According to a team of researchers at the UC San Diego Jacobs School of Engineering, the solar panels sprouting on increasing numbers of residential and commercial rooftops around the world aren't just generating green electricity, they're also helping keep the buildings cool. The news that letting photovoltaic panels take the solar beating will reduce the amount of heat reaching the roof shouldn't come as much of a surprise, but the fact no one has thought to quantify just what the effects of rooftop solar panels on a building's temperature are is a little baffling.

Although the observations for the study were taken over the short period of three days in April this year, Jan Kleissl, a professor of environmental engineering at the UC San Diego, and his team believe they the first peer-reviewed measurements of the cooling benefits provided by solar photovoltaic panels. And despite the limited time, Kleissl is confident his team developed a model that allows them to extrapolate their findings to predict cooling effects throughout the year.

Using a thermal imaging camera, the team gathered data on the roof of the Powell Structural Systems Laboratory at the Jacobs School of Engineering, which is equipped with tilted solar panels as well as solar panels that are flush with the roof, while some of the roof is not covered by any solar panels at all.

They determined that during the day, the panels reduced the amount of heat reaching the roof by about 38 percent and as a result the building's ceiling was five degrees Fahrenheit (three degrees Celsius) cooler than the ceiling under an exposed roof. Tilting panels with a gap between the building and the solar panel that allowed air to circulate were found to provide a bigger cooling effect than flush solar panels. Kleissl and his team say the amount saved on cooling the building amounts to a five percent discount on the price of the solar panels over their lifetime.

Additionally, the panels help hold heat in at night to cut heating costs in winter. On the flip side, however, the panels would also keep the sun from heating up a building in winter and would keep the heat accumulated in the building during the day in summer from escaping at night. Therefore the effects effectively cancel each other out in many climates.

"There are more efficient ways to passively cool buildings, such as reflective roof membranes," said Kleissl. "But, if you are considering installing solar photovoltaic, depending on your roof thermal properties, you can expect a large reduction in the amount of energy you use to cool your residence or business."

The UCSD Jacobs School of Engineering team's study will be published in an upcoming issue of the journal Solar Energy.