like that if you were asked to run a marathon? Not that well, right? Our body functions the best when the temperature is within our optimum range. Beyond this range we have to work much harder to maintain our performance level.

As surprising as it may sound, the same principle applies even to solar panels and their capacity to generate electricity!

Effect of temperature on solar panel efficiency

Most of us would assume that stronger and hotter the sun is, the more electricity our solar panels will produce. But that’s not the case, because one of the key factors affecting the amount of power we get from a solar system is the temperature.

The output of most solar panels is measured under Standard Test Conditions (STC) – this means a temperature of 25 degrees Celsius or 77 degrees Fahrenheit. The test temperature represents the average temperature during the solar peak hours of the spring and autumn in the continental United States [1].

According to the manufacture standards, a 25 °C (77 °F) temperature indicates the peak of the optimum temperature range of solar panels. It is when solar cells are able to absorb sunlight with maximum efficiency.

What happens when the temperature of solar panels increases?

At around 25 °C (77 °F), the electrons inside the panel are resting at a so-called ground state. When these electrons receive extra energy, they enter a new state – known as the excited state. The extra energy that elevates them to the excited state can come from two different sources – from light (sunlight) or from heat.

If you have a solar system installed at home or plan to get one in the near future, it’s useful to have a good understanding about the difference between the energy of electrons at rest and electrons in the excited state, because this energy accounts for the power output produced by solar panels.

When solar panels are absorbing sunlight, their temperature rises because of the sun’s heat. When this happens, the energy received by the resting electrons from heat is decreasing the amount of power we get from heated solar panels [2].

The common material used in solar cells, silicon, does not help this either. As a great conductor of heat, silicon actually speeds up the heat building in solar cells on hot sunny days.

In a nutshell: Hotter solar panels produce less energy from the same amount of sunlight.

The effect of temperature on solar panel output can be calculated and this can help us determine how our solar system will perform on hot summer days. The resulting number is known as the temperature coefficient.

Solar panel temperature coefficient

The temperature coefficient tells us the rate of how much will solar panel efficiency drop when the temperature will rise by one degree Celsius (33.8 °F).

For example, when the temperature coefficient is minus 0.5 percent, it means that efficiency decreases by 0.5 percent for every degree above 25 °C (or every 33.8 degrees above 77 °F).

Solar panels from different manufacturers will vary in their temperature coefficients. That is why all solar panel manufacturers have to provide a temperature coefficient (Pmax) along with their product.

In general, most solar panel coefficients range between minus 0.20 to minus 0.50 percent per degree Celsius. The closer this number is to zero, the less affected the solar panel is by the temperature rise.

How hot do solar panels get? Can they overheat?

The maximum temperature solar panels can reach depends on a combination of factors such as solar irradiance, outside air temperature, position of a panel and the type of installation, so it is difficult to say the exact number.

Solar panels fully exposed to the midday heat

Solar panels fully exposed to the midday heat

Generally, solar panels are made of dark-colored silicon cells, covered by a sheet of glass and framed in metal. Silicon and metal are good conductors of heat, contributing to faster buildup of heat inside solar cells. Even though, solar panel manufacturers and installers apply mechanisms to prevent solar panel overheating, in extremely hot conditions the energy output of solar panels might decline significantly.

In summer 2017, The Times published an article discussing the problem of Qatar being too hot for photovoltaic solar panels. According to the article, the combination of temperatures rising up to 50 °C (122 °F) and dust reduces the power output of solar panels down to less than 40 percent.

What can you do to stop your panels from getting too hot?

Being aware of the effect higher temperature has on the energy output, most certified installers take steps to support natural cooling of solar systems. A good practice is leaving at least six-inch space between roof and panels to allow air circulation from both sides.

But attaching your panels too far from the roof is not always a good idea. If the gap is too big, debris of leaves and twigs could accumulate underneath the array and cause damage to your roof or panels.

If you live in a hot climate, you should maybe consider ground-mounted solar panels, because this way they get the most airflow to keep their temperature lower. According to estimates, the temperature difference between the ground-mounted and roof attached solar panels in the same location can make up to 10 °C (50 °F) [3].

Ground-mounted solar panels

Ground-mounted solar panels

The best option is to get solar panels with temperature coefficient as close to zero as possible. The difference in total power output throughout the year can be significant. For example, if your solar panels have a coefficient of minus 0.4 percent, their output on hot days will drop nearly twice that much compared to the output of a panel with a coefficient of only minus 0.2 percent per one degree Celsius.

White or light-colored roofing also helps to lower the temperature around your panels, since these colors reflect sunlight more and do not get heated up like dark roofing.

While above mentioned points involve passive cooling methods, some people opt even for active cooling systems. For example, fans that blow air over panels, or circulating cold water which absorbs the heat from the panels and is then utilized in the household for showering or heating the building [4].

A side note: Be cautious about hosing down your panels during the hottest part of the day! It could make the glass crack and irreversibly damage your panels. The systems with water cooling do not expose solar panels to such a sudden temperature shock like you hosing them down would.

Before you decide on a solution that would work the best for you, do your research well. As you can see, there are already options to perform in different conditions and help you to save money – even on water heating.

If none of them look appealing to you at the moment, do not despair. We live in the era of an amazing development of solar energy. Just as we speak many scientists are working on tackling issues of solar panel efficiency and performance optimization.

Scientists from the Stanford University have already pioneered a concept of “self-cooling” solar cells, which will be able to re-direct the heat from the cell’s surface. This design might be just one of many future solutions to tackle the problem of solar cell overheating.

So, let’s enjoy this solar revolution.