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 residential rooftop shading in Sacramento, San Jose, Los Angeles and San Diego, CA. Our analysis
can be used to better estimate power production and/or thermal collection by rooftop solar-energy equipment. It can also be
considered when designing programs to plant shade trees.
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 well-treed 2.5–4 km2 residential neighborhood. On-hour shading of roofing planes (the
flat elements of roofs) was computed geometrically from the digital elevation model. Values in future years were determined by repeating
these calculations after simulating tree growth. Parcel boundaries were used to determine the extent to which roofing planes were shaded
by trees and buildings in neighboring parcels.
For the subset of S + SW + W-facing planes on which solar equipment is commonly installed for maximum solar access, absolute
light loss in spring, summer and fall peaked about 2 to 4 h after sunrise and about 2 to 4 h before sunset. The fraction of annual insolation
lost to shading increased from 0.07–0.08 in the year of surface-height measurement to 0.11–0.14 after 30 years of tree growth. Only
about 10% of this loss resulted from shading by trees and buildings in neighboring parcels.
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