Lawrence Berkeley National Laboratory

Shadows cast by trees and buildings can limit the solar access of rooftop solar-energy systems, including photovoltaic panels and thermal collectors. This study characterizes rooftop shading in a residential neighborhood of San Jose, CA, one of four regions analyzed in a wider study of the solar access of California homes. High-resolution orthophotos and LiDAR (Light Detection And Ranging) measurements of surface height were used to create a digital elevation model of all trees and buildings in a 4 km2 residential neighborhood. Hourly shading of roofing planes (the flat elements of roofs) was computed geometrically from the digital elevation model. Parcel boundaries were used to determine the extent to which roofing planes were shaded by trees and buildings in neighboring parcels. In the year in which surface heights were measured (2005), shadows from all sources ("total shading") reduced the insolation received by S-, SW-, and W-facing residential roofing planes in the study area by 13 - 16percent. Shadows cast by trees and buildings in neighboring parcels reduced insolation by no more than 2percent. After 30 years of simulated maximal tree growth, annual total shading increased to 19 - 22percent, and annual extraparcel shading increased to 3 - 4percent.