perfect because solar modules produce 95 percent of their full power when within 20
degrees of the sun’s direction. Roofs that face east or west may also be acceptable. As
an example, a due west facing rooftop solar PV system, tilted at 20 degrees in Salem,
Oregon, will produce about 88 percent as much power as one pointing true south at the
same location. Flat roofs work well because the PV modules can be mounted on frames
and tilted up toward true south.
Optimum orientation can be influenced by typical local weather patterns. For example,
western Washington and Oregon frequently have a marine layer of fog that burns off by
late morning and so have better solar resource after noon than before noon. Thus, west of
the Cascades, the maximum power is generated with a southwest orientation.
Tilt – Generally the optimum tilt of a PV array in the Pacific Northwest equals the
geographic latitude minus about 15 degrees to achieve yearly maximum output of power.
An increased tilt favors power output in the winter and a decreased tilt favors output in
the summer. In western Washington and Oregon, with their cloudier winters, the
optimum angle is less than the optimum east of the Cascades.
Nevertheless, it is recommended that modules be installed at the same pitch as a sloping
roof, whatever that slope is, primarily for aesthetic reasons, but also because the tilt is
very forgiving. In Salem, Oregon, for example, tilts from 20 degrees to 45 degrees will
result in approximately the same power production over the course of the year. This is
because tilts that are less than the latitude of the site increase summer production when
the solar resource is most available here, but reduce winter production when it tends to be
cloudy anyway.
Required Area – Residential and small commercial systems require as little as 50 square
feet for a small system up to as much as 1,000 square feet. As a general rule for the
Pacific Northwest, every 1,000 watts of PV modules requires 100 square feet of collector
area for modules using crystalline silicon (currently the most common PV cell type).
Each 1,000 watts of PV modules can generate about 1,000 kilowatt-hours (kWh) per year
in locations west of the Cascades and about 1,250 kWh per year east of the Cascades.
When using less efficient modules, such as amorphous silicon or other thin-film types,
the area will need to be approximately doubled. If your location limits the physical size
of your system, you may want to install a system that uses more-efficient PV modules.
Keep in mind that access space around the modules can add up to 20 percent to the
required area.
Roof Types – For roof-mounted systems, typically composition shingles are easiest to
work with and slate and tile roofs are the most difficult. Nevertheless, it is possible to
install PV modules on all roof types. If the roof will need replacing within 5 to 10 years,
it should be replaced at the time the PV system is installed to avoid the cost of removing
and reinstalling the PV system.
maps of magnetic declination at http://www.ngdc.noaa.gov/geomag/declination.shtml. In Oregon,
magnetic declinations range from about 17 to 22 degrees east.
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