Rooftop Solar Panel Layout and Row Spacing Calculator

How many solar panels fit on a roof?

It depends on the usable roof dimensions, the panel dimensions and how the panels are turned. First take off the margin left at the edges, then find how many panels fit in each direction:

panels per row = integer part of (usable width + gap) / (panel width + gap).

The same sum the other way gives the number of rows. Their product is the panel count, and times panel power it is the installed power.

Example: pitched roof

A roof face measures 12 m along the eaves and 6 m up the slope. With a 40 cm margin on every side the usable area is 11.2 m × 5.2 m. A 2278 mm × 1134 mm panel in portrait fits 9 across and 2 rows up the slope: 18 panels. At 550 W each the installed power is 9.9 kW. In landscape the same panel fits 4 × 4 = 16. Which orientation holds more depends on the roof dimensions, so try both.

Row spacing on a flat roof

On a flat roof panels sit on tilted frames and one row can shade the next. The shadow is longest at the winter solstice. The height of the row's top edge is h = L × sin β and the shade free gap is:

d = h × cos(Δγ) / tan α

α is the solar altitude and Δγ the difference between the sun's azimuth and the direction the rows face. At winter solstice noon the altitude is α = 90° − latitude − 23.44°. At 40° latitude that is 26.56°, and a row 1 m high casts a 2.0 m shadow behind it.

Keeping only noon clear is not enough. A common criterion is no shading between 9 and 15 solar time on the winter solstice. In the same example the altitude at 9 and 15 is 14° and the gap needed is 3.0 m. A larger gap fits fewer rows on the roof but protects winter output.

Why does a lower tilt fit more panels?

At a low tilt the row is lower, its shadow shorter and the rows closer. That is why flat roofs often use 10° to 15° tilt or an east west double row layout. Output per panel drops a little while output per roof rises.

Limits

The tool does geometry only. In a real project shading from obstructions, fire and maintenance walkways, parapet setbacks, roof load capacity and wind and snow loads can govern. For annual yield see the solar energy yield calculator and for string voltages the string sizing calculator.