Solar Resource and Site Assessment

Why Site Assessment Matters

Before a single module is mounted, an installer must know exactly what the sun sees at that location. A roof that faces the wrong direction, sits under a neighbor's tree for half the afternoon, or pitches at an angle that collects little winter sun will underperform regardless of equipment quality. A thorough site assessment captures roof geometry, orientation, tilt, and shading, then cross-checks those measurements against published solar resource data so the design team can predict annual energy output with confidence. NABCEP expects entry-level technicians to perform this assessment accurately and document it on a standardized form.

Solar Resource Basics

The sun's energy reaching a surface is described as irradiance (power per unit area, W/m²) and irradiation or insolation (energy over time, kWh/m²). NREL's National Solar Radiation Database underpins the PVWatts Calculator, a free online tool that retrieves typical meteorological year data for any US ZIP code and estimates annual AC energy output for a proposed array. PVWatts uses default assumptions — a south-facing array at a fixed tilt equal to site latitude — but allows you to enter your measured tilt and azimuth so you can see exactly how real-world orientation changes the output estimate.

Peak Sun Hours (PSH) is a simplified way to express daily insolation: the number of hours of equivalent full-sun (1,000 W/m²) that would deliver the same total energy. A location with 5.0 PSH receives 5.0 kWh/m² per day on average.

Roof Geometry and Orientation

Azimuth is the compass bearing of a surface's outward-facing normal, measured clockwise from true north. In the continental US, a south-facing roof has an azimuth of 180°. PVWatts uses this same convention. A small error in azimuth — even 20° to 30° off south — can reduce annual output by a few percent, so accuracy within ±5° is required for a valid assessment.

Tilt (or pitch) is the angle between the roof surface and horizontal. A flat roof has a tilt of 0°; a steep residential roof may be 30° to 40°. Tilt is measured with a digital inclinometer placed along the roof slope. The optimum fixed tilt for maximum annual production is roughly equal to site latitude for most US locations, though east- or west-facing arrays at lower tilts can still be viable.

To document geometry, measure all ridge and eave lengths with a tape measure, record setback distances (code-required clear paths for firefighter access), note any penetrations such as vents and skylights, and sketch the roof to scale on the survey form. Label each plane with its azimuth and tilt.

Shading Assessment

Shading is the most common cause of underperformance. Even partial shading of one cell in a string can disproportionately reduce string output without bypass diodes, and can still reduce output meaningfully with them. You must identify every obstruction visible from the array area: chimneys, roof vents, adjacent buildings, trees, utility poles, and antenna masts.

A Solar Pathfinder uses a convex reflective dome to show a reflected image of the full sky dome above a site. The installer positions it level and north-oriented, then reads the sun-path chart projected onto the dome to see which portions of the sun's annual path are obstructed. A Solmetric SunEye uses a fisheye camera and GPS to generate a digital sky image and calculates shading losses automatically. Either tool must be placed at multiple points across the planned array area — at least three — because obstructions vary with position on the roof.

Record the dominant obstructions (compass bearing, estimated altitude angle, and seasonal impact) on the survey form. An acceptable shading report names the obstructions and notes during which months and times of day they cast shade on the array.

Compass and Inclinometer Use

A standard magnetic compass reads magnetic north. For solar work you need true north. The difference is magnetic declination, which varies by location (east or west of true north) and changes slowly over years. Obtain the current declination for your site from NOAA's magnetic declination calculator or a printed isogonic chart, and apply the correction before recording azimuth. Failing to correct declination is a common field error that can push a reading outside the ±5° tolerance.

Place the digital inclinometer flat on the roof surface, parallel to the slope direction, and confirm it reads zero on a level surface before use. Take three readings and average them if they vary slightly.

Safety on the Roof

This field activity may take place on an actual roof. Apply the following controls:

• Fall protection: Wear a properly fitted harness and attach to an anchor rated for fall arrest whenever the roof pitch is 4:12 or steeper, or whenever work is within six feet of an unprotected edge, per OSHA requirements for construction.
• Hard hats: Required when working below other personnel or under suspended loads.
• PPE baseline: Safety glasses, work gloves, and slip-resistant footwear are the minimum for any roof activity established in Course 1.
• Tool tethering: Tape measures, inclinometers, and compasses must be tethered or pocketed when hands-free movement is needed.
• Buddy system: Never work on a roof alone. A second person on the ground or roof is required.
• Weather: Postpone roof access if the surface is wet, icy, or if lightning is possible.

Using NREL PVWatts

Navigate to pvwatts.nrel.gov, enter the site address or ZIP code, and allow the tool to locate the nearest TMY weather station. Enter your measured DC system size, module type, array type (fixed open rack, roof mount, etc.), system losses, tilt, and azimuth. Compare the annual kWh estimate using default values against your measured values to quantify the impact of orientation differences. A 10° azimuth error on a south-facing array in a mid-latitude US location typically shifts annual output by roughly 1–3%, giving students a concrete sense of measurement stakes.

Completing the Survey Form

A complete, accepted survey form includes: site address, date, assessor name, scaled roof sketch with labeled dimensions and plane orientations, tilt and azimuth readings with compass declination correction noted, shading tool used, shading data for each of the three measurement points, dominant obstructions listed, and the PVWatts output comparison table. All fields must be filled before the form is submitted for instructor review.