Helpful Tips for Solar Site Planning

By John R. Gyorki, Editorial Director

Obviously, solar panels and collectors must be located where the sunlight can reach them most of the day during all seasons of the year. Accurate procedures used to find these spots have been around for several decades, but with the recent push to install more solar panels and collectors, those methods have been fine-tuned and include some modern instruments that can simplify and accelerate the analysis.

The Basics
Trees and buildings and other fabricated structures and natural formations are the most obvious obstructions to sunlight to avoid when considering the site. After selecting the spot with the most available sunlight for your location, orient the panels or collectors to take advantage of the maximum exposure time for the season. This involves analyzing the azimuth angle and the tilt and orientation factor (TOF) as determined from a graph. Not surprisingly, critical data include the latitude and longitude of the exact site location.

The Tilt and Orientation Factors (TOF) for Oregon, West of the Cascades, comes from a plot of the Solar Azimuth angle of the collectors or photovoltaic panels and their tilt angle.

The state of Oregon provides a simple procedure for estimating the effect of TOF and shading. To begin, determine how much solar energy is available for the panels or collectors in order to calculate how effective the solar energy system will be. The graph estimates how tilt, orientation, and shading will affect the amount of solar energy the panels are expected to collect. A relative quality factor determines this effectiveness, which is known as the Total Solar Resource Fraction (TSRF). It represents the amount of energy indicated — as a percentage – that a particular panel or collector would receive when compared to a similar panel located in the same city that has optimal tilt, orientation, and no external shading. For instance, a collector with an 80% TSRF means that 80% of the total solar energy available to that city will be available to the installed panels.

How it works
TSRF may be analyzed in two parts. First, estimate the effect of TOF using a plot. The second part uses a sun chart to determine the amount of solar energy lost during the year caused by trees, buildings, and other obstructions. The information from these two constructs yields the collectors’ TSRF.

The TOF graph shown here is intended for a specific location in the state of Oregon. The graph illustrates the effect of tilt and orientation on the collectors annual output. The TOF values range from a maximum of 100% with no loss to about 60%, which represent about a 40% loss as observed in the upper two corners. The azimuth is determined from polar orientation, adjusted for magnetic declination (16 to 20° for Oregon). To estimate the TOF value to the nearest 1%, simply place a mark on the graph that identifies the collector’s tilt and azimuth, then interpolate between the nearest two lines.

The second part involves the sun chart. These charts plot the amount of sun that reaches the site by illustrating the motion of the earth relative to the sun plotted over a day. You can make your own charts using the following procedure.

Use graph paper to plot the azimuth (time of day) and elevation (angle from horizon) of the sun from sunrise to sunset at the location you plan to install your solar panels. On the horizontal axis, mark the time of day in one-hour increments from left to right with noon at the center. Mark the elevation on the vertical scale in degrees of elevation with the horizon at the bottom and 90° at the top.

The plot should represent the elevation limits between the summer and winter equinox (the longest and shortest days of the year). The line midway between them represents both fall and spring. You can add skyline obstructions to the path of direct sunlight on your graph such as trees and buildings.

Optionally, if you don’t want to spend a year making your own charts, you can purchase charts that cover your general location, or you can rent or purchase an instrument that lets you do a one-time, immediate assessment ( It is basically a highly reflective dome containing a bubble level (to align with the horizon) and a compass (to find south). The shade reflections in the vicinity appear on the dome, and a chart below the dome shows the hours and months when the obstructions are most problematic.

Light intensity
A plot of light intensity recorded in concert with measurements of azimuth vs. elevation over the same year is helpful for determining system effectiveness. The light intensity may be a little more difficult to determine unless you have a suitable instrument such as a single solar cell connected to a meter to measure its output. The most important parameter is relative intensity, so you need not calibrate the meter directly in lumens. Light intensity varies with several factors, including air density, elevation angle (more atmosphere to penetrate at lower angles), cloud cover, smog, snowfall (on solar cells), and fog. The best elevation angle setting for the solar panels is your latitude. This ensures that you will receive the maximum amount of sunlight over a year of exposure. Better yet would be a mechanism that lets you adjust the elevation of the solar panels to follow the sun’s elevation in the sky as it changes over the months.

Yet other factors influence the desirability or payoff of the location. These include temperature and wind. Photovoltaic panels provide higher output on a clear cool day than on a high-temperature, sunny day, and high winds can rip panels off roofs when they are not adequately secured.

Sources of information
Numerous state and federal government Web sites contain information that can teach you some solar power basics, as well as let you know the status of tax credits that you may be eligible to receive. For more information, see the following Web sites:

Oregon Department of Energy

Solar Pathfinder, Inc.

Solar Energy Industries Association

The National Renewable Energy Laboratory

National Climatic Data Center