Solar Site Assessment & System Design Basics

Welcome

Hello, and welcome. This is Super Structures General Contractors — a national general contractor headquartered in Powhatan, Virginia — here to help you and your clients build something that lasts. We're glad you're with us, and we look forward to connecting with you.

Alright, Solar Site Assessment & System Design Basics. Don't let the plain title fool you. Here's the part that actually matters on the job: A solar design starts on the roof and at the service panel, not in software — sun, shade, structure, electrical capacity, and the fire-code pathways decide what can actually be built, and the permit and utility approvals decide when. Learn it well and you'll stop designing systems the inspector or the utility sends back.

The site visit

Everything downstream depends on the survey. On the roof: pitch, azimuth (compass direction), dimensions, obstructions (vents, chimneys, skylights), roofing material and its remaining life, and signs of sagging or leaks. In the attic: rafter or truss size and spacing, span, and condition, because attachments will lag into them. At the service: main panel busbar rating, main breaker size, available breaker spaces, meter location, grounding electrode system, and whether the meter and panel are accessible for the disconnect the utility will want. Photograph all of it; the designer and the AHJ will ask.

Sun, orientation, and tilt

In the United States, south-facing roofs produce the most annual energy; east and west faces produce less but are routinely used, and north faces are usually avoided. Tilt near the site's latitude maximizes annual output, but most rooftop arrays simply follow the roof pitch. Shading is the killer: a chimney shadow across one module can cut a whole string's output on a string inverter. Surveyors use a shade analysis tool or software with satellite imagery to map obstructions through the year, and designers either move modules out of shade or specify MLPE.

Roof and structure

PV adds a few pounds per square foot of dead load plus wind uplift on the attachments. Building codes reference ASCE 7 for wind and snow loads, and many jurisdictions want a structural letter from an engineer, especially for older framing, ballasted commercial arrays, or ground mounts. A roof near the end of its life should be replaced first — removing and reinstalling an array later costs more than the roof. Tile, slate, and standing-seam metal roofs each have their own attachment systems.

Fire-code access pathways

Firefighters need to get on the roof, ventilate it, and get off. The International Fire Code (Section 1204) and the International Residential Code (Section R324) require clear pathways and setbacks: on most homes, pathways 36 inches wide from eave to ridge and a setback along the ridge, which can be reduced to 18 inches on each side where rapid shutdown and other conditions are met. Commercial roofs have perimeter pathways and limits on array dimensions. Local amendments vary, and the fire marshal's version controls. Layout starts by drawing the pathways, then filling in modules.

Sizing in general terms

Design begins with the customer's annual consumption in kilowatt-hours from utility bills, a production estimate based on location, orientation, tilt, and shade, and the utility's rules on system size. Then the electrical side limits the answer: a load-side connection under NEC 705.12 is bound by the 120 percent rule — the sum of the main breaker and the PV breaker cannot exceed 120 percent of the busbar rating — and a larger system needs a supply-side connection (705.11), a main-breaker downsize, or a panel upgrade.

Permits and interconnection

Two approvals run in parallel. The authority having jurisdiction (AHJ) issues building and electrical permits from a plan set: site plan with pathways, roof framing and attachment detail, one-line diagram, equipment datasheets with listings, labels, and structural calculations where required. The utility takes an interconnection application with the inverter model, system size, and one-line; it may require a site inspection and will set the meter. After the AHJ passes final inspection, the utility grants permission to operate.

Going Deeper (Intermediate)

Design software produces beautiful drawings from bad inputs. A rafter spacing measured wrong, a panel photo that hides a subpanel, or a tree the satellite image predates will all be discovered on install day. Good companies make the survey a checklist and make the designer responsible for reading it. Learn to check equipment listings on the plan: modules to UL 1703 or UL 61730, inverters to UL 1741, racking to UL 2703, and batteries to UL 9540 — an AHJ can reject unlisted equipment on sight.

Advanced / Pro-Level

Pros design to the weakest link: the busbar, the roof life, the utility's export cap, and the fire pathways. They know how to document a supply-side connection so the utility and AHJ accept it, when a line-side tap needs a service-rated disconnect, and how a main-breaker derate affects the house's loads. They design strings to the inverter's minimum and maximum voltage windows across the local temperature range, plan the conduit route before the crew is on the roof, and lay out attachment points on the plan so the roofer and installer agree.

For a GC coordinating a project, the design is also the schedule: utility applications can take weeks, and nothing is energized without PTO.

Practice Challenge

A home has a 200-amp panel with a 200-amp main breaker. What is the largest PV backfeed breaker allowed under the 120 percent rule, and roughly what inverter output does that support? (Answer: 200 A × 1.2 = 240 A; minus the 200 A main leaves a 40 A PV breaker. Since inverter output is continuous, the breaker is sized at 125 percent of output, so about 32 A — roughly 7.6 kW at 240 volts. Anything larger needs a supply-side connection, a main-breaker downsize, or a panel upgrade.)

In Practice

A designer who draws the fire pathways first and checks the busbar before promising a system size turns in plan sets that get approved. A designer who promises the kilowatts first spends the next month redrawing.

Common Mistakes to Avoid

• Skipping the attic and guessing at framing
• Ignoring the 120 percent rule until the inspector points at the panel
• Laying out modules before drawing the fire pathways
• Installing on a roof that needs replacement within a few years

Takeaway: A solar design starts on the roof and at the service panel, not in software — sun, shade, structure, electrical capacity, and the fire-code pathways decide what can actually be built, and the permit and utility approvals decide when.

Educational overview only — not hands-on installation or electrical training, a safety program, or a substitute for work under a licensed electrician or NABCEP-certified professional. Follow the NEC as adopted in your jurisdiction, the building and fire codes, OSHA, the manufacturers' instructions, and your utility's interconnection requirements.