Do solar panels keep your roof and attic cooler?
By shading your roof, solar panels can lower the temperature of your home by several degrees.
If you live in a hot climate, it’s likely that air conditioning is your largest energy consumer. Photovoltaic panels make a lot of sense in these climates, because hot climates also have higher solar energy potential.
Fortunately, solar panels can keep your house cooler. They do this by passively shading your building, helping to reduce the heat gain into your attic. Roof-mounted solar also have an air gap of several inches between the panel and the roof, permitting airflow to keep both the panels and roof cooler.
Whether or not you will will see a noticeable benefit from solar panel shading in your home depends on many factors, which this article will discuss in detail. First, let’s review how the type of roof you have can affect the temperture inside the building.
Roofs can get really hot
If you’ve ever stepped barefoot onto a sidewalk on a sunny day, you know how much heat energy a dark surface can absorb in just a few hours. Do the same on something really dark, like an asphalt driveway, and you can easily burn your feet.
Asphalt also happens to be the most popular roofing material in the United States. Made of a base material covered in asphalt and impregnated with stone granules, asphalt shingles are cheap and durable. But those dark shingles are also really great at absorbing the sun’s energy. Studies show that a traditional roof can get blisteringly hot: from 150 to 185°F in the peak of summer.
It’s not just asphalt shingles, though. Other types of roofing materials, including metal and clay tile, all get hot in the summer sun. The maximum temperature they reach depends on how much heat energy they reflect and emit.
Roof reflectance and emittance
There are two ways that a hot roof will shed heat: by reflecting the sun’s energy, and emitting heat that it’s already absorbed.
Reflectance refers to energy that a surface bounces away. A mirror doesn’t get very hot in the sun, because it’s a great reflector. The hood of a black car, however, doesn’t reflect much energy and can get searingly hot. Roofs are the same, and their reflectance depends on the color and type of material. Asphalt roofs might reflect between 5 and 15% of incoming energy, while the coolest roof materials will reflect as much as 65%.
Emittance is the heat energy that a hot surface radiates. Heat up a cast iron pan on the stove for a few minutes, and the pan will emit heat for several minutes after the stove is off. A hot roof is the same way. Emittance is one way that your roof sheds accumulated heat energy.
Here’s a graphic showing how different types of roof reflect and emit solar energy. For example, a black asphalt roof doesn’t reflect much energy, but does emit a lot of the heat it absorbs.
As you can see, even highly reflective metal roofs still absorb a lot of heat energy. It takes a roof that is specially designed to keep the building cool - a “cool roof” - to get a very substantial reduction in temperature. A roof designed to be cool can be as much as 50 – 60°F cooler than a conventional one.
Environmental benefits of cool roofs
The effect of a dark roof absorbing heat doesn’t just impact your own home. Dense cities full of houses, highrises, sidewalks, and roads can absorb so much energy that they can be 1.8 – 5.4°F warmer during the day and as much as 22°F hotter in the evening. All that heat that roofs and roads absorb during the day gets emitted at night, keeping the temperature of cities elevated long after the sun has gone down.
This is called the heat island effect, and it’s so significant that the US Department of Energy launched a cool roof initiative to help the federal government meet its greenhouse gas reduction targets.
One way they’ve implemented this is through white colored vinyl coverings, but homeowners can now get normal-looking shingles that stay cooler than conventional shingles.
How do solar panels keep your roof cool?
Solar panels will keep your roof cooler. There are four mechanisms by which they do this:
Mechanism 1: Energy conversion
Solar panels turn solar energy into electricity. Residential solar panels generally have an efficiency between 16 - 20%. This means that 16 - 20% of the energy that hits your solar panels isn’t reflected or turned into heat, but absorbed by the panel and converted into electricity, which is then carried away as DC current by wires.
In other words, solar photovoltaic panels basically wick away solar energy from your roof, carrying it off to do useful things like power your dishwasher and TV. This means that only about 80 - 84% of the remaining sunlight is left to potentially heat your roof up.
Mechanism 2: Reflectance
Like any surface, your solar panels will reflect some of the sunlight away. Even though solar cells are dark blue or black, they will still bounce away some percentage of the light that hits them.
In addition, solar panels are covered in glass and have an aluminium frame, both of which will also reflect some energy away.
Mechanism 3: Emittance
Just as an asphalt roof will heat up in the sun, a solar panel also absorbs some of the solar energy that hits it, causing it to heat up. Solar panels are dark, and some panels are deliberately made to be all-black for aesthetic reasons. They can get pretty hot, and some of that energy gets re-emitted back up to the sky.
Unfortunately, some of that heat also gets emitted downward toward the roof surface, where it can potentially heat up the building.
Mechanism 4: Convection through the air gap
A conventional home solar panel system is mounted on a racking system that is connected to the roof. That racking system holds the solar panels several inches away from the roof surface.
This standoff distance provides a gap through which air can flow. That airflow can carry away heat from the back of the panels and the roof surface, reducing the effect of heat absorbed by your solar panels.
Solar panel cooling research from UC San Diego and France
A couple studies on the effect of solar panels on the temperature of rooftops and the urban environment have been done by researchers in France and UC San Diego.
The French researchers used modelling to demonstrate that the combined effects of the four mechanisms (reflectance, emittance, convection, and energy conversion) described above not only make rooftops cooler during the day, but the cooling effect continues at night. In addition, the combined effect of many rooftop solar deployments can reduce the urban heat island effect: solar panels can help to keep not just homes but entire cities cooler.
At UC San Diego, researchers examined a laboratory building on campus that has solar panels. Collecting data with a thermal imaging camera over three days, the researchers found that the ceiling underneath the solar panels was about 5 degrees cooler compared to a section of ceiling that wasn’t covered with panels.
Their estimate was that the solar panels reduced the amount of energy reaching the roof by 38%. Over the life of the panels, the researchers estimate that this cooling effect adds an additional 5% energy benefit to the system. For a building owner, this is a great “free” bonus of having solar panels.
Will my house be cooler if I add solar panels?
The UC San Diego researchers saw a 5 degree reduction in ceiling temperature in their building, but your home might be different. How much heat is transmitted from your roof to the inside of your home depends on a lot of factors: the type of roofing material, whether you have external insulation on your roof, whether you have living space or an unoccupied attic directly underneath the ceiling, your attic ventilation, the level of insulation in your attic, and more.
There’s no doubt that solar panels can keep your house cooler, but how much cooler will depend on your situation. No matter what, it probably be a pretty minor effect. Still, it’s a “free” side effect of solar panels, with one important additional benefit: it will help your roof last longer.
UV light and heat accelerate the aging of your roof, so by keeping your roof shaded and cooler, solar panels will help prolong the life of your roof.
One last thing: this cooling effect is one additional reason to choose conventional solar panels over solar shingles, which sit directly on the building and don’t have an important air gap that aids cooling. The air gap is also a reason to not have an aesthetic frame placed around the perimeter of your array.
Some solar companies, such as GAF, offer this as a way to give a solar array a sleeker look, but blocking the air gap prevents the useful flow of air underneath your panels.
References:
- Solar Panels Keep Buildings Cool (ucsd.edu)
- Cool Roofs (Berkeley Lab)
- Cool roofing materials (Green Building Alliance)
- Heat Island Effect (epa.gov)
