Rooftop Solar PV

Rooftop solar photovoltaic (PV) systems use panels (modules) mounted on a building's roof to convert sunlight into electricity. Roofs are attractive sites because they are already built, are often unshaded, and are close to where power is used. Rooftop PV ranges from a few panels on a house to large arrays covering warehouse roofs.

Roofing considerations

Installing PV turns a roof into a platform for equipment that will stay for decades, so the roof itself becomes part of the system's success.

• Roof age and condition. PV modules are typically covered by manufacturer warranties of 20 to 25 years and often keep producing after that. Removing and reinstalling an array to replace a roof partway through its life adds cost, so the US Department of Energy notes that a roof replacement can be a good time to add solar.
• Structural capacity. Panels and racking add weight, and some roofs designed only for their own weight need strengthening. Ballasted systems on flat roofs can add considerably more load than attached systems.
• Wind loads. Arrays experience wind uplift and drag; design pressures depend on location on the roof, with edges and corners most demanding. ASCE 7-16 in the United States added specific provisions for rooftop arrays parallel to the roof.
• Waterproofing. Each attachment point is a potential leak. Flashing details matched to the roof covering are critical.
• Fire access. Many jurisdictions require pathways and setbacks so firefighters can reach and ventilate roofs.
• Snow and debris. Arrays change how snow slides and where debris collects, which can affect gutters and drainage.

Performance over time

NREL research found a median degradation rate of about 0.5 percent per year, so a typical panel produces roughly 90 percent of its first-year output after 20 years. Rates can be higher in hot climates and on rooftops, likely because of higher operating temperatures.

Mounting approaches

• Attached (penetrating) racks on pitched roofs, bolted through flashed mounts into rafters or deck.
• Clamp mounts on standing seam metal roofs that grip the seams without penetrating the panels.
• Ballasted racks on flat roofs held down by weight.
• Building-integrated PV (BIPV), where solar tiles, shingles or laminates replace part of the roof covering.

Urban effects

Panels shade the roof surface below. A simulation of London's 2018 heatwave found that rooftop solar could reduce average outdoor temperatures somewhat (about 0.5 °C), though less than cool roofs in the same study.

Policy

Several jurisdictions encourage or require rooftop solar, sometimes as an alternative to green roofs; for example, a 2015 French law required new buildings in commercial zones to be partly covered with plants or solar panels.