Yield and sizing
Number of panels that fit a roof (portrait)
- where
- columns of modules across the roof
- roof width along the eaves (m)
- edge clearance on every side (mm)
- gap between modules (mm)
- module width (mm)
- rows up the roof
- roof slope length (m)
- module length (mm)
- modules that fit
Explain this equation
Gives a quick count of how many panels fit on a plain rectangular roof in portrait, with clearance at the edges. Use it at the survey before a proper layout drawing.
The roof dimensions are converted to millimetres and the edge clearance is taken off both sides. Each panel takes up its width plus one clamp gap, but the last panel needs no gap after it, which is why one gap s is added back before dividing. Rounding down gives whole panels. Rows work the same way up the slope with the panel length.
Fitting whole items with gaps into a length: (space + gap) ÷ (item + gap), rounded down.
Sense check: an 8 × 5 m roof with 400 mm clearance and 1722 × 1134 mm panels takes 6 across and 2 up, so 12 panels.
Watch out: chimneys, vents, roof windows and the actual rail and fixing positions all reduce this. Check edge zones with the mounting system's wind design.
A first layout count for a plain rectangular roof face, with modules in portrait in a regular grid.
Enter your values to see the result.
Good to know
MIS 3002 says modules should not be within 400 mm of any edge of a domestic roof unless extra measures for wind uplift, ridge tiles, run-off, snow and noise are taken; that is the default here. The gap is the mid-clamp width from the mounting system data, typically around 20 mm. For landscape, swap L and B. Check rafter positions, roof windows, vents and the mounting system's rail overhang limits on site.
Common mistake
Measuring the roof plan width on the drawing instead of the actual slope length, which understates the rows on a pitched roof.
Limits
Rectangular, unobstructed roof face only. Hips, valleys, obstructions and fire-access or wind-zone layouts from the mounting system design (MCS 012) need a drawn layout.
Sources
- MCS MIS 3002:2025 Solar PV installation standard, Issue 1.0 (3.6.8: 1% voltage drop practicable for domestic AC cabling, up to 3% for larger systems; 3.9.6: no modules within 400 mm of any edge of a domestic roof unless extra measures are taken; BS 7671 takes precedence)
- Module manufacturer datasheet: STC values (Voc, Vmp, Isc, Imp, Pmax), temperature coefficients of Voc, Pmax and Isc, NOCT/NMOT and dimensions
Results are calculations from the published methods and figures shown, and depend on the data you enter. Follow MCS standards, manufacturer data, BS 7671, the ENA connection rules and Building Regulations. System design and DNO applications need a competent, certified installer.