PV ARRAY layout DESIGN

Purpose:

ION Solar requirements and guidelines to standardize PV array layout design.  This standard is built to be universal for Sales, Proposal, Design, and Installation to follow.  The objective is to meet all customer/sales expectations with yield or aesthetics and also be as material and labor-friendly as possible.

The first section defines parameters of negligible impact to the customer to improve the cost to ION and why we use the "3% rule", the second sections defined rules for modeling using aerial imagery, the 3rd section outlines specific layout design strategies to follow when using the 3% rule, and the last section outlines circuit / string design.

Table of Contents:

1 | THE 3% RULE

The 3% rule allows designers to adjust production estimates within a 3% margin in order to optimize material costs, installation labor, and complexity. It also helps refine the aesthetics of the array, as long as it doesn’t significantly impact the system’s yield. 

Meeting the Customer Design Objective.  The primary goal in array layout is to meet the customer's design objectives. While most customers prioritize the highest yield (maximizing production for the system size) over aesthetics, some may have specific preferences, such as avoiding module placement on front-facing roof sections or aligning the array symmetrically. These preferences, as outlined by the sales team, must be followed during design. 

Software Estimated Production Accuracy.  PV production estimate software is typically off by more than 5% when compared to actual production. This discrepancy is due to factors such as imperfect shading modeling, weather variations, future conditions, wiring resistance, and equipment changes. However, ION-installed systems often perform about 5% better than projected. 

Layout and Material Costs.  Layout decisions can significantly impact material costs, especially with racking and electrical equipment for roofs and attics. Factors like panel orientation, row staggering, array size, and the array’s location relative to the interconnection can all affect costs. These factors must be considered in the following sections, ensuring adherence to the 3% rule. 

Layout and Install Complexity.  The design layout influences installation complexity. For example, wiring each array into a junction box takes time, and layout choices like row staggering determine the number of attachment points required. The more irregular the layout, the more attachment points and installation time are needed, which increases labor costs. 

Layout and Aesthetics.  Customers often invest time in refining and visualizing their proposed layout. They typically expect the first design to be correct, but adjustments may be necessary based on new information from the site survey. A well-designed layout should balance aesthetics with system yield, providing an optimal result for the customer. 

The 3% rule ensures that any changes made during the design process provide value to ION without significantly affecting the customer’s experience. 

2 | STRUCTURE PROFILE

Accurate roof modeling is crucial for determining the available space for a solar array. Regardless of the solar design software used, ION follows standardized best practices for PV array layout on each roof section. This ensures a seamless experience for customers and the sales team while minimizing potential installation challenges. 

Find the Best Image

Ensure that the image you are using matches the correct structure. Nearby buildings may have similar roof profiles with slight differences, particularly in obstructions, so it is essential to model the correct property.

Before working with a misaligned or skewed image, check for higher-quality HD images with better alignment. Always use the image with the least distortion to achieve the most accurate design.

Roof Edges

ION utilizes a non-traditional survey approach for both sales/proposal designs and final designs after the site survey. Rather than measuring every element on the roof, we prioritize aerial imagery whenever available. This approach allows us to streamline the design process while relying heavily on accurate structure modeling software to ensure precision. 

Ridge Locations.  The ridge nearly always lands in 2 equal distances between two opposite eaves.  The only time this doesn't happen is if one pitch of the roof is different than the other or the roof drops down in height from a 2nd story to a 1st story or in between.  It is a common error to put the ridge location aligned as shown in the aerial imagery, but this is rarely the correct location.  Due to image alignment, you will need to adjust the ridge so it is the exact distance between two opposite eaves for each gable or hip roof section of the structure you are modeling.

Rakes/Gables.  Each rake / gable edge needs to land the edge at the node at the eave-gable and have a perfect 90-degree angle up to the ridge and down to the opposite eave.  Keep in mind that some shift out (not a 90-degree angle) and will only be able to catch at site survey with photos, but it's rare.

Obstructions

Obstruction locations are rarely positioned exactly as they appear in aerial imagery. Additionally, much of the best available aerial imagery is often too blurry to distinguish between actual obstructions and roof discoloration, such as stained shingles or tiles. Always verify obstruction placement using site survey data whenever possible to ensure accuracy. 

Comparing aerial imagery to site photos.  Aerial imagery is often too blurry to determine whether dark spots on a roof indicate actual obstructions or simply discoloration in the roof membrane. Site survey photos are essential for ensuring accuracy in these cases. By comparing aerial imagery with site photos side by side, obstructions can be properly identified and adjusted in the model.

