Commercial solar systems reduce electricity bills with local power generation, which is subtracted from the total consumption measured by the power meter. However, a less known fact is that photovoltaic arrays also have an insulating effect on the roof, which reduces the load on space cooling systems.
Air conditioners make indoor spaces comfortable by gathering heat and releasing it outdoors. Part of this heat comes from internal sources such as home appliances, office equipment and the building occupants themselves. However, significant heat gain also occurs through walls and windows, and especially through the rooftop.
Building upgrades that reduce heat gain have a positive effect on energy efficiency, by lowering the load on air conditioning equipment. Some common measures that improve the building envelope are triple-pane windows, thermal insulation and air sealing. However, solar panels also reduce rooftop heating by blocking sunlight.
When the rooftop is exposed directly to sunshine, its temperature can get very high. The rooftop acts as an indoor space heater, which is the last thing you would want on a hot summer day! Air conditioners are characterised by their high operating costs, and additional heating translates into higher power bills during summer.
However, after a building is equipped with solar panels, the rooftop area used by the array is no longer exposed to directly sunlight. There is still a heating effect because the rooftop is exposed to warm outdoor air, but photovoltaic modules act as a barrier against direct solar radiation.
Researchers at the University of California studied the effect of solar panels on rooftop heating, and they determined that the heat reaching the roof is reduced by around 38% with a photovoltaic array.
The insulating effect of solar panels is also useful during winter, keeping heat inside the building at night. In other words, solar panels improve building envelope performance throughout the entire year, by reducing summer heat gain and winter heat loss.
The insulating effect of solar panels is higher in buildings with a large rooftop area relative to their indoor floor space, such as low-rise shopping malls. In these buildings, the solar radiation reaching the rooftop is much higher than the radiation reaching the walls.
On the other hand, the insulating potential of solar panels is less in high-rise constructions, where the roof area is small relative to the total indoor area. In these cases, only a small fraction of solar heating occurs through the rooftop, and most of it happens when sunlight hits the walls and windows of the building.
Regardless of how effective a solar array can be as insulation, it is already a good investment based on electricity generation alone, especially with the high electricity tariffs in Australia.