Performing a comprehensive solar site analysis is the first step toward ensuring a well-performing solar-electric system. New tools and technologies are available for evaluating a site’s solar potential. But while these tools can bring us powerful information at the touch of a button, nothing can take the place of conducting a thorough assessment of the proposed array location.
As a PV array’s orientation (azimuth, the direction it faces) or tilt varies from “optimal,” potential energy harvest decreases. The optimal combination of array tilt and azimuth will depend on a few variables, including geographical location, seasonal weather patterns and system application. Azimuth and tilt are both required data for system design and energy estimating programs such as the PV Watts calculator (see Access).
A PV array’s orientation is often referenced to true south. A compass doesn’t always point to true south, so magnetic declination, the angle variation between magnetic north/south and true north/south, must be taken into account, since it varies by location. There are various resources available to determine magnetic declination for most locations around the globe (see Access). With the use of a compass and local magnetic declination, a roof’s orientation is simple to determine (see “Methods” in this issue). Since most buildings are not constructed with solar in mind, roofs that face true south are uncommon. Fortunately, most PV arrays within the continental United States oriented within 30° of true south still offer excellent potential energy harvest, often within just a few percent of the maximum.
Due to wind-loading and aesthetic concerns, most rooftop arrays are mounted parallel to the roof plane—so the roof’s pitch determines the array tilt. For grid-tied systems here in the United States, “optimal” roof angles—which maximize an array’s production—range between 25° and 35°. The roof pitch can be measured with an inexpensive inclinometer or a smartphone, which uses an app that takes advantage of the phone’s internal sensors to measure angles.
As roof pitch decreases, the roof’s orientation to true south becomes less influential on a grid-tied system’s performance. At a steeper tilt, the array will capture less energy during early mornings and late afternoons due to the sun’s oblique angle. As the array’s tilt approaches 0° (horizontal), the array is able to capture more of the early morning and late afternoon sun, which can increase overall energy production (see the “Orientation & Tilt Reference” table that shows the effects of decreasing the tilt angle with respect to azimuth). Since the goal for most grid-tied PV systems is to maximize the annual energy yield, winter production can often be “sacrificed” to maximize production in the summer, when the available peak sun-hours increase.
With your preferred solar design tool, such as Solmetric’s Annual Insolation Tool, you can create a chart for quickly evaluating potential energy production as it varies with array tilt and orientation (see “Free Tools for Estimating PV Output” in HP147). The results will reveal the ideal array orientation and tilt for an unshaded array in that particular location. This chart can be used to compare different mounting scenarios, such as multiple roof planes, as well as pole- or ground-mount options (for more information, see “Methods” in HP155).
To use Solmetric’s online tool, select your state from a drop-down menu and then select the location that best matches your local weather patterns. In our example, Grand Junction, Colorado, has very similar weather to the site’s location. Selecting the location generates an “annual insolation” graph that can be customized with specific orientation and tilt angles. To interpret the chart, find your tilt angle or roof pitch along the vertical axis and then match that up to the azimuth along the horizontal axis. This will lead to an intersecting point in the graph with a color designation that reveals a percentage value as compared to the ideal conditions, which are listed at the top of the legend area. The annual insolation value can be used to quantify the impacts of various tilt and azimuth angles. The scale along the right provides color-coordinated values expressed as a percentage of the ideal conditions and the associated insolation value in kilowatt-hours per square meter.