Solar Panel Roof Fit Calculator
Last reviewed How we calculate
Before asking for solar quotes, it helps to know roughly how many panels a roof face can take. This works it out from the size of the roof face and the panels, after the clear edges that fire codes and installers leave, in either orientation.
How many solar panels fit
How many solar panels fit on a roof face after edge clearances, in portrait or landscape.
This calculator needs JavaScript to work. The measurements below are the ones you would need to do the sum by hand.
Panels that fit
- Columns across the roof
- Rows up the slope
- System size
- Area covered by panels
A layout check, not a solar design. Vents, chimneys, skylights and shade all take space, and the roof has to carry the panels and be in good enough condition to outlast them. If the roof will need replacing within a few years, doing it before the panels go on saves paying to remove and refit them.
How the calculation works
Take the clear edge off each side of the roof face to find the usable width and slope. Count how many panels, with their gaps, fit across that width and up that slope, and multiply. Portrait panels stand with their long side up the slope; landscape panels lie with it along the eave. The system size is the panel count times the rating of one panel.
A worked example
A roof face 40 ft wide with a 16 ft slope, 18 in of clear edge all round, and panels 41 in by 69 in with 1 in gaps, takes 10 columns and 2 rows in portrait: 20 panels, or 8 kW at 400 W each. In landscape it takes 6 columns and 3 rows, 18 panels. It is always worth trying both.
What it does not tell you
How much electricity the panels would make depends on the direction the roof faces, its tilt, shade and your location. Free tools such as NREL’s PVWatts estimate that from your address. Vents, chimneys and skylights also take space, and the roof itself needs to be sound enough to carry the panels for their whole life. If it will need replacing within a few years, replacing it first saves paying to take the panels off and put them back.
Frequently asked questions
Should solar panels be portrait or landscape?
Whichever fits more panels on your roof face, within what the racking allows. The calculator shows both, and the difference is often a few panels.
How much space do fire codes need around solar panels?
Fire codes often require clear pathways along the ridge and some edges so firefighters can get onto the roof, and the required widths vary by area and roof type. Your installer or building department will give the figure for your roof.
Should I replace my roof before installing solar?
If the roof has only a few years left, usually yes. Panels normally outlast a worn roof, and taking them off and refitting them later adds cost to the roof replacement.