Solar-ready roof pre-check

What a roof must concede before solar is worth discussing

The relation is two discounts applied to one measurement. The gross candidate area is the roof plane as a tape measure or a satellite tool reports it; the shading share removes the portion that trees, chimneys, dormers and neighbouring buildings keep dark; the orientation factor scales what remains by how well the plane’s tilt and compass bearing gather the local sun. Neither discount can manufacture roof — the answer only ever shrinks from what the tape said.

Shade deserves the most suspicion, because it is the great killer of rooftop solar and the easiest term to underestimate from the ground. The share that belongs here is a year-round judgement — the low winter sun throws shadows a summer site visit never sees — taken from a shade survey or a satellite screening tool rather than a hopeful glance. A roof that loses most of its area to shade has answered the solar question already, without arithmetic.

The orientation factor is the same convention the solar pack’s own pages use for tilt and bearing, which is what makes this page a bridge rather than a rival estimate: near the optimum the factor sits close to unity, an east- or west-facing plane concedes a meaningful slice, and the input is deliberately bounded so orientation can trim an area but never invent one. Matching the solar pack’s convention means the figure this page hands over needs no translation.

Read the output honestly: it is a go or no-go screen, not a layout. The usable area says whether the solar pack’s pages are worth opening — whether an array sized from the household’s daily load could physically fit, and whether a yield estimate would be built on more than wishful thinking. What it does not say is where panels go: fire-access setbacks, mounting rows, vents and the installer’s site visit all take further bites that only a design can price.

When this calculation is used

• Deciding whether to open the solar pages at all — a quick screen from a satellite measurement before any installer is called.
• Comparing the planes of one roof — the shaded south face against the clear west one — to find which candidate deserves the survey.
• Screening a portfolio of roofs to a shortlist, spending the site visits only where the usable area survives the discounts.
• Carrying the usable area into the solar pack, where the array-size and yield pages take it as their premise.

Worked example

Run the pack’s declared reference roof: a modest suburban roof plane, lightly shaded by a neighbour’s trees, set somewhat off the local optimum bearing — the ordinary imperfect roof most households actually own.

The engine returns the usable area — the gross plane with both concessions taken. Read it against the gross figure first: the share that survives is the roof’s honest solar character, and if the discounts have taken most of the plane, that is the screen doing its job before any money moved.

Every figure in this example is produced by the certified engine when the calculator loads, checked against the signed pack’s declared test vectors; nothing on this page stores an answer. Deepen the shading share and the usable area falls away linearly — the sensitivity that makes the shade survey the one input worth paying for.

What each input represents

Assumptions and limits

• Shade is compressed to one year-round share of the area: the hourly and seasonal sun path, and partial-shading effects on string wiring, are beyond what a screen resolves.
• Tilt and bearing are compressed to one utilization factor on the solar pack’s convention; the factor discounts area rather than modelling irradiance, which the yield page does properly.
• The area is treated as geometrically available: fire-access setbacks, mounting layout, vents and structural condition are the installer’s subtractions, taken after this screen.
• This is a screen, not a design: a passing answer earns a site visit, never a purchase order.

What the guards protect against

• A shading share approaching the whole roof is refused — the pack ships the refusal as a declared test vector — because a plane that dark has already failed the screen, and discounting the sliver that remains would dress a dead site up as a small one.
• The roof area must be positive and is capped far above any building, catching a campus’s worth of roof — or square feet entered where square metres belong — pasted into a house-sized screen.
• The orientation factor’s band refuses fractions describing a plane that faces away from the sky — a mistyped decimal, not a roof — and allows only a whisker above unity, matching the solar convention, because orientation can trim an area but never manufacture one.

Provenance

Rooftop solar screening pre-check: gross roof area discounted by shading fraction and orientation utilization

The gross candidate area is multiplied by the unshaded share of the plane and by the orientation utilization factor to yield the usable photovoltaic area — the figure the solar pack’s siting pages take as their starting premise.

Screening and reference material, to be checked against the governing standard and a qualified engineer; not a design determination. The signed pack carries its own citation, which displays from the verified leaf when the calculator loads.