PV Array Siting & Mounting Considerations: Page 2 of 2

Intermediate

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Roof Mount
Roof Mount
Pole Mount
Pole Mount
Ground Mount
Ground Mount
Solar Pathfinder
Siting tools such as this Solar Pathfinder can help find the best solar window.
Solmetric SunEye
Siting tools such as this Solmetric SunEye can help find the best solar window.
Flush mounting
Flush mounting is often preferred, even when the roof isn’t oriented to true south or at an ideal tilt.
Tilted roof-mounted arrays can make sense
The increased energy production of a tilted roof mount often does not outweigh the drawbacks of rack complexity, cost, wind loading, and aesthetics. However, in off-grid systems where winter production needs to be maximized, tilted roof-mounted arrays can make sense.
Flush mounting
Flush mounting is often preferred, even when the roof isn’t oriented to true south or at an ideal tilt.
Ground mount
This ground mount is actually a multipole mount, where the racking structure spans across a single row of vertical pipe. Fencing keeps the critters and humans out, and helps meet NEC requirements for protecting exposed wire.
Aluminum wiring covering
Exposed wiring on the back of an array can be protected by an appropriate covering, such as this flat, expanded aluminum secured to the rack.
Plant growth can shade ground-mounted arrays
Plant growth can shade ground-mounted arrays, reducing their production.
Concrete pour
Civil works can increase an installation’s cost and complexity.
Different tilts show the adjustability of these top-of-pole mounts
These two arrays are shown at different tilts to show the adjustability of these top-of-pole mounts. Typically, they would be set to the same tilt.
Top-of-pole mount
Top-of-pole mounts allow users to set the array at an ideal orientation and tilt angle for their location, and can easily be adjusted to a steeper tilt angle in the winter to shed snow.
Roof Mount
Roof Mount
Installer safety equipment
Working in high places is all about safety—the equipment to avoid falls is readily available to all installers.
Airflow behind the modules
The gap between the roof and the array allows for some airflow behind the modules.
Modern flashing products
Modern flashing products decrease the chance of water leaks in the roof.
Roof obstruction
This array is still being populated with PV modules but the solar-powered attic fan means one less module will fit in the array.
Roof Mount
Pole Mount
Ground Mount
Solar Pathfinder
Solmetric SunEye
Flush mounting
Tilted roof-mounted arrays can make sense
Flush mounting
Ground mount
Aluminum wiring covering
Plant growth can shade ground-mounted arrays
Concrete pour
Different tilts show the adjustability of these top-of-pole mounts
Top-of-pole mount
Roof Mount
Installer safety equipment
Airflow behind the modules
Modern flashing products
Roof obstruction

Single Pole Mounts

Single pole-mounted arrays have the same basic advantages of ground-mounted systems, plus they can be more easily adjusted for increasing peak-sun hours captured seasonally and to shed snow. Compared to a ground mount, a pole mount releases more ground area for other uses. Tall pole mounts not only keep wires running along the backside of the array from being “readily accessible” (if the wiring is above the ground by 8 feet or more), but also could reduce theft potential. However, since tall pole mounts cannot be reached from the ground, they also require a tall ladder for array maintenance and for adjusting tilt angle.

A ground mount uses multiple, lighter-weight supports, but a single pole mount has one large steel support. Depending on the ground clearance and array size (along with wind loads and soil type), that pole can be a heavy beast, commonly 6 or 8 inches in diameter and either schedule 40 or 80 steel pipe. For example, a specific design for a 3-kilowatt array with 8 feet of clearance to the bottom edge of the modules (at a winter tilt of 55°) will require the pole to be almost 9 feet in the ground. That’s a total pole length of 22 feet of 8-inch, schedule 80 galvanized steel pipe, which will require a 9-foot-deep (3-foot-wide) hole and approximately 2.3 yards of concrete. Installing the modules on this top-of-pole mount also requires scaffolding or a scissor lift, or for the completed array to be lifted in place by a crane.

Roof Mounts

Roof-mounted arrays take advantage of space that is otherwise wasted, leaving the yard for other uses. They are less expensive to install, since no excavation or concrete work is required, and they naturally remove ready access to array wiring. (Wire management is still required; see “Array Wire Management” in HP154.)

But roof installations often take place on high and sometimes steep roofs (as does any future array maintenance) and fall arrest systems (i.e., a body harness attached to a lifeline cable and an anchor) should be used. All roof penetrations must be flashed and sealed. The combined weight of the modules and racks, along with wind loading on the building, must be appropriately engineered. Flush-mounted arrays, even with the recommended 4- to 6-inch spacing above the roof, will have less cooling airflow than a ground- or pole-mounted array, and array orientation and tilt is often less than optimal and not adjustable.

Roof mounting space is usually limited, constraining the array’s size. The array design must accommodate obstructions, such as vents, chimneys, and skylights. Often, setbacks around the array are required for firefighter access, pathways, and smoke ventilation. For example, California’s fire guidelines call for 3-foot setbacks from the top of the array to the ridge and along one (or both, depending on the roof type) sides of the array.

Working with a roofing contractor is a good idea, as they can properly secure attachments without compromising the roof structure, are familiar with the various code requirements for roofs, and can warranty the penetrations they work on. They can also evaluate the remaining life of the roof—an important consideration before installing a 20- to 25-year PV system on it.

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Justine Sanchez is a Home Power technical editor and an instructor for Solar Energy International. She is certified by ISPQ as a PV Affiliated Master Trainer. As her house hunt continues, she’s contemplating potential PV sites and mounting methods.

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