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30. Site Assessment & Survey

Learning objectives

  • Capture the field data that confirms or corrects the design.
  • Evaluate roof, shade, electrical, and structural readiness.

30.1 Measuring reality

A design built from satellite images must be verified on site. The survey captures:

  • Roof: precise measurements, pitch, true azimuth, covering type and remaining life (don’t put a 30-year array on a 5-year roof), and accessible structure (rafter/truss size and spacing).
True azimuth: compass bearing of the roof surface measured from true (geographic) north rather than magnetic north. Used to calculate the panel's actual solar exposure, since magnetic north can deviate several degrees from true north depending on location.

Asphalt-shingle roof showing multiple planes, a chimney, and vents. Figure 30.5: Roof planes, orientation, condition, and obstructions (chimney, vents).

  • Shade: a real TSRF/solar-access reading across the 9–3 window (Ch 18) with a Solar Pathfinder, drone, or app, not an assumption.
TSRF (Total Solar Resource Fraction): a site-specific percentage expressing how much of the available solar resource reaches the array after accounting for shading, tilt, and orientation losses. A TSRF of 100% means no losses; anything below roughly 80% warrants redesign or shade mitigation.

Rooftop shading-assessment tool on a tripod facing the low sun and skyline. Figure 30.3: Shading / solar-window assessment toward the equator-facing sky.

  • Electrical: service rating (busbar and main breaker → the 120% rule, Ch 16.4), meter and main locations, the interconnection point, and available breaker space.
120% rule (NEC 705.12): the limit on how much breaker capacity can back-feed a panelboard busbar. The sum of the main breaker plus all supply-side breakers (including the PV breaker) cannot exceed 120% of the busbar rating. This determines the maximum PV breaker size for a given service panel.

Opened residential service panel with a gloved hand indicating the rating label. Figure 30.1: Service panel: photograph and read the rating label (main-breaker / busbar rating): it drives interconnection sizing.

Utility smart meter beside a service disconnect on an exterior wall. Figure 30.2: Utility meter and service disconnect (interconnection point of reference).

  • Structure: confirm framing adequacy for the added dead load (Ch 25) and flag anything marginal for an engineer.

Attic interior showing roof rafters, sheathing, and structure from below. Figure 30.4: Attic / structure: rafter size and spacing for attachment.

  • Logistics: equipment access, conductor routing paths, and inverter/disconnect/storage locations.

30.2 Document everything

Photos, measurements, and notes from the survey become the basis of the final plan set and protect against disputes. A reroof recommendation, a too-small service, or a shaded ridge discovered now is cheap; discovered after install, it’s a disaster.

30.3 Site survey checklist

 ROOF      ☐ measurements ☐ pitch ☐ true azimuth ☐ covering type & remaining life
           ☐ rafter/truss size & spacing ☐ obstructions (vents, chimneys, skylights)
 SHADE     ☐ TSRF / solar access across 9–3 window (Pathfinder/drone/app)
 ELECTRICAL☐ service rating (busbar + main) ☐ meter & main location
           ☐ interconnection point ☐ available breaker spaces ☐ 120% rule headroom
 STRUCTURE ☐ framing adequacy for added dead load ☐ engineer flag if marginal
 LOGISTICS ☐ equipment access ☐ conductor routing ☐ inverter/disconnect/ESS locations
 RECORD    ☐ photos ☐ measurements ☐ notes for the plan set

Chapter 30 summary

Verify the design in the field: roof geometry/condition/structure, real shade access, service rating and interconnection point, structural adequacy, and site logistics, all documented. Catching problems here is the cheapest they’ll ever be.

  • True azimuth: compass bearing from true (geographic) north, used to quantify a roof surface’s actual solar exposure.
  • TSRF (Total Solar Resource Fraction): site-measured percentage of available solar resource reaching the array after shading, tilt, and orientation losses.
  • 120% rule (NEC 705.12): the busbar-loading limit that sets the maximum PV breaker size for a given service panel.
  • Interconnection point: the electrical connection between the PV system and the utility grid or building service, established during the survey.
  • Solar-access window (9–3): the 9 a.m.–3 p.m. period used for shade assessment; shading losses outside this window have minimal production impact.

Full definitions: Appendix A (glossary).

Practice Problems: Chapter 30

  1. Why check the roof covering’s remaining life during the survey?
  2. Which electrical figure determines how much PV you can back-feed onto the panel, and which rule uses it?
  3. A satellite-based design assumed an unshaded roof; the survey finds a tall tree to the south. What changes?
  4. Name two things that, if discovered post-install instead of at survey, become expensive disasters.

Solutions: Chapter 30

  1. So you don’t mount a 25–30-year array on a roof needing replacement in a few years (forcing a costly removal/reinstall).
  2. The service/busbar rating (with the main breaker), used by the 120% busbar rule (Ch 16).
  3. The real TSRF drops; production estimates and possibly the array layout/size must be revised, or MLPE added to limit shade losses.
  4. Any two: a too-small service, a roof needing replacement, shading that guts production, or inadequate structure: all are cheap to fix on paper, ruinous after install.