Site photos also help account for the height of tall obstructions, allowing for accurate shading analysis when placing the proposed array.

This verification must be performed for every roof section where modules are likely to be placed. However, for roof sections that will not have modules, spending time precisely modeling obstructions is unnecessary.

Shading

Modeling the Site correctly can impact the accuracy of the production layout significantly.  Some jobs don't have all the available resources, such as lidar or street view photos, to complete accurately.  It is then only possible to get shade modeling / % shading loss accurate with site survey information.  Accurate site modeling is crucial for determining the true production potential of a solar array. The precision of the production layout can be significantly impacted by how well the site is modeled.

In some cases, essential resources such as LiDAR data or street view photos may not be available. When these resources are missing, the only way to achieve accurate shade modeling and percentage shading loss calculations is through site survey information. Proper site surveys ensure that shading impacts are correctly accounted for in system design.

Roof Obstruction shading.  Without site survey photos, determining the height of roof obstructions is challenging, leaving estimation as the only option. However, when survey data is available, it is essential to compare site photos with aerial imagery and accurately model the size, shape, and height of each obstruction. This ensures that any modules placed near these obstructions have a reliable shading impact estimate.

As with obstruction location modeling, only focus on obstructions that will cast shade on roof sections where modules will be installed. Modeling shading for areas outside of the array placement is unnecessary.

Site Shading.  There are typically two types of site elements that may cast shade on a PV array: trees and adjacent structures such as homes or buildings. Properly modeling these elements is essential, but it’s also important to be strategic—only modeling the ones that will actually impact the array.

If the design software includes a sun path tool, use it to determine which trees or structures need to be modeled. If no such tool is available, understanding the sun’s path relative to the site is necessary to ensure that all potential shade sources are accounted for.

Tree Shade and LiDAR Data
When using LiDAR data, it is important to check its age, as trees may have grown significantly since the data was recorded. Adjustments should be made to account for growth, trimming, or removal.

To ensure accurate placement of trees and structures:

• Align the LiDAR image with the home so that relevant trees and buildings are correctly positioned.

• Model trees so that their tops slightly poke through the LiDAR blocks for visibility while considering trimming, removal, or growth.

• If LiDAR data is unavailable or inaccurate, compare street view and aerial photos to confirm tree and structure locations. Shadows can help estimate tree height.

Post-Site Survey Adjustments
After receiving site survey photos, verify tree and structure heights against the model. Determine if any trees have grown, been trimmed, or removed. For any trees that could shade the array, it is required to carefully review site photos and LiDAR data (if available) to ensure an accurate location and shape representation in the design.

Precautionary Offsets

Due to the inherent inaccuracies of aerial imagery and the potential for human error in measurements, ION has established a standard minimum offset of 6 inches from each roof edge and obstruction. Designers must adhere to this precautionary offset and are not permitted to encroach upon it.

This offset is not required for installation. Installers have the flexibility to position modules up to 1 inch from a roof section edge or obstruction if necessary.

For overhead service drops, a minimum clearance of 3 feet is required from the weatherhead and at least 3 feet from any service lateral running over the roof.

Fire Setback Offsets

For guidance on complying with fire setback requirements, refer to the fire setback modeling training.  Fire Setbacks

When necessary to fit additional modules on a south-facing roof, an ION designer may encroach up to 6 inches into the fire setback if required by the Authority Having Jurisdiction (AHJ). This exception is only permitted in AHJs that do not strictly enforce fire setback regulations. Installers are also allowed to follow this rule, as most AHJ inspectors do not physically measure setback distances on the roof.

The AHJ database contains records of which jurisdictions have strict fire setback enforcement. Use this strategy when designing or installing to maximize production on the highest-yielding roof sections while aligning with customer expectations for the layout.

3 | ARRAY PROFILE

The placement of the array can influence system costs. It is ideal to plan the layout carefully while adhering to the 3% rule parameter. The following guidelines for array placement should be applied as long as the production change remains within a 3% margin. 

Array Quantity and Size

The fewer arrays the better.  Minimizing the number of arrays helps reduce material and labor costs. Avoid leaving gaps between modules when designing around obstructions, and aim to place panels on as few roof sections as possible. 

Minimum Array Size.  Single-panel arrays require excessive racking, electrical materials, and labor, making them costly for the energy they produce. Each single-panel array requires four attachment points, two rails, grounding, junction box wiring, and an attic run, leading to inefficiencies. To optimize cost and performance, design arrays with at least two modules per section, effectively reducing equipment costs while maintaining system efficiency. 

