Site Assessment
What you must be able to judge
A site assessment answers four questions: how much sun the array plane receives, how much structurally supported area is actually usable, where the electrical interconnection can physically and legally land, and what the customer's real objective is. Bring tools that measure rather than guess: compass corrected for magnetic declination, an inclinometer, a tape or laser measure, a shade-measuring device (sun path or fisheye analysis), a camera, and a ladder tall enough to inspect roof condition safely.
Record true azimuth (not compass bearing), roof pitch in degrees and rise/run, and both near-field shading (chimneys, vents, adjacent roofs) and far-field shading (trees, poles, future construction). Shading is not binary: a deciduous tree that shades an array from 4-5 pm in July matters little in many rates, but the same tree shading at 9 am in December can dominate winter production.
Traps
- Reporting compass heading without declination correction; in much of North America that is 10-20 degrees of error.
- Counting module capacity from gross roof area and ignoring fire-code access pathways, ridges, valleys, and vents.
- Measuring the service panel's main breaker but not its busbar rating and existing loads.
- Ignoring roof age: putting a 25-year array on an 18-year-old shingle roof invites a costly remove-and-reinstall.
- Ignoring utility interconnection limits, which can cap system size below roof capacity.
Worked example
A south-facing plane measures 40 ft x 20 ft = 800 ft2. After subtracting a ridge setback, one side pathway, and two vent clusters you have roughly 600 usable ft2. A 400 W module covers about 18 ft2, so at a 60-70% practical packing density you plan 24-28 modules (about 10-11 kW STC), not 44. Always design to the usable plane, then confirm with racking layout software.
Rule of thumb
Assume only 55-70% of gross roof plane area becomes module area on a residential roof with setbacks and obstructions.