How much of that roof can the array actually use?

Start with the gross area of the roof plane, measured along the eave and up the slope rather than on the ground. Then subtract the clear areas the array is required to leave: a setback along the ridge, any setback at the eave, and the access pathways that have to run across the plane. What remains is the envelope an array can occupy. Dividing that envelope by the module footprint gives an upper bound on module count, which real obstructions and non-rectangular planes will reduce further.

More than most proposals assume, and disproportionately on small or cut-up roofs. A single 36 in pathway across a 30 ft plane removes a tenth of its width, and a 36 in ridge setback on an 18 ft slope removes a sixth of its depth. Because modules only fit in whole rows and columns, losing part of a row loses the entire row. That is why the same roof can lose one module or four to the same rule depending on the module size and the plane dimensions.

No. The NEC governs the electrical installation. Access pathways and ridge setbacks come from the fire code and the residential building code, most commonly the International Fire Code and International Residential Code as adopted and amended locally. This matters because the two are reviewed by different people on different schedules, and an array that satisfies the NEC can still be redrawn for a setback.

Whatever the jurisdiction has adopted, which is why the calculator makes it an input rather than a constant. Older editions of the fire code commonly required a clear setback of about three feet from the ridge. Later editions introduced a reduced setback, commonly around 18 inches, available under conditions. The reduced figure is conditional rather than automatic, so confirm which applies before designing to it.

Because aerial measurement gives the gross plane, and an array cannot use the gross plane. The calculator shows both numbers side by side for exactly this reason: the difference between them is the module count a proposal built from imagery tends to include and a permitted design cannot deliver. Discovering it at plan review means redrawing the array and re-presenting a smaller system to a homeowner who has already signed.

No, and it will always be optimistic. A grid on a clean rectangle ignores vents, skylights, chimneys, hips and valleys, plane shapes that are not rectangles, inter-row spacing, and where the equipment has to go. Every one of those subtracts. Use this to sanity-check a proposal early and to see how sensitive the answer is to the setback figures, then let the designer produce the layout.

Not usefully. Roof access pathways and ridge setbacks exist so firefighters can work on a roof, so they have no application to an array in a field. A ground mount is constrained instead by property-line setbacks, height limits, lot coverage and the zoning review, which are a different calculation on a different plane.