How to use the solar panel roof calculator
Early in a project, clients often ask how much solar a roof can take. This calculator gives a quick, defensible answer for one roof plane: how many standard modules fit inside the setbacks, the resulting system size in kilowatts peak (kWp) and a first estimate of annual generation. It tests both portrait and landscape layouts and reports the better one, so you can check whether a roof shape or dormer position is costing you panels.
- Choose units. Metric mode takes metres for the roof and millimetres for the panel. US mode takes feet and inches.
- Enter the roof plane as measured on the slope: the width along the eave and the length from eave to ridge. Measure each plane separately and add the results.
- Enter the setbacks. The edge setback applies to the eave and both sides, the ridge setback to the top. Use the margins your fire code and installer require.
- Enter the panel size, gap and rating from the module data sheet. Gaps of 10–25 mm between modules are common with standard clamps.
- Enter peak sun hours and a performance ratio for the yield estimate. Use a regional solar map or PVGIS for sun hours on the actual tilt and orientation.
Solar panel fit and yield formulas
Usable width = roof width − 2 × edge setback · usable length = roof length − edge setback − ridge setback
Columns = floor((usable width + gap) ÷ (panel width + gap)) · rows = floor((usable length + gap) ÷ (panel length + gap))
Panels = columns × rows (best of portrait and landscape)
System size (kWp) = panels × panel watts ÷ 1,000
Annual yield (kWh) = kWp × peak sun hours × 365 × performance ratio
The layout is a simple rectangular grid, which is how most residential arrays are installed. Vents, skylights, chimneys and shading from trees or neighbouring buildings will reduce the real count, so treat the result as a maximum for a clear plane. The table shows typical module sizes.
| Module type | Typical size | Typical rating |
|---|---|---|
| 60-cell / 120 half-cell | 1,650–1,760 × 1,000–1,050 mm | 300–380 W |
| 108 half-cell residential | about 1,720 × 1,134 mm | 400–450 W |
| 72-cell / 144 half-cell | about 2,000–2,100 × 1,000–1,050 mm | 400–500 W |
| Large commercial | about 2,280 × 1,130 mm | 540–600 W |
Worked examples
A metric pitched roof
A south-facing plane is 10 m along the eave and 5 m on the slope, with 0.45 m setbacks. The usable area is 9.1 × 4.1 m. In portrait, 1,134 × 1,722 mm panels with 20 mm gaps give 7 columns and 2 rows, or 14 panels. In landscape they give 5 columns and 3 rows, 15 panels, so landscape wins. At 430 W that is 6.45 kWp, and with 4 peak sun hours and a 0.8 ratio it makes about 7,530 kWh a year.
A US garage roof
A 24 × 14 ft plane with 18 in setbacks leaves 21 × 11 ft. Using 45 × 68 in panels with ¾ in gaps, portrait fits 5 columns and 1 row, while landscape fits 3 columns and 2 rows, 6 panels. At 400 W that is 2.4 kWp and, at 5 peak sun hours and 0.8, about 3,500 kWh a year.
Checking a design change
On the first roof, cutting the ridge setback to 0.3 m adds nothing, because the usable length of 4.25 m still only takes 2 rows in portrait or 3 in landscape. Cutting the edge setbacks to 0.35 m instead widens the usable area to 9.3 m, which lets portrait fit 8 columns and 2 rows, 16 panels or 6.88 kWp. Knowing which margin actually limits the array tells you which one is worth negotiating.
Who this calculator helps
- Architects testing roof forms, pitches and dormer positions for solar capacity at concept stage.
- Sustainability consultants making a first pass at on-site renewable targets before detailed modelling.
- Builders and developers checking whether a roof can meet a planning or energy code renewable requirement.
- Homeowners sanity-checking a solar quote against the size of their roof.
Practical tips
- Get the sloped roof length from plan dimensions with the roof pitch calculator, and the total roof area with the roofing calculator.
- Check orientation and seasonal sun angles with the sun path calculator; low winter sun increases shading from nearby objects.
- Pair panels with water reuse on the same roof using the rainwater harvesting calculator.
- Keep the array as one clean rectangle where possible. It looks better and usually costs less to mount and wire.
- Confirm the roof structure can carry the extra dead load, typically 10–20 kg/m² for panels and racking.
Want to show a client how the array will look? Render it onto a photo of the house with AI Exterior Redesign.
