Successful PV Site Evaluation

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Successful PV Site Evaluation
Successful PV Site Evaluation
Solmetric’s Roof Azimuth Tool Screenshot
Solmetric’s Roof Azimuth Tool can determine orientation from aerial photos. Google Earth has a similar tool that can determine approximate roof area.
An inclinometer
An inclinometer is an inexpensive tool for quickly determining the a surface’s slope. The needle is pointing to the angle in degrees.
Smartphone app
An inexpensive smartphone app can measure the roof angles in degrees, percentage, rise-to-run ratio, and even radians.
Solmetric's free online tool
Solmetric offers a free online tool for determining annual insolation based on various combinations of azimuth and tilt. This example roof is within 5% of the ideal tilt and orientation.
Solmetric’s SunEye 210
The SunEye 210 is Solmetric’s latest solar siting tool, which displays an annual sunpath view, showing annual and seasonal solar access values.
Solmetric Screenshot
The SunEye 210's accompanying software includes additional displays, such as the obstruction elevation.
The Solar Pathfinder
The Solar Pathfinder uses a highly reflective dome to reveal obstructions, which can be outlined on a paper sun-path chart. It has a built-in level and compass, and the base can be adjusted for the site’s magnetic declination.
Solar Pathfinder Screenshot
Digital photos taken of the Pathfinder reading can be used with the Assistant PV software to generate detailed summary reports.
Capture readings from all four corners of the intended array area.
To properly evaluate a proposed PV array location, capture readings from all four corners of the intended array area.
A solar siting tool can be used to determine a “shade-free” distance from roof obstacles
A solar siting tool can be used to determine a “shade-free” distance from roof obstacles, such as this plumbing vent. This should be recorded on the site evaluation form.
Check the roof’s condition
Check the roof’s condition—repairs or reroofing should happen before installing a PV system.
Assess the roof’s structural condition
Inspecting the attic is another way to assess the roof’s structural condition, check for leaks, and map rafters for future array mounting.
Plan for wire and conduit runs in advance
Plan for wire and conduit runs in advance. The path between array and balance-of-system components needs to meet the Code as well as logistical requirements. This junction box makes a transition between PVC conduit coming from the pole-mounted array to EMT conduit running through the building interior to the inverter.
Check for obstacles
Check for obstacles—a propane tank in the yard indicates underground pipe runs that should be avoided when trenching for wire runs.
AC service entrance
Generally, a new grid-tied system will connect to the existing electrical infrastructure at the AC service entrance. Besides checking equipment condition, look for service amperage rating, main breaker rating, busbar ratings, and adequate breaker space. Incompatibility with any of these things may dictate upgrading the panel.
Balance-of-system components
Balance-of-system components take up space, and have Code and practical access requirements. Map their placement in advance.
Successful PV Site Evaluation
Solmetric’s Roof Azimuth Tool Screenshot
An inclinometer
Smartphone app
Solmetric's free online tool
Solmetric’s SunEye 210
Solmetric Screenshot
The Solar Pathfinder
Solar Pathfinder Screenshot
Capture readings from all four corners of the intended array area.
A solar siting tool can be used to determine a “shade-free” distance from roof obstacles
Check the roof’s condition
Assess the roof’s structural condition
Plan for wire and conduit runs in advance
Check for obstacles
AC service entrance
Balance-of-system components

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.

Orientation & Tilt

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.

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