Solar Panel Installation Engineer Letter — Structural and Electrical

Rooftop solar PV installations trigger two parallel engineering questions: does the roof structure have the capacity to carry the additional dead and wind load, and does the electrical design meet the NEC. Most jurisdictions want both questions answered by a licensed professional engineer before the permit is issued, and increasingly before the final inspection is approved.

We write both sides of that letter.

Structural analysis

The structural side addresses four factors:

Dead load of the PV system. Modern crystalline silicon systems typically add 3–5 psf to the roof dead load. This is small in absolute terms but must be combined with the existing dead load, the live load, and the environmental loads per ASCE 7-22 load combinations.

Wind load at the array. The panels and racking see their own wind uplift and shear. For array-specific wind calculations we work from either ASCE 7-22 Chapter 29.4 (rooftop solar arrays) or the SEAOC PV2 guidelines, depending on the applicable code edition.

Attachment capacity. Lag screws, standoffs, or flashed mounts attaching to the truss top chords must develop the required withdrawal, shear, and combined capacity with appropriate factors of safety. The attachment schedule — fastener type, size, embedment, spacing — is spelled out in the letter.

Existing truss or rafter capacity. For sites built to a prescriptive code path, we verify the existing members have reserve capacity for the added load. For sites with engineered trusses, we request the truss specs and confirm the as-built members against the original design loads.

Electrical analysis

The electrical side addresses NEC Article 690 and 705 requirements:

String sizing, voltage at lowest expected ambient, conductor sizing, overcurrent protection per inverter and manufacturer recommendations.

Rapid shutdown and labeling per 690.12 and 690.56.

Point of connection — supply side tap vs. load side breaker vs. panel upgrade — per 705.12 with supporting load calculations.

Grounding and bonding of the array per 690.43 and 690.45, with specific attention to the metallic framing and mounting system.

When homeowners call us

The three most common solar engineer letter scenarios:

The contractor’s PE-of-record let the project lapse and the jurisdiction wants a new sealed letter before final inspection.

The homeowner bought a home with an existing PV system and the permit was never closed.

The existing PV system is being expanded or a battery is being added, requiring updated structural and electrical documentation.

In each case, the letter path is faster and cheaper than re-engineering the whole system. Contact us with the array data sheet, inverter spec, and a few roof photos — we usually quote within an hour.