Solar Panel Roof Area Calculator

Each row is one panel as its maker prints it on the datasheet or product page. Each area is worked out from the printed millimetre dimensions at 0.09290304 square metres per sq ft. REC's own datasheets print an area of 20.8 sq ft for both REC panels. Panel dimensions vary by manufacturer. Always check the spec sheet for exact measurements before planning your layout. The table shows the printed sizes of these named panels only, not every panel on sale.

1. Start with total roof dimensions. Measure or estimate the length and width of each roof face. For a simple gable roof, each side is roughly half the total footprint. A 30' x 50' house covers 1,500 sq ft, so each face of a simple gable covers 750 sq ft in plan, seen from above. The sloped surface the panels sit on is larger: plan area x the square root of (1 + pitch squared), with pitch as rise over run. At a 6/12 pitch (0.5) that is 750 x 1.118 = about 839 sq ft per face.
2. Subtract obstacles. Vents, plumbing stacks, chimneys, skylights, and dormers all eat into panel space. Each one takes its own footprint plus the buffer zone around it where panels cannot sit, so measure them on your own roof.
3. Apply fire code setbacks. Fire codes keep clear pathways on the roof for firefighter access, and your local fire department sets the rule. As one example, for solar photovoltaic systems on one and two family dwelling units, the Los Angeles Fire Department requires panels on a hip roof to leave one three-foot wide clear access pathway from the eave to the ridge on each roof slope that carries panels, and panels on a building with a single ridge to leave two three-foot wide access pathways from the eave to the ridge on each slope that carries panels. How much area the pathways take depends on your roof's shape and your local code, so check it before you plan the layout.
4. Consider shading. Any area shaded during the middle of the day, when the sun is strongest, loses output, and the longer the shade lasts the more it loses. Nearby trees, neighbouring buildings, and even other roof features cast shadows that shift throughout the day and year.
5. Work out your usable percentage. Divide the area left after these deductions by the total roof area. NREL's rooftop study (Technical Report NREL/TP-6A20-65298) found that only 26% of the total rooftop area on small buildings, those with a footprint smaller than 5,000 sq ft, is suitable for PV, and it cites earlier studies that put the share at 22%-27% of residential rooftop area and 60%-65% of commercial rooftop area. The calculator starts at 26%. Your own roof decides, so enter the share you measured. A professional site survey can give you an exact number using satellite imagery and shade analysis tools.

A south-facing roof (in the northern hemisphere) produces the most energy. But not everyone has a perfect south-facing roof, and the output difference between orientations matters.

Southeast or southwest-facing roofs give up a little against true south. East or west-facing roofs produce less, and how much less depends on your latitude and tilt. A north-facing roof produces the least and is generally not worth installing panels on unless no other option exists.

Roof pitch matters too. The ideal tilt angle equals your latitude, so it changes from south to north across the country. A flat roof produces less than a roof tilted toward the sun. A steep 45-degree roof aimed south works well at northern latitudes but loses output at southern ones where the sun is higher.

The practical takeaway: if your south-facing roof is small but your east/west faces are large, the east/west faces often produce more total energy despite lower per-panel efficiency. Maximise total array size, not per-panel perfection.