Sunlight is the fuel for all solar technologies, and the term “solar resource” refers to identifying how much of it is available to a given collector area.
The most rudimentary analysis requires standing on your site with your arms up in a “Y” form and looking to see what obstructions there are in the field of vision between your arms. This basically represents the productive sun hours of 9:00 am to 3:00 pm.
There are professional devices, such as the Solar Pathfinder and Solmetric SunEye, which calculate the Total Solar Resource Fraction (TSRF), which accounts for shading, panel tilt and azimuth (direction relative to south).
Systems applying for Energy Trust Incentives must have 75% TSRF, meaning the solar array will have at least 75% of the sunlight available compared to a completely unshaded and perfectly oriented array on the same site. IPhone Apps such as Sun Surveyor and Sun Seeker will also provide a solar assessment. If you live in Washington County in Oregon, you can also use a free online tool “Mapdwell Solar System” to determine a rough estimate of your solar resource and TSFR percentage.
The tilt of the earth on its axis and our location north of the equator result in a specific sun path that changes throughout the year. The sun is at its highest point at noon on the Summer Solstice, or June 21st.
Most photovoltaic and solar hot water systems are stationary, and having unobstructed solar access between the hours of 9 am and 3 pm is ideal. Tracking systems usually follow the sun path from east to west during the day, and are seasonally adjusted for altitude angle.