This is the most asked question — and the most often answered wrongly. Dividing roof area by module area always overstates the count: it ignores the parapet setback, chimney and HVAC shading, service walkways and module orientation. SolarSimPro takes the roof with its real geometry and finds the module count by actually drawing the layout.
Modules sit differently on flat, monopitch, gable, hip, sawtooth and terrace roofs. A flat roof needs a mounting-table tilt and row spacing; on a pitched roof the modules run parallel to the surface.
Modules cannot butt up against the roof edge. A parapet casts a long shadow in winter, and a clearance is left at the edge for wind load. These two allowances alone can remove a tenth of the rough estimate.
Chimneys, HVAC units, vents, roof windows and stair openings both take up space and cast shade. Each one leaves an unusable area around it.
Portrait or landscape changes how many fit. Module length varies by manufacturer; the difference between 2.28 m and 1.76 m shifts the number of rows on the same roof.
The common shortcut is roof area ÷ module area. For a 300 m² roof and a 2.2 m² module that gives 136 modules. In practice such a roof usually takes 90-110. That gap is not just disappointment — it means the wrong capacity and the wrong price in your quotation.
The loss comes from three places: the edge and parapet setback, the dead area around obstacles, and the leftover strips where rows do not meet the roof edge exactly. None of these appear in a division.
The software takes the roof boundary, the parapet and the obstacles as geometry, and places modules inside those limits at their real dimensions. Once the layout exists, the module count is not an estimate — it is a countable result.
On a flat roof, row spacing is set by the shadow length at the winter solstice. Shorten the spacing and more modules fit, but the rows shade each other; the software shows that trade-off together with the yield loss.
No separate spreadsheet is needed; the number comes out of the layout itself.
As a rough upper bound you get 45 modules, but on most roofs the realistic figure is 30-38. Parapet setback, obstacles and module orientation create that gap. The exact number only comes from generating a layout on the roof's own geometry.
Module count × module rating gives the DC capacity. Forty 610 W modules make 24.4 kWp. The software shows this alongside the layout; you do not need to work it out separately.
If the modules lie parallel to the roof surface there is no inter-row shading, so no spacing is required. Row spacing becomes relevant on a flat roof when a tilt is created with mounting tables.
The shaded area can be excluded from the layout, or modules can be placed there and the loss measured. Near-shading analysis computes, hour by hour, how much each module is shaded across the year.
No installation, runs in the browser. There are only minutes between drawing the roof outline and having the module count.