Structural Influence

Avoid Hip-Roof Box Trusses. Manufactured truss homes, typically built after 1990, often have "hip roofs" with triangle-shaped sections. In these areas, the top chords (rafters) run horizontally instead of vertically, making it difficult to mount solar panels without additional attic blocking during installation. Designers should avoid placing panels on these sections unless it would reduce system production by more than 3%. 

Avoid Structural Upgrades. Older homes may not be able to support the additional weight of a solar array. Some roof sections may require rafter upgrades to meet structural requirements, while others may not. To minimize costs and complexity, avoid placing panels on sections that would require reinforcement. 

Disqualified Roof Sections and Structures.  In some cases, a roof section or an entire structure may fail structural analysis and cannot be upgraded to support solar. When this occurs, those sections or structures must be excluded from the design. 

Racking Influence on Portrait vs Landscape

Production Impact.  Panel orientation typically does not affect system production and is primarily adjusted based on roof space constraints or the type of racking used. 

Preferred Orientation. Rail-mounted racking: Portrait orientation is preferred.  Rail-free racking: Landscape orientation is preferred.

Avoid Mixing Orientations. Arrays should minimize mixing portrait and landscape orientations within the same array. Mixing orientations should only be considered if it prevents a production loss exceeding 3% by avoiding placement in lower-yield areas. 

Horizontal Row Alignment / Minimize Gaps in Rows

Avoid Horizontal Row Staggering. When a roof section has many obstructions or an irregular shape, it may be tempting to shift panels vertically, disrupting the alignment of horizontal rows. However, this adds significant complexity to installation, increases material costs, and results in poor aesthetics. In most cases, horizontal row staggering can be avoided while still meeting customer expectations.  Only use this approach when necessary.

Avoid Gaps Between Rows. Rows should remain continuous whenever possible. Keeping rows intact reduces the need for additional rail cuts, end clamps, and attachments, ultimately lowering material and labor costs.

Detached Structures / Ground Mounts

Detached Structures. Placing panels on a detached structure often presents challenges. Trenching is usually required either because the utility does not allow two points of PV interconnection or because the detached structure lacks electrical equipment capable of handling a solar interconnection. Additionally, many detached structures were built without proper permits, making them difficult to modify and bring up to code for inspection approval.

If a customer prefers or requests solar on a detached structure, the initial sales or proposal design can include it. However, there is a high likelihood that the design will need modifications based on later survey findings, which may include additional costs for trenching and equipment.

Ground Mounts. Ground-mounted solar arrays require specific conditions, such as adequate clearance from property lines, trenching to the main electrical service, and higher installation costs. While ground mounts should generally be avoided due to these added expenses, they are an option for customers who have sufficient space, prefer not to install panels on their home, or cannot install on their main structure due to roof membrane incompatibility or structural concerns.

4 |CIRCUITS AND STRINGING

Circuit and string design will not affect customer expectations but will influence installation cost and ease. 

Junction Box Locations

Place at the top of each Array.  Junction box locations should always be placed at the top of each array, regardless of whether the homerun wiring is run inside (in the attic) or outside (on the roof). This ensures easier servicing if issues arise in the future.

Center between circuits in an array. If an array exceeds the panel limit for a single circuit, place the junction box between the two circuit loops. If the entire array fits within a single circuit or shares part of a circuit with another array, position the junction box closest to the point of interconnection to minimize homerun raceway length.

Homerun Circuit Quantity

The fewer circuits the better.  Always design/install with as few circuits as possible unless using a true standard inverter without module-level optimization. This minimizes wiring costs for each project.

Different rules apply to Standard Inverter Types.  When designing with true standard inverters, consider shading impacts and apply smart string-level design. Optimize additional MPPTs if necessary. Running strings in parallel on the roof can help reduce the number of wire runs back to the inverter.

Circuit / String Loops

Create loops to each junction box.  Each array should create loops by starting with modules adjacent to the junction box, connecting adjacent panels in any preferred direction, and looping back up to the junction box to close each circuit. This minimizes the need for additional wiring or splicing, streamlining installation and wire management. 

Circuit and String Array Jumping

Avoid jumping circuits between arrays.  If multiple arrays are placed on a structure and require multiple circuits due to the number of modules, keep each circuit within an array boundary. Only jump between arrays through the attic or roof raceway if necessary.

For standard string type inverters, arrays cannot be jumped unless the roof sections have the same tilt and azimuth within 3 degrees.